Results of the flow cytometry ACTG quality control program: analysis and findings

A Critical Review on the Standardization and Quality Assessment of Nonfunctional Laboratory Tests Frequently Used to Identify Inborn Errors of Immunity

Inborn errors of immunity (IEI), which were previously termed primary immunodeficiency diseases, represent a large and growing heterogeneous group of diseases that are mostly monogenic. In addition to increased susceptibility to infections, other clinical phenotypes have recently been associated with IEI, such as autoimmune disorders, severe allergies, autoinflammatory disorders, benign lymphoproliferative diseases, and malignant manifestations. The IUIS 2019 classification comprises 430 distinct defects that, although rare individually, represent a group affecting a significant number of patients, with an overall prevalence of 1:1,200-2,000 in the general population. Early IEI diagnosis is critical for appropriate therapy and genetic counseling, however, this process is deeply dependent on accurate laboratory tests. Despite the striking importance of laboratory data for clinical immunologists, several IEI-relevant immunoassays still lack standardization, including standardized protocols, reference materials, and external quality assessment programs. Moreover, well-established reference values mostly remain to be determined, especially for early ages, when the most severe conditions manifest and diagnosis is critical for patient survival. In this article, we intend to approach the issue of standardization and quality control of the nonfunctional diagnostic tests used for IEI, focusing on those frequently utilized in clinical practice. Herein, we will focus on discussing the issues of nonfunctional immunoassays (flow cytometry, enzyme-linked immunosorbent assays, and turbidimetry/nephelometry, among others), as defined by the pure quantification of proteins or cell subsets without cell activation or cell culture-based methods.

MiSet RFC Standards: Defining a Universal Minimum Set of Standards Required for Reproducibility and Rigor in Research Flow Cytometry Experiments

Poor adherence to best practices, insufficient training, and pressure to produce data quickly may lead to publications of suboptimal biomedical research flow cytometry data, which contributes to the body of irreproducible research findings. In addition, documentation of compliance with best flow cytometry practices for submission, visualization, and publication of flow cytometry data is currently endorsed by very few scientific journals, which is particularly concerning as numerous peer-reviewed flow cytometry publications emphasize instrumentation, experimental design, and data analysis as important sources of variability. Guidelines and resources for adequate reporting, annotation and deposition of flow cytometry experiments are provided by MIFlowCyt and the FlowRepository database, and comprehensive expert recommendations covering principles and techniques of field-specific flow cytometry applications have been published. To facilitate the integration of quality-defining parameters into manuscript and grant submission and publication requirements across biomedical fields that rely on the use of flow-cytometry-based techniques, a single comprehensive yet easily and universally applicable document is needed. To produce such a list of gold-standard parameters that assess whether a research flow cytometry experiment has been planned, conducted, interpreted, and reported at the highest standard, a new initiative defining the minimum set of standards a robust and rigorous research flow experiment must fulfill (MiSet RFC Standards) was proposed at CYTO 2019. MiSet RFC Standards will integrate and simplify existing resources to provide a universal benchmark a flow cytometry experiment can easily be measured against. The goal of MiSET RFC Standards is its integration into peer-review and publication procedures through partnership with stakeholders, journals and publishers in biomedical and translational research. This article introduces the aims and anticipated timeline and discusses strategies for interdisciplinary consensus and implementation. A single-resource broadly applicable guideline will harmonize standards across different fields of biomedical research and lead to publication of more robust research findings. © 2019 International Society for Advancement of Cytometry.

Impact of blood storage and sample handling on quality of high dimensional flow cytometric data in multicenter clinical research

Obtaining reliable and reproducible high quality data in multicenter clinical research settings requires design of optimal standard operating procedures. While the need for standardization in sample processing and data analysis is well-recognized, the impact of sample handling in the pre-analytical phase remains underestimated. We evaluated the impact of sample storage time (≈transport time) and temperature, type of anticoagulant, and limited blood volume on reproducibility of flow cytometric studies. EDTA and Na-Heparin samples processed with the EuroFlow bulk lysis protocol, stained and stored at 4 °C showed fairly stable expression of cell surface markers and distribution of the major leukocyte populations for up to 72 h. Additional sample fixation (1% PFA, Fix & Perm) did not have any beneficial effects. Blood samples stored for <24 h at room temperature before processing and staining seemed suitable for reliable immunophenotyping, although losses in absolute cell numbers were observed. The major losses were observed in myeloid cells and monocytes, while lymphocytes seemed less affected. Expression of cell surface markers and population distribution were more stable in Na-Heparin blood than in EDTA blood. However, storage of Na-Heparin samples was associated with faster decrease in leukocyte counts over time. Whole blood fixation strategies (Cyto-Chex, TransFix) improved long-term population distribution, but were detrimental for expression of cellular markers. The main conclusions from this study on healthy donor blood samples were successfully confirmed in EDTA clinical (patient) blood samples with different time delays until processing. Finally, we recognized the need for adjustments in bulk lysis in case of insufficient blood volumes. Despite clear overall conclusions, individual markers and cell populations had different preferred conditions. Therefore, specific guidelines for sample handling should always be adjusted to the clinical application and the main target leukocyte population.

A Brief Chronicle of CD4 as a Biomarker for HIV/AIDS: A Tribute to the Memory of John L. Fahey

Foundational cellular immunology research of the 1960s and 1970s, together with the advent of monoclonal antibodies and flow cytometry, provided the knowledge base and the technological capability that enabled the elucidation of the role of CD4 T cells in HIV infection. Research identifying the sources and magnitude of variation in CD4 measurements, standardized reagents and protocols, and the development of clinical flow cytometers all contributed to the feasibility of widespread CD4 testing. Cohort studies and clinical trials provided the context for establishing the utility of CD4 for prognosis in HIV-infected persons, initial assessment of in vivo antiretroviral drug activity, and as a surrogate marker for clinical outcome in antiretroviral therapeutic trials. Even with sensitive HIV viral load measurement, CD4 cell counting is still utilized in determining antiretroviral therapy eligibility and time to initiate therapy. New point of care technologies are helping both to lower the cost of CD4 testing and enable its use in HIV test and treat programs around the world.

