[The ways of harmonization of clinical laboratory measurement techniques]

The results of implementation of different clinical laboratory techniques are to be equal in clinically significant limits to be optimally applied in diagnostics of diseases and treatment of patients. When the results of laboratory tests are not standardized and harmonized for the very same clinical assay the results can be expressed by unmatched numbers. Unfortunately, in some handbooks the values are presented based on the results of application of specific laboratory techniques without considering possibility or likelihood of differences between various techniques. When this is a case, accumulation of data of diferent clinical research studies and working out of clinical handbooks on this basis will be inconsistent. Inadequate understanding of issue that the results of laboratory tests are not standardized and harmonized can lead to incorrect clinical, financial, managerial or technical decisions. The standardization of clinical laboratory techniques was applied to many measurands related to primary referent techniques (standard specimen of pure substance) or/and developed referent measurement techniques. However, harmonization of clinical laboratory techniques for those measurands which are not related any developed measurement techniques is quite problematic due to inadequate determination of measurand, its inadequate analytical specificity, insufficient attention to commutability of referent materials and poor systematic approach to harmonization. To overcome these issues an infrastructure is to be developed to support systematic approach to identification and prioritization of measurands which are to be harmonized on the basis of clinical importance and technical applicability. The management of technical implementation harmonization process for specific measurands.

High-performance liquid chromatography method to analyze free and total urinary pyridinoline and deoxypyridinoline

The pyridinium cross-links pyridinoline (PYD) and deoxypyridinoline (DPD) are established markers of bone resorption measured in blood and urine and are used to investigate bone metabolism and manage bone diseases. Unfortunately, the currently observed interlaboratory variability caused by inconsistent assay calibration limits the optimal use of these markers. A high-performance liquid chromatography (HPLC)-based assay was developed using synthetic PYD and DPD as calibrators to analyze free and total PYD and DPD in urine. The spectroscopic characteristics of the synthetic calibrators were identical to those of calibrators isolated from bone. The mean intraassay variabilities of the HPLC method were 4.1 and 3.8%, respectively, for total DPD and PYD and 9.8 and 9.5%, respectively, for free DPD and PYD. The mean interassay variabilities were 9.1 and 8.2% for total DPD and PYD and 8.6 and 7.0% for free DPD and PYD, respectively. The mean recoveries were 98.1% for total DPD, 100.8% for total PYD, 98.6% for free DPD, and 94.9% for free PYD. The method exhibits a good correlation with a commercial immunoassay and with other HPLC assays currently used in hospital laboratories.

The national glycohemoglobin standardization program: a five-year progress report

BACKGROUND

The Diabetes Control and Complications Trial (DCCT) and the United Kingdom Prospective Diabetes Study (UKPDS) demonstrated conclusively that risks for complications in patients with diabetes are directly related to glycemic control, as measured by glycohemoglobin (GHB). In 1994, one year after the DCCT results were reported, the American Diabetes Association (ADA) set specific diabetes treatment goals. However, 1993 College of American Pathologists (CAP) Survey results indicated a lack of comparability of GHB test results among methods and laboratories that represented a major obstacle to meaningful implementation of the ADA guidelines. Thus, an AACC subcommittee was formed in 1993 to develop a standardization program that would enable laboratories to report DCCT-traceable GHB results. This program was implemented in 1996 by the National Glycohemoglobin Standardization Program (NGSP) Steering Committee.

APPROACH

We review the NGSP process and summarize progress in standardization through analysis of CAP data.

CONTENT

Since 1996, the number of methods and laboratories certified by the NGSP as traceable to the DCCT has steadily increased. CAP GH2-B survey results reported in December 2000 show marked improvement over 1993 data in the comparability of GHB results. In 2000, 90% of surveyed laboratories reported GHB results as hemoglobin A(1c) (HbA(1c)) or equivalent, compared with 50% in 1993. Of laboratories reporting HbA(1c) in 2000, 78% used a NGSP-certified method. For most certified methods in 2000, between-laboratory CVs were <5%. For all certified methods in 2000, the mean percent HbA(1c) was within 0.8% HbA(1c) of the NGSP target at all HbA(1c) concentrations.

A reference method laboratory network for cholesterol: a model for standardization and improvement of clinical laboratory measurements

BACKGROUND

Accurate and precise measurement of blood cholesterol plays a central role in the National Cholesterol Education Program's strategy to reduce the morbidity and mortality attributable to coronary heart disease. Matrix effects hamper the ability of manufacturers to adequately calibrate and validate traceability to the National Reference System for Cholesterol (NRS/CHOL). CDC created the Cholesterol Reference Method Laboratory Network (CRMLN) to improve cholesterol measurement by assisting manufacturers of in vitro diagnostic products with validation of the traceability of their assays to the NRS/CHOL.

METHODS

CRMLN laboratories established the CDC cholesterol reference method (modification of the Abell-Levy-Brodie-Kendall chemical method) and are standardized using CDC frozen serum reference materials. CRMLN laboratories use common quality-control materials and participate in monthly external performance evaluations conducted by CDC. The CRMLN performance criteria require member laboratories to agree with CDC within +/-1.0% and maintain a CV < or =2.0%.

