Low hemoglobin density potential marker of iron availability

INTRODUCTION

Low hemoglobin density (LHD%) is a new parameter provided by Beckman-Coulter derived from the mean cell hemoglobin concentration, using the mathematical sigmoid transformation LHD% = 100×√(1-(1/(1 + e(1.8(30-MCHC)))). This study investigated the reliability of LHD% for the assessment of iron status in the presence of inflammation.

METHODS

Healthy subjects (n = 90) and patients with iron deficiency (IDA, n = 110), chronic kidney disease (CKD, n = 65) and anemia of chronic disease (ACD, n = 85; 24 were iron deficient and 61 were iron sufficient) were analyzed on a LH 780 analyzer (Beckman Coulter Inc., Miami, FL, USA). Independent samples U test and receiver operating characteristic (ROC) curve analysis were applied. To determine the concordance between LHD% and soluble transferrin receptor (sTrR) Cohen's κ index was calculated.

RESULTS

LHD % values showed no statistical difference in patients with IDA and patients with ACD accompanied with IDA (P = 0.6427); LHD% values in these patients were significantly different (P < 0.0001) compared with the iron-sufficient patients with ACD. ROC analysis for LHD% in the detection of iron deficiency showed the following: area under curve 0.903; cut off 5.5%, sensitivity 88.6%, specificity 76.9%; κ index, 0.65.

CONCLUSION

LHD% is a reliable parameter for the detection of iron deficiency in patients with anemia in the presence of inflammation.

Comparison of BNP and NT-proBNP assays in the approach to the emergency diagnosis of acute dyspnea

N-terminal pro-brain natriuretic peptide (NT-proBNP) and BNP measurement could have a significant role in differentiating dyspnea between cardiac or pulmonary origin in the emergency room. The development of new and different commercial assays for these B-type natriuretic peptides offers the possibility of improving and simplifying their measurements but this could be defaulted due to the differences in methodology and the lack of assay standardization. We compared four available methods of measuring NT-proBNP and BNP and evaluated their usefulness in diagnosing the causes of breathlessness in the emergency room. The correlation of BNP with different assays was strong with r>0.98 (P<0.0001). Comparison studies between NT-proBNP and BNP procedures were in good agreement with r>0.87. The area under the receiver-operating characteristic curve (ROC) for BNP or NT-proBNP for detecting any cardiac dysfunction was higher than 0.96 (95% CI). A BNP value of 116 pg/mL measurement with the Access BNP assay (Beckman Coulter Inc., Fullerton, CA), a BNP value of 79 pg/mL with Advia Centaur BNP assay (Bayer Diagnostics, Tarrytown, NY), and an NT-proBNP level of 817 pg/mL in Elecsys NT-proBNP assay (Roche Diagnostic, Mannheim, Germany), showed both high sensitivity (>92%) and high specificity (>93%). We have found that NT-proBNP and BNP present similar diagnostic accuracies for the differential diagnosis of dyspnea.

[Susceptibility of Enterococcus genus to new antimicrobial agents]

The aim of this study was to determine the prevalence of resistance to different antibiotics in 109 clinical strains of Enterococcus isolated consecutively in hospital over a four-month period in 2002. The strains were identified by species using the semiautomatic system Microscan walk away 40 (Dade Behring) and the API20 STREP system (Biomerieux). Three different methods for the susceptibility study were used: Sensititre, E-test and disc diffusion. The percentage of Enterococcus faecalis and Enterococcus faecium isolated was 92% and 8%, respectively. The following resistance to Enterococcus faecalis was detected: erythromycin (53%), telithromycin (36%), penicillin (2%), ampicillin (1%), vancomycin, teicoplanin and linezolid (0%), high level resistance to streptomycin (51%) and gentamicin (32%). A high percentage of strains resistant to penicillin and ampicillin was detected in E. faecium (six of nine strains). For two species, high susceptibility to linezolid and glycopeptides was found.