Imunidade celular em caninos neonatos - do nascimento ao 45° dia de idade

O objetivo do presente trabalho foi acompanhar o desenvolvimento imunológico dos neonatos caninos, a fim de avaliar a imunidade celular pela análise dos leucócitos e linfócitos totais e das subpopulações de linfócitos T (CD4+ e CD8+) pela técnica de citometria de fluxo. Foram utilizados 30 cães neonatos de ambos os sexos, sem raça definida, aos três, 10, 17, 24, 31, 38 e 45 dias de idade. A contagem de leucócitos totais aos 45 dias (11.639±3.574) foi significativamente maior que no terceiro dia de idade (8.740±1.812) (P<0,05); não houve diferença entre a contagem total de linfócitos aos 45 dias em relação ao terceiro dia de idade. Quanto às subpopulações de LT CD4+ e LT CD8+, os percentuais de LT CD4+, aos três dias de idade (24,9±16,8%), foram inferiores quando comparados à média entre o 10°, o 24° e o 31°dia (35,5%), e os de CD8+, ao terceiro dia, menores em relação às médias do 10° e do 31° dia de idade. Pode-se concluir que as subpopulações de LT CD4+ e CD8+ sofrem oscilações durante o desenvolvimento pós-natal, sendo estas crescentes em relação aos níveis obtidos aos três dias de idade. A relação CD4+:CD8+ mostrou superioridade para o primeiro tipo celular, sendo que a maior relação entre CD4+ e CD8+ ocorreu no terceiro dia de idade. Com base nos resultados obtidos neste estudo, notaram-se as diferenças semanais nas populações linfocitárias, o que demonstra a dinâmica dessas células durante o período neonatal.

Statistical methods for the assessment of EQAPOL proficiency testing: ELISpot, Luminex, and Flow Cytometry

In September 2011 Duke University was awarded a contract to develop the National Institutes of Health/National Institute of Allergy and Infectious Diseases (NIH/NIAID) External Quality Assurance Program Oversight Laboratory (EQAPOL). Through EQAPOL, proficiency testing programs are administered for Interferon-γ (IFN-γ) Enzyme-linked immunosorbent spot (ELISpot), Intracellular Cytokine Staining Flow Cytometry (ICS) and Luminex-based cytokine assays. One of the charges of the EQAPOL program was to apply statistical methods to determine overall site performance. We utilized various statistical methods for each program to find the most appropriate for assessing laboratory performance using the consensus average as the target value. Accuracy ranges were calculated based on Wald-type confidence intervals, exact Poisson confidence intervals, or via simulations. Given the nature of proficiency testing data, which has repeated measures within donor/sample made across several laboratories; the use of mixed effects models with alpha adjustments for multiple comparisons was also explored. Mixed effects models were found to be the most useful method to assess laboratory performance with respect to accuracy to the consensus. Model based approaches to the proficiency testing data in EQAPOL will continue to be utilized. Mixed effects models also provided a means of performing more complex analyses that would address secondary research questions regarding within and between laboratory variability as well as longitudinal analyses.

Early immunological predictors of neurodevelopmental outcomes in HIV-infected children

BACKGROUND

A previous analysis of children infected with human immunodeficiency virus (HIV) in the Women and Infants Transmission Study showed a strong correlation between low activated CD8(+) T lymphocytes in the first 2 months of life and good immunological prognosis. We sought to extend these observations to neurodevelopmental prognosis.

METHODS

Ninety-eight HIV-infected children born before 1994 with flow cytometric data from the first 2 months of life and adequate neurodevelopmental testing through age 30 months were studied. Children were divided into those with low (<or=5% CD8(+)HLA-DR(+) cells or <or=25% CD8(+)CD38(+) cells) or high (>5% CD8(+)HLA-DR(+) cells or >25% CD8(+)CD38(+) cells) immune activation at 1 and/or 2 months of age. Analysis was performed using survival analysis, Cox's proportional hazard regression, and longitudinal regression models.

RESULTS

Absence of immune activation, measured as <or=5% CD8(+)HLA-DR(+) cells, was strongly associated with better performance on the psychomotor developmental index of the Bayley scales of infant development through the third year of life. This association persisted after adjustment for CD4 cell count, viral load, and progression to acquired immunodeficiency syndrome (P= .005). An association with the mental development index was also present (P= .048). Significant association between neurodevelopmental outcomes and <or=25% CD8(+)CD38(+) cells was not seen.

CONCLUSIONS

In this prospective cohort study of HIV-infected children, there was a significant favorable association of low immune activation in peripheral T cells at age 1 or 2 months, measured by a low percentage of CD8(+)HLA-DR(+) cells, with subsequent psychomotor and mental development. This association was independent of other indices of severity and progression of HIV infection.

Immunophenotyping of T cell subpopulations in HIV disease

This unit illustrates the challenges associated with measurements of T cell subpopulations, and describes how those challenges can be overcome by using polychromatic (5+ color) approaches to flow cytometry. Such approaches are immensely powerful and directly applicable to studies of T cell biology in HIV disease; however, their development requires careful consideration of the antibodies and fluorochromes employed, as discussed here. Furthermore, factors generally known to influence T cell counts can also govern the success or failure of polychromatic experiments; therefore, these factors are reviewed and provide recommendations for minimizing their influence on polychromatic measurements of T cell subsets.

Association of cells with natural killer (NK) and NKT immunophenotype with incident cancers in HIV-infected women

Evidence indicates that immunosupression is associated with the development of certain cancers. The pathogenesis of HIV disease includes an alteration in innate immunity, mediated through NK and NKT cells. The evaluation of innate immune status in HIV patients prior to cancer diagnosis may identify the specific immunological events preceding the development of malignant disease. We evaluated the association between immunophenotypically defined NK, NKT, and CD8(+) cell percentages and incident malignancies in 1817 HIV(+) women in the Women's Interagency HIV Study (WIHS) who were followed for a median of 7.5 years. A total of 52 incident cancers of 20 different sites were identified. Compared to cancer-free women, cancer cases were older (p < 0.01), more likely to be anti-HCV(+) (p = 0.02), and had higher baseline median HIV RNA levels than controls. The CD8(+), NK, and NKT percents at baseline were not related to cancer risk. However, when time-dependent values for NKT cells were used, higher levels of NKT cells were associated with a reduced risk of cancer (adjusted hazard ratio = 0.67, 95% CI = 0.50, 0.89 per NKT percentage point). In addition to the loss of CD4(+) lymphocytes and an increased risk of opportunistic infections, HCV coinfected individuals may also experience alterations in innate immunity, including reduced NKT and NK cell number and possibly their function. In time-dependent analyses, increased numbers of NKT cells were associated with a reduced risk of cancer. HIV-induced innate immune dysfunction may contribute to the eventual emergence of cancer in the setting of existing coinfections and altered immunosurveillance.

The 2005 Italian Quality Control Study for the evaluation of CD4 cells in centers involved in the treatment of HIV type 1 patients

We report the results of an external quality control program, including 17 Italian centers involved in the care of patients infected by HIV, to evaluate CD4 T cell count proficiency and reproducibility. The centers received two commercial stabilized blood preparations, one with "normal" and one with "low" CD4 T cell content. The centers were asked to process the samples two times, 1 week apart, with the same procedure used for samples from HIV patients. Most centers showed a good performance of CD4 frequency and absolute count determinations. In particular, the "low" sample was correctly analyzed by all centers; only two underestimated the "normal" sample CD4 frequency, and only one underestimated the CD4 absolute count by >100 CD4 cells/microl. Overall, our data suggest that most Italian laboratories provide reliable and reproducible results in evaluating CD4 T cells in HIV(+) samples.