RESULTS

From 1995 to 200 the CRMLN laboratories met the accuracy criterion 97% of the time and the precision criterion 99% of the time. During this time period, the CRMLN maintained an average bias to CDC of 0.01% and an average collective CV of 0.33%.

CONCLUSIONS

CDC established the CRMLN as the first international reference method laboratory network. The CRMLN assists manufacturers in the validation of the calibration of their diagnostic products so that clinical laboratories can measure blood cholesterol more reliably. The CRMLN can serve as a model for other clinical analytes where traceability to a hierarchy of methods is needed and matrix effects of the field methods with processed calibrators or reference materials are present.

Estimating the long-term effects of storage at -70 degrees C on cholesterol, triglyceride, and HDL-cholesterol measurements in stored sera

We estimated the effects of long-term storage at -70 degrees C on serum total cholesterol, HDL-cholesterol, and triglycerides in specimens that had been stored for up to 7 years. These estimates were made using measurements in serial specimens collected from the placebo control group of the Air Force/Texas Coronary Atherosclerosis Prevention Study over a period of approximately 5 years. We compared the group means for pairs of serial specimens taken at 6- and 12-month intervals, assuming that (a) a negligible placebo effect occurred between the serial specimen pairs; (b) in the absence of storage effects, the variation in the group means would reflect only normal biological variation and would not materially affect the group means for the serial specimens; (c) any systematic changes in these group means would reflect storage-related changes; and (d) storage-related changes are cumulative, i.e., the overall changes for a given storage period are the sum of the changes during previous storage periods. We observed average decreases of 2.0% per year for total cholesterol over 7 years and 2.8% per year in triglycerides for the first 5 years. HDL-cholesterol decreased by 1.3% per year, but this change was not statistically significant. This approach may be useful for estimating storage-related changes for studies in specimens stored for a period of years and for which stability data may not be available.

Current activities at the Centers for Disease Control and Prevention's National Diabetes Laboratory

In 1997, the Centers for Disease Control and Prevention established the National Diabetes Laboratory in order to help prevent and treat type 1 diabetes. This state-of-the-art laboratory collaborates with research scientists and key national and international organizations throughout the world to identify and study risk factors for type 1 diabetes by developing measurements for glycosylated proteins, developing and evaluating technology for measuring genetic risk factors for the disease, and working to standardize autoantibody measurements. Developing improved technologies for diagnosing and managing diabetes and developing reference materials for properly calibrating and standardizing blood glucose meters are also critical aspects of the laboratory's work. In addition, the laboratory provides quality storage for valuable collections of biologics and other materials and facilitates sharing of specimens, associated epidemiologic data, and test results. Working with our partners in diabetes research, we are improving the diagnosis, treatment, and prevention of type 1 diabetes.

Assessment of current National Cholesterol Education Program guidelines for total cholesterol triglyceride, HDL-cholesterol, and LDL-cholesterol measurements

We examine the effect of systematic bias and random error, quality control, and intraperson biological variation on the National Cholesterol Education Program (NCEP) clinical classifications for reported lipid measurements. We consider misclassification to occur if a true lipid homeostatic set point is within a desirable range but the reported lipid value is in a high-risk range, or if a true lipid homeostatic set point is in a high-risk range but the reported lipid value is in a desirable range. To evaluate the overall adequacy of the NCEP guidelines to ensure correct patient classification, we construct operating characteristic curves for total cholesterol, triglycerides, high-density lipoprotein cholesterol, and low-density lipoprotein cholesterol. We demonstrate that if laboratories are meeting the NCEP guidelines for inherent bias and analytic precision and are using standard quality-control (QC) procedures incorporating at least two QC samples per analytical run from each of two QC pools (for a total of 4 QC samples), the current NCEP guidelines are adequate to ensure (probability >0.90) correct patient classifications regardless of the size of the systematic bias of the laboratory or increased random analytic error. Thus we suggest that at least two concentrations of QC material be included in the QC scheme to ensure that the measurement system is operating within desired specifications across the entire range of desirable and high-risk lipid concentrations and to ensure with high probability that patients are correctly classified.

The accuracy of laboratory measurements in clinical chemistry: a study of 11 routine chemistry analytes in the College of American Pathologists Chemistry Survey with fresh frozen serum, definitive methods, and reference methods

CONTEXT

Better procedures are needed whereby national proficiency testing survey providers can assess and improve the accuracy of laboratory measurements in clinical chemistry.

SETTING

The 1994 College of American Pathologists Comprehensive Chemistry Survey.

DESIGN

This study of matrix effects and the accuracy of laboratory measurements for 11 analytes linked the logistics of the Survey to definitive methods at the National Institutes of Standards and Technology, reference methods at the Centers for Disease Control and Prevention, proficiency testing materials, and a fresh frozen serum sample. The data were analyzed with a statistical model of laboratory measurements.