Amino, chloromethyl and acetal-functionalized latex particles for immunoassays: a comparative study

Latex particles with different functionalized surface groups (amino, acetal and chloromethyl) for the covalent linking of protein molecules were synthesized and characterized. Immunopurified anti-ferritin antibodies were then covalently coupled with a mean efficiency rate (protein covalently bound to latex particles with respect to the total amount of protein added) of 60%. The reagents developed were applied to the measurement of serum ferritin concentration in a turbidimetric procedure, showing a good measuring range and a lowest detection limit of 3.5 ng/ml in the case of the amino-modified particles. These immunological reagents were compared with a commercial nephelometric method, showing a good linear correlation in all cases but no transferability in the acetal and chloromethyl latex with additional carboxyl groups, probably due to interference with other serum components. The differences among latex found in this study indicate that it would be necessary to optimize the assay conditions for each type of particle, in order to achieve a maximum immunoreactivity.

Longitudinal study of serum copper and zinc levels and their distribution in blood proteins after acute myocardial infarction

The levels and distribution of serum Cu and Zn were studied in patients diagnosed with acute myocardial infarction from the day of admission to the Cardiovascular Intensive Care Unit until the 10th day following the attack. The results obtained show that Cu increases significantly (p < 0.01) after the 5th day after the acute myocardial infarction, while Zn decreases significantly (p < 0.01) with respect to the control group from the first day on, with the lowest values being found on the 3rd day after the attack. Further, total serum Cu showed an excellent correlation with the albumin-bound and globulin-bound Cu (ceruloplasmin), as well as with the concentrations of both serum protein fractions. In contrast, total serum Zn only presents this correlation with Zn bound to albumin, but not with Zn bound to globulin nor with the albumin concentration. These findings suggest the existence of some type of relationship between the two fractions of the element bound to protein. This relationship is probably different for both metals.

Development of a microparticle-enhanced nephelometric immunoassay for quantitation of human lysozyme in pleural effusion and plasma

A microparticle-enhanced nephelometric immunoassay, based on polystyrene beads coated with antihuman lysozyme antibody, has been developed for lysozyme quantification in sera and pleural effusions. The standard curve extends from 0.58 mg/l to 18.75 mg/l and no antigen effect was observed. The results showed a good serial precision. The intra-assay precision (n = 20) expressed as CV was between 2.2 and 4.2 in three different concentrations. The inter-assay precision, with different calibration curves (n = 12) was between 6.4 and 7.1. The analytical assay showed a sufficient linearity (r > 0.999). There were no interferences either with haemoglobin (up to 4 g/l), lipids (up to 0.5%, expressed as 1% Lipofundina content), or bilirubin (up to 5 mg/dl). The analytical sensitivity was lower than 0.6 mg/l. The correlation with a Micrococcus lysodeikticus turbidimetric assay showed a correlation coefficient of 0.915. We have studied 92 patients with pleural effusion. In each case, pleural fluid adenosine deaminase activity and pleural fluid to plasma lysozyme ratio were determined. The lysozyme ratio showed similar clinical sensitivity and specificity as to adenosine deaminase.