Twenty years of external quality assurance in clinical cell analysis--a tribute to Jean-Luc D'Hautcourt

External quality assurance (EQA) programs in clinical cell analysis are now a consolidated item of laboratory practice. All the flow cytometric testings with an impact on clinical decision making have been submitted to regular EQA programs during the last 20 years, and this has produced internationally homogeneous guidelines, with a remarkable improvement in result reproducibility.Jean-Luc D'Hautcourt was a pioneer in this field, and his valuable contributions to flow cytometric method standardization and to the dissemination of the educational aspects of EQA programs are recognized. The different methodological approaches undertaken in the United States and Europe are discussed. The educational role of SIHON in the Benelux Countries and of UKNEQAS for Leucocyte Immunophenotyping worldwide is emphasized. Accredited and accreditating EQA programs require an impressive degree of organization and technical knowledge, so that only major international providers can afford such a task nowadays. However, small local studies still provide the necessary stimulus to the continuous improvement of the scientifical aspects of EQA schemes.

Correlates of CD4+ and CD8+ lymphocyte counts in high-risk immunodeficiency virus (HIV)-seronegative women enrolled in the women's interagency HIV study (WIHS)

Studies of human immunodeficiency virus (HIV) infection often compare values from HIV-uninfected controls, including CD4 and CD8 lymphocyte counts. Nonetheless, little is known regarding factors associated with CD4 and CD8 cell numbers in HIV-uninfected individuals. To ascertain potential factors associated with differences in CD4 and CD8 cells among HIV negative women, we studied these cells in a group of 953 women, enrolled as HIV-negative comparators in the Women's Interagency HIV Study. Using standard techniques, we measured CD4 and CD8 cells obtained during study-related visits every six months through visit 20 (maximum of 9.5 years). Results were correlated with demographic and behavioral factors, and data were analyzed using a multiple linear regression approach with generalized estimating equations. At baseline, the median age was 32.4 years, body mass index (BMI) was 26.4 kg/m(2), CD4 cell count was 1010 (range 214-2705)/microL, and CD8 cell count was 542 (range 72-2448)/microL. African-Americans comprised 54%, 24% were Hispanic, and 19% were Caucasian. In multivariate analysis, increasing age (p = 0.0006), increasing BMI (p = 0.001), and current smoking status (p = 0.03) were independent predictors of higher CD4 counts. Multivariate analyses of CD8 cells revealed that lower age (p = 0.001), higher BMI (p = 0.03), Hispanic race/ethnicity (p = 0.01); current smoking (p = 0.006), injection drug use (p = 0.02), and Hepatitis C infection (p = 0.01) were independent predictors of higher CD8 cell counts. Multiple demographic and behavioral factors may influence CD4 and CD8 counts in HIV negative women. These factors must be considered in future analyses comparing lymphocyte subsets in HIV positive and negative women.

Predictors of immunologic long-term nonprogression in HIV-infected children: implications for initiating therapy

BACKGROUND

Early markers that predict immunologic long-term nonprogression in infants with perinatally acquired HIV infection might assist in subsequent antiretroviral treatment decisions.

OBJECTIVES

We sought to identify early markers of immunologic long-term HIV disease nonprogression.

METHODS

We analyzed immunologic and virologic characteristics at 1 and 2 months of age in HIV-infected children who were enrolled in the Women and Infants Transmission Study and born before 1995, comparing immunologic long-term nonprogressors (ILTNPs; n = 10) with non-ILTNPs (n = 127). ILTNPs were children who survived to 8 years or older with CD4 percentages of 25% or greater and counts of 500 cells/mm 3 or more without receiving highly active antiretroviral therapy. Non-ILTNPs were defined as all other HIV-infected children. Receiver operating characteristic curve analysis was used to assess combined sensitivity and specificity for each of these characteristics and to determine potential threshold values to discriminate between ILTNPs and non-ILTNPs.

RESULTS

Characteristics in the first 2 months of life associated with ILTNP status in univariate analysis included higher CD4 percentages, lower CD8 + percentages, lower CD8 + HLA-DR + percentages, and lower HIV-1 RNA PCR values. In receiver operating characteristic analysis CD8 + HLA-DR + percentage had the best combined sensitivity and specificity for discriminating between ILTNPs and non-ILTNPs. CD8 + HLA-DR + percentages of 5% or less predicted ILTNP status with 80% sensitivity and 80% specificity. In multivariate analysis CD8 + HLA-DR+ percentage of 5% or less remained a significant predictor of ILTNP status after adjusting for CD3 + CD4 + percentage and HIV-1 RNA PCR value (odds ratio, 15.4; 95% CI, 1.9-124.7).

CONCLUSION

CD8 + HLA-DR + T-lymphocyte percentage of less than 5% at 1 to 2 months of age might be predictive for ILTNP status but should not be used at this time to make treatment-deferral decisions. Immune activation in HIV-infected infants might herald more disease progression. Further study of the use of this subpopulation in early infancy to predict ILTNP status is warranted.

Morbidity and mortality during the first two years of life among uninfected children born to human immunodeficiency virus type 1-infected women: the women and infants transmission study

OBJECTIVE

We evaluated morbidity and mortality during the first 2 years of life among children born to human immunodeficiency virus-(HIV) type 1-infected women enrolled in the Women and Infants Transmission Study (WITS) during an 11-year period (1990-2001).

DESIGN AND METHODS

As part of WITS, evaluations were performed at birth and at 1, 2, 4, 6, 9, 12, 18 and 24 months of age. Growth, hospitalization and the incidence of clinical disease were assessed regularly.

RESULTS

Data regarding 1118 children born to HIV-infected women (955 HIV-uninfected children and 163 HIV-infected children) were analyzed. Fewer changes in the caretaker of the child and fewer in utero exposures to drugs, tobacco and alcohol occurred in the latter periods of the study (all P values for time trend analyses <0.01). The percentages of HIV-uninfected children with poor weight gain (44 of 767; 5.7%), short stature (32 of 703; 4.5%) and wasting (27 of 792; 3.4%) were higher than expected for the general population. Two or more changes in caretaker were associated with all growth deficiencies except wasting, and fetal exposure to tobacco was associated with height abnormalities. Anemia was common and was associated with receipt of zidovudine prophylaxis. Morbidity and mortality decreased during the study period. For the uninfected children, a decrease in class A events (Kaplan-Meier rates: group 1, 22.3%; group 2, 6.8%; group 3, 4.2%; P < 0.001) and class C events and death (Kaplan- Meier event rates: group 1, 2.0%; group 2, 1.7%; group 3, 0.2%; P = 0.062) during the first 2 years of life account for the differences in the curves over time.