RESULTS

(1) Matrix biases affected the results reported from 69% of the 644 peer group/survey specimen pairs evaluated. (2) Because of matrix biases, the reference value was the correct target value only 32% of the time; thus, the traceability established by definitive method and reference method value assignments on Chemistry Survey specimens did not assure accuracy on patient samples. (3) In contrast to matrix biases, the error caused by random matrix effects with proficiency testing samples was about the same as that caused by random specimen effects with fresh frozen serum, and both were less than within-run random analytic error. (4) Calibration biases occurred in 73% of the 180 peer groups evaluated, and, after matrix biases were removed, the total variance of interlaboratory measurements was due to peer group calibration bias (48%), within-peer-group random calibration error (31 %), within-run random error (14%), and random specimen effects (7%).

CONCLUSIONS

An opportunity exists to improve method calibration accuracy in clinical chemistry. With improved design, national proficiency testing surveys can monitor and help reduce method calibration error by converting reported survey results to a true accuracy base that predicts accuracy on patient samples. For medical purposes, the correct target values on artificial (matrix-modified) chemistry materials are reference values adjusted for the matrix bias of each peer group. Matrix biases estimated by the use of fresh frozen serum can be used as factors to transfer the accuracy of definitive methods from artificial reference materials to patient samples.

Point: status of lipid and lipoprotein standardization

Cholesterol and triglyceride standardization procedures have been used extensively and continuously since the 1950s. Definitive and Reference Methods, as well as primary and secondary standards, have been developed and maintained as the basis for evaluating the accuracy of results by various methods in many laboratories. But, although standardization efforts for apolipoprotein A-I and B measurements have been reported in detail in the scientific literature, much less has been reported in the area of total and lipoprotein cholesterol and triglyceride standardization efforts. Standardized cholesterol and triglyceride concentrations, determined in multiple large epidemiological and clinical studies, have been instrumental to the National Cholesterol Education Program panels that have assessed the lipoprotein values associated with risk of coronary disease, and have determined the cutpoints that are now used extensively by physicians to guide diagnosis and treatment of individual patients.

Evaluating lyophilized human serum preparations for suitability as proficiency testing materials for high-density lipoprotein cholesterol measurement

OBJECTIVE

To evaluate the suitability of various commercial preparations for use by the College of American Pathologists as survey materials in assessing high-density lipoprotein cholesterol measurement performance.

DESIGN

Lyophilized human serum preparations from six vendors (vendors A through F) were evaluated to determine which material(s) best mimicked the commutability of fresh human serum. Two freshly collected unfrozen pools prepared from donor specimens were analyzed concurrently with the vendor materials to identify sources of variation and possible matrix bias. Each material was evaluated using 5 common precipitation reagents (phosphotungstate-magnesium, phosphotungstic acid, dextran sulfate [50K and 500K], and heparin-manganese). To evaluate how each reagent separates lipoproteins in each material, the lipoprotein separation patterns were profiled using high-pressure liquid chromatography and compared with separation patterns observed for the fresh human serum pools.

MAIN OUTCOME MEASURES

Similarities in performance characteristics of vendor material(s) were compared with fresh human serum.

RESULTS

Two of the six materials gave separation profiles for the lipoproteins similar to the typical patterns observed for human serum. Material from vendor B showed the best commutability across all of the precipitation reagents and had the best combination of low overall variability (10% for level 1 and 9.4% for level 2) and minimal concentration differences among reagents.

CONCLUSIONS

Vendor B was selected by the College of American Pathologists to provide materials for use in assessing performance of lipid and lipoprotein testing in the 1994 Comprehensive Chemistry Surveys. This study demonstrates the great variability that different vendor preparations introduce into the measurement of high-density lipoprotein cholesterol. It also emphasizes the effort required to evaluate the suitability of processed materials for use in proficiency testing.

Traditional lipoprotein profile: clinical utility, performance requirement, and standardization

The lipid and lipoprotein parameters which are predominantly measured and effectively comprise the traditional lipoprotein profile include total cholesterol, high density lipoprotein (HDL) cholesterol, low density lipoprotein (LDL) cholesterol, and triglyceride. Total cholesterol is accepted as the initial entry point in a case finding approach such as that recommended by the National Cholesterol Education Program (NCEP). HDL cholesterol, known to be a strong inverse predicator of risk, is an additional measurement to total cholesterol to improve risk assessments. The evidence for triglyceride association remains mixed: although strong associations are found in some studies, the evidence as an independent risk factor is still incomplete. Triglyceride is therefore measured primarily for LDL estimation. Final classification and potential intervention is ultimately based on the measurement of LDL cholesterol. Reliability in the measurement of total cholesterol, HDL, LDL, and triglyceride is especially important if the uniform decision points established by the NCEP are to be properly implemented. Attention must be placed on controlling preanalytical sources of variation, which can account for as much as 60% of the total measurement variability. The major analytical source of error comes from matrix effects, which results in problems of proper analytical calibration. Instrument system calibration should be verified by a comparison with an accuracy base using fresh patient specimens. CDC has established a network of reference method laboratories to provide access to these lipid and lipoprotein accuracy bases.