Development of an automated immunoturbidimetric ferritin assay

We have developed a new procedure for turbidimetric measurement of ferritin concentration in human serum, based on latex microparticle agglutination technology. The procedure has been automated using the Falcor 300 analyzer. Carboxilated latex particles (336 nm in diameter) were covalently coupled with immunopurified F(ab')2 fragments of anti-ferritin IgG antibodies. Coated microparticles were automatically mixed with undiluted sample and the resulting absorbance due to agglutination was measured at 550 nm. The procedure generated a calibration curve with a measuring range of 0 to 558 microg/l, showing a day-to-day imprecision lower than 5.7%. The detection limit was 4 microg/l. There were no interferences from bilirubin, hemoglobin or rheumatoid factors. Turbid and lipemic samples caused an important interference which could be avoided by pretreating those samples prior to measurement. A prozone effect was provisionally obtained with ferritin concentrations over 1800 microg/l. The results suggested a hook-like effect due to a rapid microparticle precipitation in the reaction media, that could be avoided by increasing the reaction medium density by adding sucrose to the buffer, up to 150 g/l concentration. This sucrose addition resulted in a displacement of the Heidelberger curve with a prozone phenomenon occuring at concentration higher than 3000 microg/l of ferritin. Results obtained with the present procedure correlated well with those obtained by a nephelometric procedure and with those obtained by an RIA. We conclude that this latex turbidimetric immunochemical procedure is simple, rapid, has a good analytical and operational performance on the Falcor 300 analyzer and is well suited for the measurement of ferritin concentration in human serum.

Evaluation of an immunoturbidimetric assay of serum digoxin without sample pretreatment

We have evaluated a new turbidimetric immunoassay test (On Line, Roche) for the quantification of digoxin in human serum without sample pretreatment. The assay, automated on a Cobas-Mira clinical analyser, is based on the measurement of the changes in absorbance that occur when digoxin-coated microparticles aggregate with digoxin antibody; the aggregation reaction is partially inhibited by digoxin in the sample. The calibration curve extends up to 6.4 nmol/l, with a wide measuring range, and was stable at least for 10 days during the studied period. The intra- and inter-assay coefficients of variation were in all cases lower than 7%. The detection limit of the assay was 0.256 nmol/l and the linearity, assessed by measuring serial dilutions of poisoned patient sera, showed correlation coefficients higher than 0.998. There is no interference from bilirubin (up to 340 mumol/l) or haemoglobin (up to 7900 mg/l), although a positive interference by lipids was found, which can be avoided by previous delipidation of these sera. No digoxin-like factors were identified affecting digoxin values in a concentration higher than that of the sensitivity of the assay in samples of premature infants, pregnant women, or patients with renal or hepatic failure. Correlation coefficients between the turbidimetric assay and two immunoassays (fluorescence polarization and RIA) were 0.983 and 0.955 respectively, calculated from the assay of 102 and 98 samples. Nevertheless the values obtained by the evaluated assay were proportionally 20% higher, a fact probably related to differences in calibrators.

Development and validation of an automated particle-enhanced nephelometric immunoassay method for the measurement of human plasma C1q

We have developed a sensitive immunoassay based on latex particle agglutination for measuring C1q concentrations in human plasma. In this simple and fast particle-enhanced immunoassay, we used carboxylated latex particles (diameter 210 nm) covalently coated with F(ab')2 fragments of anti-C1q antibodies. These particles are incubated with diluted sample (400-fold) for 6 min at room temperature, with the resulting agglutination quantified by measuring the change of light-scatter produced. The assay has been automated on the Behring nephelometer analyzer with a sampling rate of 150 samples/hr. This assay generates a standard curve in the range of 24-775 mg/L, showing intraassay and interassay precision of < 8% and < 10%, respectively. Dilution linearity was validated throughout the dynamic range of the assay. There were no interferences from bilirubin, Intralipid, haemoglobin, and rheumatoid factor. Results obtained in 45 clinical samples correlated well with those obtained by a commercial radial immunodiffusion method (r = 0.936), and with those obtained by the Behring immunoprecipitation nephelometric test (r = 0.950). The mean concentration in plasma from healthy subjects was 180 mg/L and the reference interval was from 128 to 237 mg/L. This latex nephelometric procedure is a convenient method and an interesting alternative to other immunoassays for routine measurement of human C1q.