CONCLUSIONS

During an 11-year period, morbidity and mortality during the first 24 months of life decreased substantially for children born to HIV-infected women.

National external quality assessment scheme for lymphocyte immunophenotyping in Belgium

In 2000, the Belgian Scientific Institute of Public Health introduced a voluntary external quality assessment scheme for lymphocyte immunophenotyping. This paper provides an analysis of the first six surveys. Specimens consisted of fresh EDTA-anticoagulated whole blood and were sent by overnight mail. The 41 participants were surveyed for methodology and were asked to report white blood cell count, percentage of lymphocytes, and percentages and absolute numbers of CD3+, CD4+, CD8+, and CD19+ cells. Median intralaboratory coefficients of variation were 1.0, 1.3, 1.7, and 3.2% for CD3+, CD4+, CD8+, and CD19+ cell percentages, respectively. Interlaboratory variability was consistently lower than 6.5% for CD3+ and CD4+CD3+ cell percentages, and lower than 9.5% for CD8+CD3+ cell percentages. Median coefficients of variation for the absolute values were higher, ranging from 10.1% for CD4+CD3+ cells to 16.5% for CD19+ cells. The percentage of CD4+CD3+ and CD8+ CD3+ cells was in several samples significantly lower than the percentage of total CD4+ and CD8+ cells. The number of laboratories measuring total CD4+ and CD8+ cells decreased by 30% during the programme. Between-laboratory variability remained stable over time. Analysis of individual laboratory performance indicated that some laboratories markedly improved their results.

Impact of the international program for Quality Assessment and Standardization for Immunological Measures Relevant to HIV/AIDS: QASI

Measurements of CD4 T-cell levels are essential for the assessment of human immunodeficiency virus (HIV) disease course, clinical staging, epidemiological studies, and decisions regarding prophylactic therapies against opportunistic infection. Until now, only in the industrialized countries was T-cell subset monitoring considered a practical option to assess disease progression. The Quality Assessment and Standardization for Immunological Measures Relevant to HIV/AIDS (QASI) program was established in 1997 to meet performance assessment for immunophenotyping laboratories in countries where such service is not available. The QASI program is provided at no cost to any laboratory in a resource-poor setting that wishes to participate. This report describes the beneficial impact of participation in the QASI program. Carefully selected commercial stabilized whole blood preparations were sent regularly to participating laboratories. Participants reported the T-cell subset values they obtained by flow cytometry. Once the aggregate mean values for the T-cell subsets were established for the shipment, a comprehensive and confidential report was sent to each laboratory. The results from five consecutive shipments were analyzed. The coefficient of variation decreased from 7.2% to 4.7% and from 14.2% to 8.8% for percent and absolute CD4 T-cell counts, respectively. With the implementation of the QASI program using commercial stabilized whole blood specimens, it is possible to reduce interlaboratory error. This study illustrates that a quality assessment program can improve the overall performance of laboratories. Reducing interlaboratory variation can enhance significantly the effectiveness of multicenter HIV vaccine or drug trial evaluation.

CD45-assisted PanLeucogating for accurate, cost-effective dual-platform CD4+ T-cell enumeration

BACKGROUND

North American and European guidelines for dual-platform (DP) flow cytometry recommend absolute CD4 T-cell counts to be calculated from two parameters: the absolute lymphocyte counts obtained on a hematology analyzer and the percentages of CD4+ cells among lymphocytes (CD4%/lympho) obtained by flow cytometry. Nevertheless, the identification of lymphocytes is error-prone: a poor match between these common denominators in the two systems is the main source of inaccuracy. In contrast, total leucocyte counts (white cell counts [WCC]) and CD4% among the gated CD45+ leucocytes (CD4%/leuco) can be determined with greater accuracy.

METHODS

We introduced "PanLeucogating," i.e., we used total leucocytes as the common denominator for improving the precision of DP absolute CD4 counting. Correlations and Bland-Altman tests were used for statistical analysis.

RESULTS

First, 22 stabilized blood product samples were provided by U.K. National External Quality Assessment Scheme (NEQAS) and a higher accuracy and precision of CD4 counts were documented using PanLeucogating compared with lymphocyte gating. Next, 183 fresh and 112 fixed (TransFix) whole blood samples were used to compare DP methods and single-platform (SP) methodology, including both volumetric and bead-based techniques. A particularly high correlation and comparable precision of absolute CD4 counts were observed between the SP volumetric method and DP PanLeucogating (R(2) = 0.990; bias 6 +/- SD 17%). The SP volumetric method showed lower levels of agreement with the DP lymphocyte gating (R(2) = 0.758; bias 14 +/- SD 51%) and with the SP bead-based method (R(2) = 0.923; bias 4 +/-SD 31%).

CONCLUSIONS

These observations show that DP leucocyte counts (WCC) should replace lymphocyte counts as the "common denominator" although CD4%/lympho values can, as an extra step, be also provided readily if requested. When coupled with quality control for WCC on hematology analyzers, the DP method with CD45 PanLeucogating represents a robust CD4 T-cell assay that is as accurate as the SP volumetric technique. This DP method uses only two, CD45 and CD4, antibody reagents and can be run on any pair of hematological analyzer plus flow cytometer.

Evaluation of stabilized blood cell products as candidate preparations for quality assessment programs for CD4 T-cell counting

BACKGROUND

Exceptionally robust cell preparations are needed for quality assessment programs (QAPs) such as the International Program for Quality Assessment and Standardization for Immunological Measures (QASI) relevant to HIV/AIDS. A suitable product must withstand environmental stress related to transportation for a minimum of 6 days. The two objectives of this study are (1) to evaluate the performance of various commercial preparations with multicenter participation and (2) to evaluate the robustness of stabilized blood cell products.

METHODS

Phase 1: The performance of stabilized blood cell products was evaluated in a multicenter QAP utilizing various staining procedures and flow cytometers. Absolute cell enumeration was achieved using single-platform T-cell subset methodology. Phase 2: The robustness of stabilized blood cell products was evaluated by monitoring T-cell subset values from samples stored at 4 degrees C, 22 degrees C, and 37 degrees C for up to 10 days.

RESULTS

The largest interlaboratory variation in both absolute and relative T-cell values was 16% in samples with CD4 levels > or =400 cells per microliter and 21% in samples with CD4 levels <400 cells per microliter. Six preparations retained their phenotypic expression for 7 days at 4 degrees C and 22 degrees C. However, only two preparations remained stable for 4 days at 37 degrees C.

CONCLUSION

Some stabilized cell preparations are more robust and therefore more suitable for quality assessment purposes.