Estimating and minimizing effects of biologic sources of variation by relative range when measuring the mean of serum lipids and lipoproteins

Biologic intraindividual variation (CVb) is a major source of inaccuracy in current lipid and lipoprotein measurements. Metaanalysis has been used to estimate the average CVb of serum total cholesterol (TC), high-density lipoprotein cholesterol (HDLC), low-density lipoprotein cholesterol (LDLC), and triglyceride (TG). These CVb values are larger than the National Cholesterol Education Program-accepted and -proposed analytic (CVa) goals. Measuring serial specimens reduces the error in determination of the mean concentration used in classification of the patient by cutoff points. We show (a) a convenient technique, based on the relative range, to qualitatively estimate and interpret biologic variation of TC, HDLC, LDLC, and TG, and (b) the number of serial specimens required to meet a total variation goal for measurements of mean lipid and lipoprotein values. A total variation goal has been selected that can be met by two serial specimens for a majority of individuals.

Estimating and minimizing effects of biologic sources of variation by relative range when measuring the mean of serum lipids and lipoproteins

Biologic intraindividual variation (CVb) is a major source of inaccuracy in current lipid and lipoprotein measurements. Metaanalysis has been used to estimate the average CVb of serum total cholesterol (TC), high-density lipoprotein cholesterol (HDLC), low-density lipoprotein cholesterol (LDLC), and triglyceride (TG). These CVb values are larger than the National Cholesterol Education Program-accepted and -proposed analytic (CVa) goals. Measuring serial specimens reduces the error in determination of the mean concentration used in classification of the patient by cutoff points. We show (a) a convenient technique, based on the relative range, to qualitatively estimate and interpret biologic variation of TC, HDLC, LDLC, and TG, and (b) the number of serial specimens required to meet a total variation goal for measurements of mean lipid and lipoprotein values. A total variation goal has been selected that can be met by two serial specimens for a majority of individuals.

Assay instrument-dependent matrix effects in standardization of cholesterol measurements

Human serum-based frozen reference materials have been used by the Centers for Disease Control and Prevention (CDC)-National Heart, Lung and Blood Institute Lipid Standardization Program to improve the precision and accuracy of blood cholesterol measurements. Occasionally, laboratories in the program have had problems obtaining results for patients' fresh serum samples equivalent to those obtained with frozen CDC standardization pools. This incompatibility of sample, reagent, instrument, and assay characteristics has been labeled broadly as a "matrix effect," which usually is attributed to unknown characteristics of the processed pool material. In this study we showed that a large negative bias obtained with CDC pools was attributable to use of the sample blank mode on the Cobas-Bio analyzer. However, under the same conditions, fresh patients' serum samples were analyzed accurately. The use of a blank absorbance immediately after mixing sample and reagents (the "autoblank" mode) allowed the instrument to accurately analyze both fresh serum samples and CDC standardization pools and thus allowed the documentation of traceability of the cholesterol measurements to the National Reference System for Cholesterol.

Assay instrument-dependent matrix effects in standardization of cholesterol measurements

Human serum-based frozen reference materials have been used by the Centers for Disease Control and Prevention (CDC)-National Heart, Lung and Blood Institute Lipid Standardization Program to improve the precision and accuracy of blood cholesterol measurements. Occasionally, laboratories in the program have had problems obtaining results for patients' fresh serum samples equivalent to those obtained with frozen CDC standardization pools. This incompatibility of sample, reagent, instrument, and assay characteristics has been labeled broadly as a "matrix effect," which usually is attributed to unknown characteristics of the processed pool material. In this study we showed that a large negative bias obtained with CDC pools was attributable to use of the sample blank mode on the Cobas-Bio analyzer. However, under the same conditions, fresh patients' serum samples were analyzed accurately. The use of a blank absorbance immediately after mixing sample and reagents (the "autoblank" mode) allowed the instrument to accurately analyze both fresh serum samples and CDC standardization pools and thus allowed the documentation of traceability of the cholesterol measurements to the National Reference System for Cholesterol.

Biological variability in concentrations of serum lipids: sources of variation among results from published studies and composite predicted values

To obtain the best estimates of the average intraindividual biological variability (CVb) in the concentrations of total cholesterol (TC), low-density lipoprotein cholesterol (LDLC), high-density lipoprotein cholesterol (HDLC), and triglyceride serum lipids in a person's blood, we evaluated results from 30 studies published from 1970 to 1992. The usually more applicable random-effects model estimated an average CVb of 6.1% for TC, 7.4% for HDLC, 9.5% for LDLC, and 22.6% for triglyceride. Composite estimates of the average CVb from all evaluated published studies by different models of estimation ranged from 6.0% to 6.4% for TC, from 6.2% to 7.5% for HDLC, from 7.0% to 9.6% for LDLC, and from 22.4% to 22.9% for triglyceride. Two important factors influenced the reported biological variation of the study subjects: (a) the magnitude of the variability of the analytical method used and (b) the design characteristics of the study--primarily the number of subjects, the sampling interval, and the number of measurements per subject. For TC, we found a statistically significant positive correlation between the reported mean CVb and both the number of study subjects and the analytical variation. For TC and LDLC we estimate CVb as a function of the study design features. The number of patient specimens required to obtain reliable estimates for serum lipid concentrations are determined from the CVb and the current analytical variation.