Multicentre evaluation of the Boehringer Mannheim Hitachi 911 system for homogeneous immunoassays

A multicentre evaluation of the new analyser, Hitachi 911, is reported for three different classes of homogeneous immunoassays (latex assays, immunoprecipitation assays, and CEDIA assays). The evaluation protocol follows ECCLS, IFCC and NCCLS guidelines. Using patient samples and commercial controls, within run and between run coefficients of variation were less than 3% in most cases, but as high as 9.7% for some CEDIA and latex assays. All the assays were linear, either in the reference or the therapeutic range of the analytes. No interference by haemolysis, lipaemia or icterus was observed. The methods were compared with other commercial methods. Coefficients of correlation were higher than 0.94 for all the methods. However, there were differences of slope and intercept for rheumatoid factor, apolipoprotein A-I and apolipoprotein B. On the Hitachi 911, all of the eight methods give precise and accurate results, and compare well with other established methods on immunoassay dedicated analysers. The discrepancies observed could be ascribed to current problems of immunoassay standardization.

Automated turbidimetry of serum lipoprotein(a)

We describe a simple immunoturbidimetric method for quantifying lipoprotein(a) in serum based on latex-enhanced particle agglutination technology. Carboxylated latex particles (diameter 240 nm) covalently coated with F(ab')2 fragments of anti-lipoprotein(a) antibodies are incubated with the sample for 5 min at 37 degrees C, and the resulting agglutination is quantified by measuring the change of turbidity produced at 700 nm. The assay is rapid, precise and fully automated on the Hitachi 911 analyser. The assay range is about 0.03-0.9 g/l. Average analytical recovery was 97.8%. Precision (CV) ranged from 1.9 to 3.1% at different lipoprotein(a) values. There was no interference from bilirubin, Intralipid, haemoglobin, plasminogen or apolipoprotein B. Comparisons with a latex nephelometric assay carried out on the Behring nephelometer analyser, and with three commercially available methods, a radioimmunoassay and two ELISA assays, gave good correlations (r > 0.95), although a large among-method variation in lipoprotein(a) values was found. We conclude that the proposed latex turbidimetric immunoassay method is suitable for routine use in clinical laboratories.

Automated determination of apolipoproteins A-I and B with an improved immunoturbidimetric assay using Boehringer Mannheim and Hitachi analyzer systems

Results from an evaluation of immunoturbidimetric methods (Tina-quant) for apolipoprotein A-I (apo AI) and apolipoprotein B (apo B), are presented and compared with results from six other commercial immunoassays. These apo AI and B procedures are fully automated on the Boehringer Mannheim (BM) and Hitachi 704 and 717 analyzers and use undiluted serum. The methods provide an analytical range from 0.002 to 2.3 g/L for apo AI and from 0.003 to 2.2 g/L for apo B and are precise with maximal CVs of 4%. Neither hemoglobin nor intralipid interfered with the assays. Bilirubin concentrations higher than 376 mumol/L for apo AI and 444 mumol/L for apo B, produce a negative interference. In the apo AI method a positive interference caused by rheumatoid factor was found for concentrations higher than 440 x 10(3) IU/L. All comparisons showed an excellent correlation (r > 0.93) with all methods, and slopes ranged from 1.01 to 1.25 for apo AI, and from 0.62 to 1.15 for apo B.

IgG interference in second generation enzymeimmunoassays for anti-hepatitis C virus antibodies

The interference of endogenous IgG in the identification of anti-HCV antibodies was studied in three second-generation enzymeimmunoassays. The addition of increasing concentrations of this immunoglobulin led to the appearance of false positives. The results obtained confirm the hypothesis that a non-specific binding of the IgG with the support material used was responsible for this interference.

[Lymphocyte subpopulations, neopterin, and beta-2-microglobulin: relationship with clinical stage, risk of progression to AIDS and presence of active infection in HIV infection]

BACKGROUND

The aim of the present study was to evaluate the relation between the number of CD4 lymphocytes, the CD4/CD8 ratio and the plasma levels of neopterin and beta-2 microglobulin, and the clinical status, risk of progression without of active opportunistic infections, among HIV-infected patients.