Quality control of CD4+ T-lymphocyte enumeration: results from the last 9 years of the United Kingdom National External Quality Assessment Scheme for Immune Monitoring (1993-2001)

The human immunodeficiency virus (HIV) global epidemic has necessitated the routine enumeration of T-lymphocyte subsets, which has created a need for external quality assurance (EQA). The United Kingdom National External Quality Assessment Scheme (UK NEQAS) for Immune Monitoring provides EQA for 296 laboratories in 40 countries. In 1993, UK NEQAS developed and incorporated into its program stabilized whole blood that enables the accurate monitoring of laboratory performance. Overall, the mean interlaboratory coefficient of variation (CV) for percentage CD4(+) T-lymphocyte subset enumeration has fallen from 15% to less than 5%, as a direct result of the increased use of CD45/ side scatter (SSC) gating. Laboratories using alternative gating strategies (i.e., CD45/CD14 or forward scatter [FSC]/SSC) were about 7.4 times more likely to fail an EQA exercise. Furthermore, the adoption of single-platform technology resulted in a reduction of the overall mean interlaboratory CV for absolute CD4(+) T lymphocytes from 56% (prior to the widespread use of single-platform technology) to 9.7%. Individual laboratory deficiencies were also identified using a performance monitoring system and, through re-education by collaboration with the coordinating center, satisfactorily resolved. In conclusion, during the last 9 years, the UK NEQAS for Immune Monitoring program has highlighted the significant technological advances made by laboratories worldwide that undertake lymphocyte subset enumeration.

Grading of laboratories on CD4+ T-lymphocyte evaluations based on acceptable data boundaries defined by the measurement error

BACKGROUND

We addressed the definition of limits of error of %CD4+ and CD4+ counts (AbsCD4+) typical of laboratories of excellence, as well as the grading of laboratories based on the decision to take these limits as boundaries of unacceptable data.

METHODS

We studied the 99.9% confidence intervals of the means of 24 human immunodeficiency virus (HIV)+ and HIV- blood samples analyzed by 18 laboratories of the Liguria Region Quality Assessment Program (Liguria Region QALI). Regression equations of lower (L1) and upper (L2) confidence limits over the means of data cleared of unusual results were used to interpolate limits of error for mean values in the tested range.

RESULTS

L1 and L2 were symmetric around the mean and a single absolute difference (Abs Res) between the limits and the mean was found. Abs Res significantly increased over mean values (P = 0.0005 for %CD4+, P < 0.0001 for AbsCD4+). Limits were compatible with errors shown with blind replicates. Unacceptable results, outside the limits, accounted for 25% and 30% of %CD4+ and for 18% and 35% AbsCD4+ in the Liguria Region QALI and in the Piemonte Region QA Program, respectively. Limits interpolated over the median showed a similar grading. A comparable fraction of unacceptable data was also found with the method used in the U.K. National External Quality Assessment Scheme (NEQAS) immune monitoring scheme.

CONCLUSIONS

We propose the general use of these regression equations to determine bounds for unacceptable data in proficiency testing and to identify laboratories of excellence.

Validation and quality control of immunophenotyping in clinical flow cytometry

Clinical flow cytometry has evolved from two-parameter quantitative assessment of peripheral blood lymphocytes to six-parameter qualitative evaluation of bone marrow for hematopathology. Leukemia and lymphoma immunophenotyping represent an extremely important complement to morphology in the diagnosis and monitoring of hematopoietic malignancies. The complexity of five- and six-parameter analyses and the interpretation of the data rely on standardization and validation of the instrument, the reagents and the procedure. In addition, flow cytometry laboratories in the U.S. are required to document proficiency testing, sample preparation, method accuracy, specificity, sensitivity and precision. NCCLS and the U.S.-Canadian Consensus Conference have provided recommendations, but each laboratory is ultimately responsible for validating its own qualitative and quantitative procedures. This paper reviews procedures for validation and quality control of all aspects of the operation of a clinical flow cytometry service.

Need for an external proficiency testing program for cytokines, chemokines, and plasma markers of immune activation

An external evaluation program for measuring the performance of laboratories testing for cytokines and immune activation markers in biological fluids was developed. Cytokines, chemokines, soluble cytokine receptors, and other soluble markers of immune activation (CSM) were measured in plasma from a healthy human immunodeficiency virus (HIV)-seronegative reference population and from HIV-seropositive individuals as well as in supernatant fluids from in vitro-stimulated human immune cells. The 14 components measured were tumor necrosis factor (TNF) alpha, gamma interferon, interleukin-1 (IL-1), IL-2, IL-4, IL-6, IL-10, Rantes, MIP-Ia, MIP-Ibeta, soluble TNF receptor II, soluble IL-2 receptor alpha, beta(2)-microglobulin, and neopterin. Twelve laboratories associated with the Adult and Pediatric AIDS Clinical Trial Groups participated in the study. The performance features that were evaluated included intralaboratory variability, interlaboratory variability, comparison of reagent sources, and ability to detect CSM in the plasma of normal subjects as well as the changes occurring in disease. The principal findings were as follows: (i) on initial testing, i.e., before participating in the program, laboratories frequently differed markedly in their analytic results; (ii) the quality of testing of a CSM in individual participating laboratories could be assessed; (iii) most commercial kits allowed distinction between normal and abnormal plasma CSM levels and between supernatants of stimulated and unstimulated cells; (iv) different sources of reagents and reference standards frequently provided different absolute values; (v) inexperienced laboratories can benefit from participating in the program; (vi) laboratory performance improved during active participation in the program; and (vii) comparability between analyses conducted at different sites can be ensured by an external proficiency testing program.

Standardization of lymphocyte antibody binding capacity - a multi-centre study

As quantitative flow cytometry is being increasingly used to characterize non-malignant and malignant disorders, interlaboratory standardization becomes an important issue. However, the lack of standardized methods and process controls with predefined antibody binding capacity values, limits direct interlaboratory comparison. The present study has addressed these issues using a stable whole blood product and a standardized antigen quantification protocol. It was demonstrated that: (i) a standard technical protocol can result in a high degree of interlaboratory concordance; (ii) interlaboratory variation of less than 12% can be achieved for CD4 antibody binding capacity values; and (iii) stable whole blood can be used as a process control with predefined antibody binding capacity values. Furthermore, using such an approach, a normal range was established for CD3, CD4 CD8 and CD19. These antigens appear to be expressed in a hierarchical manner, a factor that could be used as a procedural quality control measure.