Biological variability in concentrations of serum lipids: sources of variation among results from published studies and composite predicted values

To obtain the best estimates of the average intraindividual biological variability (CVb) in the concentrations of total cholesterol (TC), low-density lipoprotein cholesterol (LDLC), high-density lipoprotein cholesterol (HDLC), and triglyceride serum lipids in a person's blood, we evaluated results from 30 studies published from 1970 to 1992. The usually more applicable random-effects model estimated an average CVb of 6.1% for TC, 7.4% for HDLC, 9.5% for LDLC, and 22.6% for triglyceride. Composite estimates of the average CVb from all evaluated published studies by different models of estimation ranged from 6.0% to 6.4% for TC, from 6.2% to 7.5% for HDLC, from 7.0% to 9.6% for LDLC, and from 22.4% to 22.9% for triglyceride. Two important factors influenced the reported biological variation of the study subjects: (a) the magnitude of the variability of the analytical method used and (b) the design characteristics of the study--primarily the number of subjects, the sampling interval, and the number of measurements per subject. For TC, we found a statistically significant positive correlation between the reported mean CVb and both the number of study subjects and the analytical variation. For TC and LDLC we estimate CVb as a function of the study design features. The number of patient specimens required to obtain reliable estimates for serum lipid concentrations are determined from the CVb and the current analytical variation.

Matrix effects and the accuracy of cholesterol analysis

We found evidence of bias due to matrix effect in 70% of 37 instrument/reagent-specific systems analyzing the total cholesterol content of a lyophilized proficiency testing material. We used a computational method to remove bias due to matrix effect from the proficiency testing database. After correction for matrix effect bias and when compared with the reference method, 92% to 93% of results for three lyophilized proficiency testing samples analyzed in 1989 and 1990 met the 1992 National Cholesterol Education Program total error goal of 8.9%, and 94% to 95% met the Clinical Laboratory Improvement Amendments of 1988 (CLIA '88) goal of 10%. However, compared with the definitive method for total cholesterol, the calibration bias of 41% of 37 peer groups exceeded the 1992 National Cholesterol Education Program goal for bias of 3%. Because the calibration bias of the method is incorporated into the peer group mean, use of peer group means as target values to assess result acceptability hinders advancement of the state of the art in interlaboratory comparability and the clinical effectiveness of laboratory testing. The prevalence of matrix effects has prevented successful application of accuracy-based evaluation of cholesterol test proficiency. The establishment of predictable recovery, preferably complete recovery, of cholesterol from reference materials is an important priority for cholesterol test methods. However, adjustment of proficiency testing results to remove the average bias due to matrix effects can help assess the actual state of the art in cholesterol test accuracy.

Matrix effects on proficiency testing materials. Impact on accuracy of cholesterol measurement in laboratories in the nation's largest hospital system

The objective of this collaborative study with the Department of Veterans Affairs (VA), College of American Pathologists (CAP), and the Centers for Disease Control and Prevention (CDC) was to quantitate the matrix-induced biases of cholesterol measurements on the CAP Comprehensive Chemistry Surveys materials used in proficiency testing (PT). A total of 174 VA Medical Centers outpatient clinics and clinical laboratories participate in the VA-CDC National Cholesterol Standardization and Certification Program. This study was conducted in 112 VA laboratories that have been standardized for measuring cholesterol accurately (within +/- 3.0% of the CDC reference-method values) using fresh, unfrozen, unadulterated human serum samples. Fresh serum samples and 1990 CAP Surveys materials were sent by overnight mail, and the laboratories were asked to analyze them simultaneously in triplicate in a single analytic batch run. The results showed significant matrix-effect biases with the CAP Surveys materials with six of the eight major peer groups, despite the fact that accuracy of cholesterol measurements was maintained with fresh serum samples. The magnitude and direction (positive or negative) of the matrix-effect biases were instrument, reagent, and method specific using the following peer groups: du Pont Dimension (-8.9%); Beckman CX4, CX5, and CX7 (-5.5%); Kodak Ektachem 400, 500, and 700 (+4.4%); Instrumentation Laboratory Monarch (-3.1%); Baxter Paramax (-2.4%); Technicon SMAC and RA (+1.3%); Hitachi/BMD 704 through 747 (+0.4%); and Abbott Spectrum (-0.3%). The CAP PT materials used currently do not behave in a manner identical to fresh human serum when measuring cholesterol on many, but not all, analytic systems. The observed biases due to "matrix effects" with PT materials will cause incorrect conclusions about the accuracy of many laboratory procedures performed on fresh patient specimens. This matrix-effect phenomenon will severely hamper interlaboratory accuracy transfer, standardization efforts, and monitoring performance of a laboratory's testing accuracy with the use of the current survey materials used in PT programs. Collaborative efforts are needed to (1) improve PT fluids to analytically behave more like fresh, human serum; (2) improve instrument design and reagent formulation; and (3) select methods and methodologic parameters that are more "robust" and less sensitive to the exact character of processed calibrators, quality control, and PT materials.