METHODS

Seventy-two patients infected by HIV in different clinical groups were evaluated upon entering the study and following a mean follow up of 6 months for the parameters studied.

RESULTS

The values of CD4 lymphocytes and neopterin were related with the clinical status according to the CDC's classification, with no significant differences existing in the beta-2 microglobulin level. The CD4 count as well as the neopterin and the beta-2 microglobulin levels differed significantly when classified to the patients with regard to the risk of progression to AIDS throughout the study. The presence of active opportunistic infections was related with significantly higher concentrations of neopterin without differences recorded for the remaining parameters.

CONCLUSIONS

The parameters studied are good markers or both clinical status and/or the risk of short-term progression to AIDS. Neopterin levels are high during acute infections. Therefore, its prognostic value should be cautiously evaluated in this situation.

Automated latex nephelometric immunoassay for the measurement of serum lipoprotein (a)

A sensitive immunoassay based on latex particle agglutination has been developed for measuring lipoprotein Lp(a) concentrations in serum or plasma. Carboxylated latex particles (diameter 240 nm) covalently coated with F(ab')2 fragments of anti-lipoprotein Lp(a) antibodies are incubated with diluted sample (400-fold) for 12 min at room temperature, with the resulting agglutination quantified by measuring the change of light-scatter produced. The assay has been automated on the Behring nephelometer analyzer with a sampling rate of 150 samples/hour. This assay generates a standard curve in the range of 27 to 1750 mg/L, showing inter-assay precision of less than 8%. There were no interferences from plasminogen, bilirubin, Intralipid, haemoglobin, rheumatoid factor, and apolipoprotein B. No significant differences were observed when fresh and frozen samples were compared. Sample pretreatment with "Lipoclean" clearing agent and sample lyophilization decreased the agglutinating reaction. In two separate studies using 77 and 112 patient sera the Lp(a) values, determined by the latex nephelometric method, the Terumo Macra Lp(a) ELISA test, and the Pharmacia Apo(a) radioimmunoassay method, gave correlation coefficients of 0.948 and 0.974, respectively. Physiological lipoprotein (a) values were determined in a blood donor group, with the distribution of serum Lp(a) highly skewed, with a mean (SD) and median values of 213(236) mg/L and 116 mg/L, respectively. Concentrations of Lp(a) were found to be age- and sex-independent. This latex nephelometric procedure is a convenient method and an interesting alternative to other immunoassays for routine measurement of human lipoprotein (a).

Measurement of serum beta 2-microglobulin by a latex nephelometric immunoassay

A rapid and simple technique for the measurement of beta 2-microglobulin on the Behring nephelometer was developed using a commercially available latex-anti beta 2-microglobulin. This is a fully automated assay and no pretreatment of sample is necessary. An excellent correlation with radioimmunoassay (r = 0.972) and time-resolved fluoroimmunoassay (r = 0.914) was obtained. Haemolysis, lipemia, bilirubin, and rheumatoid factor do not cause interference because of the high dilution of samples and the use of Fab fragments of the antibody. The analytical range extends from 0.6 to 10.7 mg/L. Between-run imprecision (CV) ranged from 6 to 8%.

Automated latex nephelometric immunoassay of theophylline in human serum

A latex test was adapted for the nephelometric quantitation of theophylline in human serum. The assay is fully automated on the Behring Nephelometer Analyser with a sampling rate of 150 samples per hour. There is no interference from bilirubin (up to 340 mumol/l), haemoglobin (up to 7000 mg/l), Intralipid (up to 5 g/l), or rheumatoid factor (up to 1100 x 10(3) IU/l). The theophylline standard curve extends from 1.25 to 40 mg/l. The coefficient of variation ranged from 2.4 to 7.4%. The correlation coefficient between the latex immunoassay and the TDx theophylline procedure was 0.988, calculated from the assay of 74 samples.