A standardized gating technique for the generation of flow cytometry data for normal canine and normal feline blood lymphocytes

Flow cytometry is becoming a commonly used technique to characterize a variety of cells. It provides a powerful application to rapidly determine the relative percentages of T-lymphocyte subsets and B-lymphocytes. The effectiveness of its application, however, is dependent on standardization, especially in a clinical setting. Application of flow cytometry to veterinary diagnostics has been limited by the unavailability of reagents and by the unstandardized characterization of normal values using antibodies not commercially available, but typically provided through the generosity of other researchers. This paper presents a standardized gating protocol, and average values and ranges observed for normal canine and feline blood lymphocytes using commercially available antibodies to cell surface markers for CD5, CD3, CD4, CD8, MHC II, and B lymphocytes. The averages for these markers on gated lymphocytes were as follows: Canine CD5 83.3%, Canine CD4 45.0%, Canine CD8 28.8%, Canine MHC II 98.0%, Canine B Cell 12.9%, Canine CD4/CD8 ratio 1.87, Feline T lymphocytes 77.3%, Feline CD4 44.5%, Feline CD8 25.7%, Feline B Cell 24.1%, Feline CD4/CD8 Ratio 1.75. Normal values were also established for a mixed breed group of dogs, and old versus young dogs. This information will provide researchers and clinicians with a standardized protocol for gating, which establishes a basis for comparison between techniques, and a measure of phenotypic percentages for flow cytometry in normal dogs and cats based on this standardization and commercially available antibodies.

Laboratory and clinical markers of HIV infection in a national haemophilia cohort treated with recombinant factor VIII concentrate. The Association of Hemophilia Clinic Directors of Canada

Over an interval of approximately six months beginning in October 1993, most haemophilia A patients in Canada were switched from a plasma-derived intermediate-purity factor VIII concentrate (i.p. VIII) to a recombinant factor VIII (rVIII). In order to determine the consequence of this change in therapy on progression of HIV infection, we gathered surveillance data on clinical status and CD4 and CD8 cell counts in those patients who were HIV seropositive at the time of switching concentrates. Data were recorded at the time of switchover, annually for 2 years thereafter, and retrospectively at a point 1 year prior to the switch. CD4 cells fell significantly over the study period. Multiple direct comparisons revealed that this decline was restricted to the time intervals which included the final year in which patients received intermediate-purity factor VIII concentrate (i.p. VIII). In the 2 year interval in which rVIII was used exclusively, there was a nonsignificant fall in CD4 cells. Changes in CD4 cells did not correlate with the intensity of exposure to either i.p.VIII or rVIII. CD8 cells did not fall significantly over the study period. There was no obvious reduction in the incidence of death or clinical progression over the 2 years in which rVIII was used. However, we are hopeful that the stabilizing trend in CD4 cell counts which followed the introduction of rVIII will be predictive of corresponding clinical stabilization over the coming years.

Quality assessment of CD34+ stem cell enumeration: experience of the United Kingdom National External Quality Assessment Scheme (UK NEQAS) using a unique stable whole blood preparation

CD34+ peripheral blood stem cell (PBSC) mobilization and harvesting has rapidly replaced autologous bone marrow as a source of stem cells for transplantation. Timing and adequacy of harvests rely upon the accurate enumeration of circulating CD34+ cells. However, previous EQA programmes have reported interlaboratory CVs as high as 284%, suggesting the need for greater standardization. In addition the routine use of fresh and/or frozen cells as analytes also introduces antigen instability as a variable factor. To circumvent this problem and achieve a true reflection of interlaboratory variation, we have used a novel whole blood preparation in which the antigenic profiles of PBSCs, as determined by flow cytometry, are retained for > 200 d. This international scheme, currently the largest in the world, distributes aliquots of stabilized whole blood bi-monthly to 91 laboratories in 20 countries (44 U.K., 47 overseas). Participants are required to determine the percentage and absolute values for CD34+ PBSCs using in-house techniques. Adopting such a preparation, a more accurate determination of interlaboratory variation has been possible when compared to previous EQA studies, with CVs as low as 22% and 24% for percentage and absolute counts. In addition the programme has established that a wide range of methods are in routine use, emphasizing the urgent requirement for national/international consensus guidelines.

Instrument-dependent fluorochrome sensitivity in flow cytometric analyses

Flow cytometry has become the preferred technique by which critical clinical evaluations are made such as CD4 counts and aneuploid analyses. Mounting concern has arisen over the numerous techniques, reagents, and different flow cytometric employed to determine these data. Several studies have documented significant differences in results when different flow cytometers are utilized to analyze the same sample. Fluorochrome-dependent instrument sensitivity also has been reported by numerous investigators. As more and more procedures are performed by cytometric analysis, light scatter and fluorescence limitations, which appear to be instrument dependent, demonstrate that not all flow cytometers have the same capabilities. Attempts were made to calculate molecules of equivalent soluble fluorochrome (MESF) values on nine different flow cytometers using fluorescein isothiocyanate (FITC) and R-phycoerythrin (R-PE) labeled microsphere reference standards produced by Flow Cytometry Standards Corporation (FCSC). Dramatic differences were observed in the ability of some cytometers to resolve these microspheres. The diminished resolution appeared to be instrument model and fluorochrome dependent. We propose that diminished fluorescence resolution in certain flow cytometers could be responsible for significant variability in clinical values reported from laboratories utilizing different flow cytometers.

Interlaboratory variability of CD8 subset measurements by flow cytometry and its applications to multicenter clinical trials. NAID/NICHD Women and Infants Transmission Study Group

Recent studies have demonstrated the utility of measuring subsets of CD8+ T cells as prognostic markers in epidemiology cohort studies of human immunodeficiency virus (HIV)-infected patients. Most of these studies evaluating the value of CD8+ T-cell subsets have been performed at single centers, and few data are available on variability in the measurement of the CD8+ cell populations in multicenter trials. In the current study, we addressed this question by evaluating interlaboratory variability from the five laboratories enrolled in the Women and Infants Transmission Study sponsored by the National Institutes of Health. This study evaluated 35 HIV-positive and 28 HIV-negative proficiency testing samples sent to the laboratories for evaluation. The study focused on the robust coefficient of variation (RCV) for CD38 (11%), HLA-DR (21%), and CD57 (15%) expression on the CD8+ population. Data from the current study indicated that the variability in these measurements is greater than that for CD3+ CD4+ (RCV, 5%) and CD3+ CD8+ (RCV, 5%) cells. Knowledge of the variability of the CD8+ subset measurements should guide investigators in the design and analysis of clinical trials and epidemiology studies. Ability to obtain improved interlaboratory agreement on CD8+ subset measurements will facilitate further evaluation of these markers in HIV studies.