Blood lipid measurements. Variations and practical utility

OBJECTIVE

To describe the magnitude and impact of the major biological and analytical sources of variation in serum lipid and lipoprotein levels on risk of coronary heart disease; to present a way to qualitatively estimate the total intraindividual variation; and to demonstrate how to determine the number of specimens required to estimate, with 95% confidence, the "true" underlying total cholesterol value in the serum of a patient.

DATA SOURCES

Representative references on each source of variation were selected from more than 300 reviewed publications, most published within the past 5 years, to document current findings and concepts. Most articles reviewed were in English.

STUDY SELECTIONS

Studies on biological sources of variation were selected using the following criteria: representative of published findings, clear statement of either significant or insignificant results, and acquisition of clinical and laboratory data under standardized conditions. Representative results for special populations such as women and children are reported when results differ from those of adult men.

DATA EXTRACTION

References were selected based on acceptable experimental design and use of standardized laboratory lipid measurements.

DATA SYNTHESIS

The lipid levels considered representative for a selected source of variation arose from quantitative measurements by a suitably standardized laboratory. Statistical analysis of data was examined to assure reliability. The proposed method of estimating the biological coefficient of variation must be considered to give qualitative results, because only two or three serial specimens are collected in most cases for the estimation.

CONCLUSIONS

Concern has arisen about the magnitude, impact, and interpretation of preanalytical as well as analytical sources of variation on reported results of lipid measurements of an individual. Preanalytical sources of variation from behavioral, clinical, and sampling sources constitute about 60% of the total variation in a reported lipid measurement of an individual. A technique is presented to allow physicians to qualitatively estimate the intraindividual biological variation of a patient from the results of two or more specimens reported from a standardized laboratory and to determine whether additional specimens are needed to meet the National Cholesterol Education Program recommendation that the intraindividual serum total cholesterol coefficient of variation not exceed 5.0. A National Reference Method Network has been established to help solve analytical problems.

Factors influencing the accuracy of the national reference system total cholesterol reference method

Previous comparisons between the Reference and Definitive Methods for measuring serum cholesterol have demonstrated a small but persistent positive bias in the Reference Method, averaging about +1.6%. Here we describe the results of further investigations designed to better characterize the nature of this bias. Analysis of a well-characterized model serum sample (SRM 909) suggests that more than half of the difference in cholesterol values determined by the two methods is the result of small contributions from cholesterol precursor sterols and phytosterols, which are also measured for the Reference Method. An additional significant contribution may be from cholesterol oxidation products, particularly 7-hydroxycholesterol isomers, which are active in the Liebermann-Burchard reaction. The 7-hydroxycholesterol in SRM 909, most of which appeared to be already present in the serum rather than formed during saponification, may account for as much as 20% of the observed difference between the methods. Contributions from other possible sources, including impurities in the cholesterol standard and incomplete saponification of cholesteryl esters, are very small. Because the observed bias is both quite small and consistent among samples, the cholesterol Reference Method continues to meet all of the requirements generally expected for a dependable and effective Reference Method.

A comparison of weights of debris extruded apically by conventional filing and Canal Master techniques

Sixty extracted human teeth were divided into three groups of 20 each. Apically extruded debris and irrigant were collected, dried, and weighed by the following three instrumentation techniques: (a) group 1, filing 1 mm short of the foramen; (b) group 2, Canal Master instrumentation to the foramen; and (c) group 3, filing to the foramen (for a relative comparison). The results indicated that all three groups were significantly different from one another. Group 1 had the least amount of debris extruded. Of the two groups instrumented to the foramen, group 3 had twice as much debris extruded as group 2. An apical dentinal plug was frequently found in group 1 and was probably a major reason why this group had the least amount of extruded debris. The significance of this dentinal plug and possible indications for instrumentation to the foramen are discussed.

Clinical issues in cholesterol testing

Lipid investigators have begun to examine the biological sources of variation in serum cholesterol levels and to seek ways to accurately measure the total cholesterol (TC) level in the serum of a patient. A person's TC level varies primarily because of the effects from seasonal changes, behavioral changes, and illness. Results of studies of the effect of seasonal changes indicate that serum TC and obesity increase during winter and decrease during summer. Behavioral sources of variation include diet, alcohol intake, smoking, and exercise. Clinical sources of TC level variation include all illnesses. The physician can help control sources of variation by recognizing their causes, by advising of the effect that behavioral risk factors have on cholesterol levels, and by using the average of results for multiple specimens to estimate the true value of serum cholesterol in a patient.