Automated quantitative nephelometric latex immunoassay for determining ferritin in human serum

We describe a rapid and sensitive latex nephelometric immunoassay for quantifying ferritin in human serum. This latex immunoassay procedure uses commercially available ready-for-use reagents [Tina-Quant (a) Ferritin, Boehringer Mannheim] that have a long shelf life. The assay consists of incubating the diluted serum sample (5-fold) for 12 min at room temperature with latex particles covalently coated with anti-ferritin antibodies, and then quantifying the change of light-scatter produced. The assay is fully automated on the Behring nephelometer analyzer with a sampling rate of 150 samples/hour. The method has an analytical range of 3 to 260 micrograms/l. Maximal intra- and inter-assay CVs were 4.0 and 6.2%, respectively. Analytical recoveries ranged from 91.3 to 103.6%. Assay detection limit was less than 3 micrograms/l. Linearity of the test is given throughout the measuring range. There was no interference from bilirubin (up to 340 mumol/l), haemoglobin (up to 7 g/l), or rheumatoid factor (up to 1,100 IU/ml). Turbid and lipemic samples interfere. This interference may be avoided by pretreating these samples prior to assay. Results correlated well with those obtained by an automated ELISA test (r = 0.995) and with those of two commercial RIA methods (r greater than 0.97). This latex nephelometric procedure is a convenient method and represents an interesting alternative to other immunoassays for measuring ferritin in human serum.

Rheumatoid arthritis and hepatitis C virus antibodies

The prevalence of antibodies against the hepatitis C virus (anti-HCV) in patients seropositive for rheumatoid arthritis (RA) and suffering from this disease has been analyzed in comparison to a control group. Our results suggest that the high rate of prevalence found in RA patients is due to IgG interference in the measuring method used.

Automated turbidimetry of rheumatoid factor without heat inactivation of serum

We describe an improved turbidimetric procedure for rheumatoid factor measurement in human serum. Heat-aggregated human IgG is used as antigen. Interference produced by the complement component C1q, which required a previous heat pretreatment of sera, is avoided by the addition of the chemical inhibitor, poly(vinyl sulphonic acid). This inhibitor eliminates C1q interference without affecting the attachment of rheumatoid factor to the Fc part of IgG, thus permitting full automation of the assay. After studying the reactivity of different heat-aggregated IgG preparations, we optimized the procedure for preparing the antigen in order to attain maximal reactivity: IgG was heated at 63 degrees C until the absorbance at 340 nm of a 1/10 dilution of the antigen lay between 0.65 and 0.95 absorbance units. The study of antigen stability showed a maximum aggregation two weeks after preparation. The antigen was then stable and could be used in the assay. The standard curve extends from 30 to 500 x 10(3) IU/l. Intra- and inter-assay CV were less than 5.5%. Relative analytical sensitivity and specificity were 98.3% and 95.2%, respectively. The results agreed well with those obtained by the non-improved turbidimetric procedure (r = 0.996) and with nephelometry values on the Behring nephelometer analyser (r = 0.964). The correlation with an enzyme immunoassay was also good (r = 0.923).

Quantitative automated latex nephelometric immunoassay for determination of myoglobin in human serum

We have evaluated a new latex nephelometric test for the quantitation of myoglobin in human serum. The assay consists of incubating the diluted serum sample (20-fold) for 12 min at room temperature with latex particles covalently coated with anti-myoglobin antibodies and then quantifying the change of light-scatter produced. The assay is fully automated on the Behring nephelometer analyzer with a sampling rate of 150 samples/hour. There is no interference from bilirubin (up to 340 mumol/l), haemoglobin (up to 7,000 mg/l), or rheumatoid factor (up to 1,100 int. units/ml). Myoglobin standard curve extends from 20 to 380 micrograms/l. Assay detection limit lies around 6 micrograms/l. Coefficient of variation ranged from 2.7 to 7.6%. Correlation coefficient between latex immunoassay and an RIA method was 0.987, calculated from the assay of 37 samples. A statistically significant difference was found between the distribution for females and males. The serum level of myoglobin showed an age-dependent variation. Concentrations up to 60 micrograms/l are considered to be normal.