Multicolor flow cytometric analysis of the CD45 antigen provides improved lymphoid cell discrimination in bone marrow and tissue biopsies

Samples from bone marrow or non-hematopoietic tissue such as solid organ biopsies often contain an excess of non-leukocytes exhibiting lymphocyte-like light scatter characteristics, making it sometimes difficult to define satisfactory light scatter lymphocyte gates. To circumvent this, we describe here a multiparametric method of identifying lymphoid cells by expression of the CD45 antigen, in conjunction with light scatter parameters. A 'third color'-conjugated anti-CD45 antibody was included with every FITC/PE double staining, thereby permitting live or list mode analysis gating on CD45 positive cells. The triple-staining technique was applied to (a) human bone marrow, showing that special attention has to be given to the enumeration of B cells, and (b) to liver biopsies, where gating on CD45 fluorescence and orthogonal light scatter was shown to clearly resolve all lymphocyte subsets from debris. All cell types examined in tissue biopsies as well as T and NK cells in bone marrow were best distinguished by gating on bright CD45 expression in conjunction with low orthogonal light scatter, while accurate identification of marrow B cells relied upon including all levels of CD45 intensity. The multicolor gating procedure, aimed mainly at immune-monitoring of non-malignant tissues, is applicable to most kinds of single cell samples, and may prove to be an aid for lymphocyte gating in cases where leukocyte populations are not clearly resolved on a light scatter basis alone.

Evaluation of a dual-color flow cytometry immunophenotyping panel in a multicenter quality assurance program

A basic immunophenotyping panel that employed dual-color combinations of fluorescein isothiocyanate (FITC) and phycoerythrin (PE) conjugated monoclonal antibodies (mAb; FITC-CD45/PE-CD14, FITC-IgG1/PE-IgG2, FITC-CD3/PE-CD8, FITC-CD3/PE-CD4, FITC-CD3/PE-CD16 + PE-CD56, and PE-CD19) was utilized in a quality assurance program to determine whether the 4 laboratories participating in a multicenter AIDS study obtained similar lymphocyte subset percentage values for T cells, B cells, NK cells, and CD4+ and CD8+ T cells. Over a 1 1/2 year period, 78 shared peripheral blood specimens were prepared and analyzed in each laboratory. The CD45bright CD14- percentage for each specimen was used to correct that individual's lymphocyte subset values. Interlaboratory coefficients of variation (CV) for the human immunodeficiency virus type I (HIV) seronegative (n = 38) and HIV-seropositive (n = 40) specimens using this panel were < 3% for total T cells; < 5% for CD4+ T cells and CD8+ T cells; < or = 17% for B and NK cells; and < 8% for CD4T/CD8T ratios. The 6-tube basic immunophenotyping panel has several notable features: a) for clinical studies, it permits comprehensive evaluation of an individual's major lymphocyte subsets, i.e., T, B, NK, and CD4+ and CD8+ T cells; b) for interlaboratory proficiency testing programs, it allows the detection of differences among laboratories in measurements of several functionally distinct cell populations; and c) for within-sample quality assurance, it provides several quality control checks, including the lymphosum, i.e., the sum of an individual's corrected T+B+NK values, a sum that was generally 100 +/- 5% on the HIV-seronegative specimens analyzed in this study.

A simultaneous three-color T cell subsets analysis with single laser flow cytometers using T cell gating protocol. Comparison with conventional two-color immunophenotyping method

We describe a method for simultaneous analysis of CD3, CD4, and CD8 positive cells from whole blood utilizing single laser flow cytometers. All three T cell values are attained from a single test tube. CD4 and CD8 positive cells are identified only if they are CD3 positive. Thus the values obtained by this method for T helper/inducer and T cytotoxic/suppressor cells can be reported directly as a percentage of T lymphocytes. Analysis for CD4 and CD8 positive cells is accomplished, by first gating on CD3 positive T lymphocytes, hence the approach is referred to as a T gating method. As the third dye, conjugated to anti-CD3 monoclonal antibodies (MAbs), we utilized peridinin chlorophyll protein (PerCP), a new red fluorochrome. The proposed method may prove to be practical for monitoring disease progression in AIDS, where longitudinal T helper/inducer and T cytotoxic/suppressor cell enumeration must be performed unambiguously by a simple, reproducible, and fast method.

Laboratory control values for CD4 and CD8 T lymphocytes. Implications for HIV-1 diagnosis

With the advent of standard flow cytometric methods using two-colour fluorescence on samples of whole blood, it is possible to establish the ranges of CD3, CD4 and CD8 T lymphocyte subsets in the routine laboratory, and also to assist the definition of HIV-1-related deviations from these normal values. In 676 HIV-1-seronegative individuals the lymphocyte subset percentages and absolute counts were determined. The samples taken mostly in the morning. The groups included heterosexual controls, people with various clotting disorders but without lymphocyte abnormalities as well as seronegative homosexual men as the appropriate controls for the HIV-1-infected groups. The stability of CD4% and CD8% values was demonstrated throughout life, and in children CD4 values less than 25% could be regarded as abnormal. The absolute counts of all T cell subsets decreased from birth until the age of 10 years. In adolescents and adults the absolute numbers (mean +/- s.d.) of lymphocytes, CD3, CD4 and CD8 cells were 1.90 +/- 0.55, 1.45 +/- 0.46, 0.83 +/- 0.29 and 0.56 +/- 0.23 x 10(9)/l, respectively. In patients with haemophilia A and B the mean values did not differ significantly. In homosexual men higher CD8 levels were seen compared with heterosexual men and 27% had an inverted CD4/CD8 ratio but mostly without CD4 lymphopenia (CD4 less than 0.4 x 10(9)/l). However, some healthy uninfected people were 'physiologically' lymphopenic without having inverted CD4/CD8 ratios. When the variations 'within persons' were studied longitudinally over a 5-year period, the absolute CD4 counts tended to be fixed at different levels. As a marked contrast, over 60% of asymptomatic HIV-1+ patients exhibited low CD4 counts less than 0.4 x 10(9)/l together with inverted CD4/CD8 ratios. Such combined changes among the heterosexual and HIV-1-seronegative homosexual groups were as rare as 1.4% and 3%, respectively. For this reason, when the lymphocyte tests show less than 0.4 x 10(9)/l CD4 count and a CD4/CD8 ratio of less than unity, the individuals need to be investigated further for chronicity of this disorder, the signs of viral infections such as HIV-1 and other causes of immunodeficiency.

Human mesangial cells are resistant to productive infection by multiple strains of human immunodeficiency virus types 1 and 2

Human immunodeficiency virus-associated nephropathy (HIVAN) is a recognized clinical entity of unknown pathogenesis. A role for viral infection of renal cells in the initiation of this process at present is an intriguing but untested hypothesis. Studies in primate models of acquired immunodeficiency syndrome (AIDS) suggest that injury to the mesangial cell may be central to the sclerosing glomerular lesion characteristic of HIVAN. We therefore tested the infectibility of human mesangial cells (HMC) in vitro by a variety of strains of HIV chosen to include a spectrum of tropisms for different cell types. Productive infection of mesangial cells could not be demonstrated using any of the virus strains. Nonetheless, HIV infection of intrinsic renal cells remains an attractive area of inquiry for understanding the natural history of HIVAN.