Establishment of reference methods for lipids, lipoproteins and apolipoproteins

Reference methods for lipids, lipoproteins, and apolipoproteins have been developed for use as part of an accuracy base for institutional, national, or international reference systems. A widely accepted reference method exists only for total cholesterol. Well described interim or institutional in-house reference methods have been established for the other lipids, lipoproteins, and apolipoproteins. The major criteria for a reference method are 1) scientific basis, 2) sound principles, 3) available calibration and control materials, 4) traceability to a definitive method or a point of reference, and 5) applicability to reference materials that provide traceability to clinical methods and transferability to other reference laboratories. The total cholesterol reference method of the U.S. National Reference System demonstrates how a reference method can be developed and applied. Reference methods now available can lead to an accepted international accuracy base for the clinically useful lipid, lipoprotein, and apolipoprotein measurements.

Analyzing quality-control trends with moving slope charts

We have developed and evaluated a new procedure for detecting trends in quality-control measurements and applied it to laboratory data. The method requires the use of sequential or "moving" slope estimates to identify trends. Formulae are derived to estimate the regression error for the moving slope directly from the standard deviation of the analytical measurements obtained during characterization runs. Control limits for the moving slope depend only on this regression error, the span of the slope, and the desired statistical level of control. The moving slope can be plotted with control limits to determine out-of-control points. The statistical power of the moving slope is found to be much greater than that of an often-used test for trends. An example of the use of the moving slope is shown for quality-control measurements for total cholesterol obtained over several years. We conclude that the moving slope procedure has considerably more statistical power than trend rules and that it yields more useful information to the analyst.

College of American Pathologists-Centers for Disease Control collaborative study for evaluating reference materials for total serum cholesterol measurements

Several recent studies to evaluate the performance of laboratory instruments have shown that with some instrument systems processed (lyophilized, frozen, and stabilized) materials exhibit matrix effects that cause the assay for cholesterol to respond differently for them than for patient specimens. To understand this phenomenon better the College of American Pathologists, Northfield, III, and the Centers for Disease Control, Atlanta, Ga, have conducted a collaborative study with 44 laboratories where 16 instruments manufactured by nine companies are evaluated. The purposes were to assess measurement variation on several reference materials used for standardizing total cholesterol measurements and to evaluate a new stabilized liquid serum as a potential reference material. Lypophilized, frozen, fresh-frozen, and stabilized materials at three concentrations were measured for total cholesterol. The results show that the average coefficient of variation of measured total cholesterol for all instruments, laboratories, vials, and replicates is 3.6% to 4.1% for each of the materials measured (excluding the results for one instrument). For one instrument, however, significant bias was found on the stabilized liquid serum material. Results from the fresh-frozen materials indicate that the instrument systems evaluated allow laboratories to attain the National Cholesterol Education Program analytical performance goals.

A comparison of results for cholesterol in human serum obtained by the Reference Method and by the Definitive Method of the National Reference System for cholesterol

Here we compare the Reference Method (I) and the Definitive Method (II) for measurement of cholesterol in serum. For pure cholesterol solutions, values by I agree with values by II, but for fresh, frozen, or lyophilized sera, values by I average 1.6% higher than values by II. We conclude that an undetected interference is associated with I. However, the observed difference does not diminish the usefulness of I as an accuracy base for cholesterol measurements, because it is clinically insignificant at present.

A comparison of results for cholesterol in human serum obtained by the Reference Method and by the Definitive Method of the National Reference System for cholesterol

Here we compare the Reference Method (I) and the Definitive Method (II) for measurement of cholesterol in serum. For pure cholesterol solutions, values by I agree with values by II, but for fresh, frozen, or lyophilized sera, values by I average 1.6% higher than values by II. We conclude that an undetected interference is associated with I. However, the observed difference does not diminish the usefulness of I as an accuracy base for cholesterol measurements, because it is clinically insignificant at present.

Progress in lipid reporting practices and reliability of blood cholesterol measurement in clinical laboratories in Nebraska. Efforts to align results with the Centers for Disease Control, and feasibility of meeting National Cholesterol Education Program Guidelines

The National Cholesterol Education Program has recommended that all laboratories be consistent, precise, and accurate in the reporting and measurement of blood cholesterol levels. In a follow-up to a 1984 survey study, we assessed the changes in reporting procedures for measurements of blood lipid levels in 16 clinical laboratories in Nebraska. Using human serum reference materials of known cholesterol concentrations provided by the Centers for Disease Control, we also assessed the precision and accuracy of measurement of blood cholesterol levels in clinical laboratories in Nebraska. Fourteen of the 16 laboratories restudied in 1987 had altered the reference range for total serum cholesterol since 1984, 86% of whom lowered the upper limit of the reference range. Eleven of 16 laboratories expressed reference ranges for total serum cholesterol in terms of patient age in 1987, while only 7 of 20 did in 1984. Gender-based reference ranges increased from 0 to 5 from 1984 to 1987. Similar trends were seen in the reporting of high-density lipoprotein cholesterol and triglyceride concentrations. Reporting procedures varied greatly; only 1 laboratory used National Cholesterol Education Program risk levels for measuring total serum cholesterol levels. Fifteen laboratories met the National Cholesterol Education Program recommendation for precision (coefficient of variation, less than or equal to 5%) and 78% of laboratories obtained results that satisfied the current recommendation for accuracy (within 5% of "true value," as determined by the Centers for Disease Control).