Immunoassay by particle counting for coagulation testing: application to the determination of antithrombin III, von Willebrand factor antigen (vWF:Ag) and plasminogen

Based on immunoassay by particle counting, three methods for antithrombin III, von Willebrand factor and plasminogen were developed on an automated IMPACT machine and on a semi-automated MULTIPACT system. Precision of the techniques, measured at low, medium and high level of the calibration curve showed coefficients of variation varying from 4.3 to 13.8%. Accuracy was evaluated by dilution recovery test and by correlation with rocket immunoelectrophoresis and chromogenic substrate techniques. The results show that the proposed methods correlate well with existing techniques and that immunoassay by particle counting is applicable to several coagulation tests.

Turbidimetry of rheumatoid factor in serum with a centrifugal analyzer

We describe the simple, rapid turbidimetry of IgM rheumatoid factor in human serum by use of the Cobas-Bio centrifugal analyzer. Heat-aggregated human IgG is used as the antigen. The immunoturbidimetric reaction is monitored at 340 nm for 300 s, and the changes in absorbance after the antigen is added are used to prepare the standard curve. Test results are calculated from the stored curve and reported in int. units/mL, based on comparison with the WHO reference serum for rheumatoid factor. There is no interference from bilirubin (up to 340 mumol/L) or hemoglobin (up to 5600 mg/L). Serum samples with a triglyceride concentration greater than 2.20 mmol/L must be cleared of lipids before analysis. The standard curve is linear from 30 to 500 int. units/mL. Precision, accuracy, linearity, and sensitivity are quite acceptable. The CV was generally less than 5% for different concentrations of rheumatoid factor. Results agree well with those by a rate-nephelometric procedure on the Beckman ICS system (rs = 0.932). However, both correlate poorly with a modified classical Waaler-Rose test. Of 47 patients with rheumatoid arthritis, 34 had IgM rheumatoid factor in their serum, but the measured value did not reflect the activity of the disease.

Turbidimetry of rheumatoid factor in serum with a centrifugal analyzer

We describe the simple, rapid turbidimetry of IgM rheumatoid factor in human serum by use of the Cobas-Bio centrifugal analyzer. Heat-aggregated human IgG is used as the antigen. The immunoturbidimetric reaction is monitored at 340 nm for 300 s, and the changes in absorbance after the antigen is added are used to prepare the standard curve. Test results are calculated from the stored curve and reported in int. units/mL, based on comparison with the WHO reference serum for rheumatoid factor. There is no interference from bilirubin (up to 340 mumol/L) or hemoglobin (up to 5600 mg/L). Serum samples with a triglyceride concentration greater than 2.20 mmol/L must be cleared of lipids before analysis. The standard curve is linear from 30 to 500 int. units/mL. Precision, accuracy, linearity, and sensitivity are quite acceptable. The CV was generally less than 5% for different concentrations of rheumatoid factor. Results agree well with those by a rate-nephelometric procedure on the Beckman ICS system (rs = 0.932). However, both correlate poorly with a modified classical Waaler-Rose test. Of 47 patients with rheumatoid arthritis, 34 had IgM rheumatoid factor in their serum, but the measured value did not reflect the activity of the disease.

Rapid turbidimetric determination of serum C3c and C4 by end point centrifugal analysis

An end point turbidimetric method for the determination of C3c and C4 in serum using a centrifugal analyzer (Cobas Bio) is described. Several analytical factors were evaluated -pH, temperature, PEG and antibody concentration. Wide variations of temperature and pH did not significantly affect the turbidimetric reaction. A 20 g/L PEG concentration and 25-fold antiserum dilution were found satisfactory for the analysis. Precision of the assay was good and comparison with a RID method yielded an r value of 0.97. The procedure is simple and reliable.