Integration of a barcode reader with a commercial flow cytometer

This report describes the application and installation of a barcode reader on a standard EPICS Elite flow cytometer. The barcode reader system eliminates keyboard entry of sample information on the cytometer. The system automates the transfer of sample information already present in our laboratory database to the cytometer at run time. The system uses a standard "off-the-shelf" bar code wand with a personal computer keyboard interface and requires no additional software at run time. No typing of sample information is required by the operator at any stage of normal sample operation at the cytometer. All operations are automatically coded into the cytometry software using the macro functions of the software. Tubes are inserted into the tube reader and sample information is transferred automatically into the cytometer. We have found that the system allows rapid and continuous operation of routine clinical and research samples. This automated data entry also reduces the possibility of data input errors.

Rapid multivariate analysis and display of cross-reacting antibodies on human leukocytes

We present an application which can rapidly determine the binding patterns of monoclonal antibodies on mixed populations of cells simultaneously in a single rapid analysis. It is an application of the tube identifier parameter (TIP) system which can provide fully correlated list-mode data of the entire patient phenotype in a single file. Using the phenogram analytical display, we are able to determine the cross-reacting antibodies for an entire antibody panel for each cell type. This information can be displayed in a single plot. Using light scatter gating to select different populations of lymphocytes, monocytes, and neutrophils, phenograms can be simultaneously generated. This provides a directly comparable means of displaying the positive and negative binding characteristics of each antibody on each cell population. Any marker combination that is abnormal will be identifiable in the phenogram. Additionally, by plotting the fluorescence distributions of each marker beside one another (termed overview), quantifiable differences in intensity can be determined. There are 3 major benefits of the proposed analysis. By using the TIP concept, several sets of antibodies can be compared simultaneously. Any light scatter gate can be used and this gate can be changed on one histogram or plot, yet apply to the total analysis. Data analysis is particularly rapid since the entire phenotype of a patient can be evaluated by performing a single rapid analysis.

Measuring percent lymphocytes by flow cytometry to calculate absolute lymphocyte subset counts for HIV+ specimens

A comparison was made of lymphocyte percentages from an automated hematology analyzer (ELT 15) vs. a fluorescence flow cytometer (Cytofluorograf). The hematology values were consistently higher than the flow (by greater than 10% for 13 of 50 HIV+ specimens). The findings were similar to three other pairings: H*1 vs. Cytofluorograf, Cell Dyn 3000 vs. Cytofluorograf, and Cell Dyn 3000 vs. EPICS Profile. In another comparison manual percent lymphocytes matched much better with flow values. Factors of sample preparation and instrument analysis as they relate to nonrandom cell loss and lymphocyte resolution are examined.

Flow cytometric analysis of T-lymphocyte subsets in cats

We report a rapid, reliable method for the immunophenotype analysis of feline lymphocytes. Fluorescein isothiocyanate (FITC) conjugated to murine monoclonal antibodies f43, Fel 7 and fCD8 was used to identify phenotypes corresponding to feline T-cells, CD4+ T cells and CD8+ T cells. For isolation of white blood cells, whole blood lysis was faster, less variable and required much less sample than density gradient separation. To identify feline CD4+ and CD8+ cells simultaneously, directly conjugated FITC-fCD8 and phycoerythrin (PE) fCD4 (Fel 7) were used in two-color analysis. The two T cell sub-populations were non-overlapping. Dual-label and single-label values were not significantly different. Mean lymphocyte subset percentages in conventional and specific-pathogen-free (SPF) cats did not differ significantly. These values were: pan T lymphocytes (f43), 54.8%, CD4+ cells (Fel 7), 33.9%, and CD8+ cells (fCD8), 19.1%. Mean CD4/CD8 ratio was 1.9 in normal cats; the range was 1.2-2.6.

Quality control in the flow cytometric measurement of T-lymphocyte subsets: the multicenter AIDS cohort study experience. The Multicenter AIDS Cohort Study Group

Since 1984, the Multicenter AIDS Cohort Study (MACS) has utilized four flow cytometry laboratories to measure T-lymphocyte subset levels semiannually in a large cohort of homosexual men. This report summarizes the steps taken in the MACS laboratories to attain comparability of lymphocyte subset determinations across the centers and over time. Identical flow cytometers, monoclonal antibodies, and analytic procedures have been used, and over a period of time, the procedure for sample preparation was also standardized. Interlaboratory proficiency testing utilizing identical specimens analyzed in the four laboratories was performed to evaluate the comparability of the data among the laboratories. Our results verify that such testing can identify technical bias in flow cytometric evaluations performed at different laboratories. Temporal laboratory consistency in flow cytometric measurements was evaluated using data from each site's HIV-seronegative homosexual reference group. Both sequential 95% confidence intervals (mean +/- 2 x SEM) and the within-person standard deviations of the immune measurements were considered. Significant variation in CD3, CD4, and CD8 lymphocyte subset percentages over time in the seronegative reference population was observed. Our observations indicate that the lymphocyte subset values of this seronegative group should be used to adjust those obtained on the seropositive study participants during a particular time period, thereby allowing improved discrimination of the effects of HIV on T cells in infected individuals. The data presented are of use for designing epidemiologic and intervention studies in HIV-1-infected individuals, especially for calculating sample sizes. The methods we have used to assess the quality of data in the MACS have general application to quality control programs in flow cytometry laboratories. In particular, comparison of sequential confidence intervals and within-person standard deviations for lymphocyte subset determinations on control populations are essential to a comprehensive proficiency testing program because they permit assessment of consistency within a laboratory over time.

Procedural guidelines for performing immunophenotyping by flow cytometry

Flow cytometry is a rapidly expanding technology that is moving from the research laboratory into the clinical laboratory. Recent advances in availability and reproducibility of monoclonal antibody reagents specific for a wide range of cell types coupled with lower costs for increasingly automated flow cytometers with powerful and user friendly data analysis capabilities have made flow cytometry the method of choice for immunophenotyping in the clinical laboratory. However, there is great variability in the level and type of quality assurance procedures used from laboratory to laboratory. A subcommittee established by the National Committee for Clinical Laboratory Standards (NCCLS), composed of representatives from industry, academia, professional societies, and regulatory agencies, has drafted consensus procedures which address specific problems and suggested solutions for performance of immunophenotyping by flow cytometry. This paper is based on the authors' discussions with the NCCLS Committee but does not represent an official NCCLS position. The official NCCLS document on this subject (H42) is expected to be published in 1989.