The Centers for Disease Control-National Heart, Lung and Blood Institute Lipid Standardization Program. An approach to accurate and precise lipid measurements

In collaboration with the National Heart, Lung, and Blood Institute, the CDC has supported programs for standardizing lipid measurements for more than 30 years. These programs were begun because comparable and accurate quantitative measurements were needed for epidemiologic studies of coronary heart disease. Since the first program was initiated, over 500 national and international laboratories have participated in the various CDC lipid standardization programs. The cornerstone of these standardization programs has been an accuracy base of lipid reference materials and methods developed by CDC. Specifically, CDC has developed human, serum-based reference materials for cholesterol, HDL, triglyceride, and apolipoproteins A-I and B and reference methods for total cholesterol, HDL, and triglyceride. The CDC reference method for cholesterol has been adopted as the national reference method for cholesterol by the National Reference System for the Clinical Laboratory Council of the National Committee for Clinical Laboratory Standards. The approved CDC reference method along with an approved NBS definitive method, an approved NBS certified Reference Material, and the CDC certified serum-based secondary reference materials make up the accuracy base for serum cholesterol measurements in the United States, and together they are recognized as the National Reference System for Cholesterol. The NCEP Laboratory Standardization Panel recommends that cholesterol measurements made by all clinical laboratories should be standardized so that cholesterol values are traceable to the National Reference System for Cholesterol. In support of the NCEP's efforts, CDC will establish a standardization program permitting the laboratory and manufacturing community to trace cholesterol measurements and the development of cholesterol diagnostic products back to the national reference system. The major emphasis of this standardization effort is to establish a network of reference method laboratories (1) to assign cholesterol values to all commercially prepared lots of calibrators and control materials and (2) to provide reference measurements on individual "fresh" human serum specimens to manufacturers and clinical laboratories. CDC is also working to (1) provide reference materials to manufacturers, (2) collaborate with NBS to maintain documentation of the national reference system accuracy base, (3) cooperate with proficiency testing organizations to assist in the accurate labeling of reference materials, and (4) provide training and education pertinent to cholesterol standardization.(ABSTRACT TRUNCATED AT 400 WORDS)

A laboratory for handling chemical toxicants safely

The Centers for Disease Control has a special Chemical Toxicant Laboratory (CTL) for handling very hazardous chemicals. It is designed to protect the workers, prevent the release of the chemical toxicant into the surrounding environment, and provide for the scientific integrity of the experiments conducted. A discussion of laboratory ventilation and special containment devices is presented. The design of the CTL, coupled with a realistic set of safety guidelines, provides for the safe conduct of research involving highly toxic chemicals.

Insulin resistance in total lipodystrophy: evidence for a pre-receptor defect in insulin action

The cause of insulin resistance in lipodystrophic diabetes is unknown but has generally been ascribed to dysfunction at either the receptor or post receptor level. In a 14 year-old girl with total acquired lipodystrophy, subcutaneous and intravenous insulin requirements approximated 600 units daily. However, circulating total and free insulin levels were not increased, and during testing by the euglycemic clamp method, the glucose response to increasing free insulin concentrations was within the range found in eight subjects with insulin-dependent diabetes. Insulin clearance during the euglycemic clamp was 43, 98, 115, and 116 mL/kg/min at each of four insulin infusion rates compared to means of 13, 13, 12, and 11 in the control subjects with diabetes. No detectable degrading activity was present in serum, and serum inhibited insulin degradation normally. Binding of insulin to IgG, IgM, and IgE was not increased, insulin binding to monocytes and erythrocytes was not sufficiently abnormal to account for the the insulin resistance, and insulin receptor increased insulin clearance or accelerated degradation of insulin by tissues.

Use of a glucose-controlled insulin infusion system in children and adolescents with insulin-dependent diabetes

Seven children and adolescents (aged 8 to 23 years, mean 16 years) with poorly controlled insulin-dependent diabetes mellitus were evaluated with a glucose-controlled insulin infusion system (GCIIS) to determine whether its use could help define appropriate long-term insulin treatment regimens and increase patient understanding and compliance with such regimens. The GCIIS-derived insulin regimen was characterized by a mean insulin requirement of 1.0 +/- 0.1 units/kg/day, with 55% of the total insulin dose given in the morning and 45% in the evening. Forty-eight percent of the total insulin was short acting. Both clinical and biochemical indices of metabolic control (HbA1c levels) improved after insulin dose readjustment based on GCIIS data. The continuous documentation of blood glucose levels provided by the GCIIS was valuable in educating patients about the interrelationship between insulin, exercise, and diet. Using GCIIS data, patients were taught to apply the new regimen flexibly to variations in activity and diet. Thus, the GCIIS can aid in the management of selected children and adolescents with insulin-dependent diabetes millitus. Insulin patterns derived from its use suggest that a flexible, twice daily regimen characterized by nearly equal proportions of short- and intermediate-acting preparations may be beneficial for other pediatric patients.