Commutability of serum protein values: persisting bias among manufacturers using values assigned from the certified reference material 470 (CRM 470) in the United States

Significant among-manufacturer differences in values for serum proteins persist 7 years after the introduction of the international reference material (Certified Reference Material 470; Reference Preparation for Proteins in Human Serum). In some cases, such as transthyretin and C4, the biases actually continue to increase. Further efforts at standardization are needed in order to improve commutability of results among laboratories and are essential if universal reference intervals are to be developed.

High sensitivity immunoassays for C-reactive protein: promises and pitfalls

C-reactive protein (CRP) has historically been measured in the clinical laboratory for the detection and monitoring of occult infection and inflammation, using immunoturbidimetric or immunonephelometric techniques. The recent commercial availability of automated high-sensitivity assays has enabled investigators to measure CRP at levels previously unattainable on a routine basis and to explore its clinical utility in apparently healthy individuals. CRP concentrations increased above the individuals' baselines but still within the normal reference intervals have been observed in association with increasing age, obesity, and smoking and in individuals with chronic infections such as Chlamydia pneumoniae and Helicobacter pylori. More importantly, however, data from prospective studies have shown CRP to be a strong and independent predictor of future coronary events in subjects with and without coronary heart disease. An algorithm for risk assessment of coronary risk employing both CRP and lipid concentrations has recently been proposed. However, in order for this approach to be incorporated into clinical practice, agreement among the various CRP methods must be achieved. Of critical importance to this process is a basic understanding of issues affecting assay performance. Factors such as assay precision, sensitivity, matrix effects, calibration, and standardization need to be addressed adequately by the in vitro diagnostic industry and the clinical laboratory.

Effect of a new international reference preparation for proteins in human serum (certified reference material 470) on results of the College of American Pathologists Surveys for plasma proteins

OBJECTIVE

To examine the impact of a new international reference preparation for serum proteins on the among-manufacturer variance in the College of American Pathologists Surveys.

DATA SOURCE

The results of the Surveys for the years 1993-1998, supplied by the College of American Pathologists.

DATA EXTRACTION AND SYNTHESIS

Mean values for manufacturers' assays were compared for each protein in the quality control samples. Results were separated by the reported reference material from which values for calibrators had been transferred. Standard statistical parameters (means, standard deviations, and coefficients of variation) were calculated from the reported means.

CONCLUSIONS

Among-manufacturer coefficients of variation have dropped significantly for most serum proteins since the introduction of the new reference material. Possible reasons for continued differences are discussed.

The distribution of antinuclear antibody titers in "normal" children and adults

OBJECTIVE

To characterize the antinuclear antibody (ANA) titer distributions and patterns in normal subjects, segregated by age and sex.

METHODS

Sera were obtained from 183 blood donors (130 females, 53 males) aged 20-63 years, from 200 schoolchildren (100 females, 100 males) aged 10-19 years, and from 237 children (102 females, 135 males) aged 3 months to 9 years whose sera were received for unrelated clinical testing. ANA was assayed by indirect immunofluorescence using HEp-2 cells as substrate.

RESULTS

In adults, ANA titers were slightly higher in females than in males (p=0.053); there was no sex effect in subjects aged <20 years. ANA titer increased significantly with age only among females (p<0.01). Homogeneous staining was associated with lower titers than speckled or nucleolar staining (p=0.058), at least in part because of antigen density in the test substrate itself. The frequency of cytoskeletal staining decreased (p<0.01) with age, while that of nucleolar staining increased (p<0.01).

CONCLUSION

Reference ranges for ANA vary by age, sex, and immunofluorescence pattern. Therefore, all these variables must be considered in the interpretation of ANA results.

Reference distributions for the negative acute-phase serum proteins, albumin, transferrin and transthyretin: a practical, simple and clinically relevant approach in a large cohort

Inflammation is associated with diverse clinical conditions accompanied by characteristic changes in serum levels of the acute-phase proteins that can be used to stage the inflammatory process and evaluate the impact of treatment. Some acute-phase proteins increase during inflammation, while others, such as albumin, transferrin, and transthyretin, decrease. The current study reports reference ranges for serum levels of albumin, transferrin, and transthyretin based on a cohort of over 124,000 Caucasian individuals from northern New England, tested in our laboratory between 1986 and 1998. Measurements were standardized against CRM 470 (RPPHS) and analyzed using a previously validated statistical approach. Individuals with laboratory evidence of inflammation (C-reactive protein of 10 mg/L or higher) were excluded. The levels of all three analytes varied by age, generally rising until the second or third decade of life and then decreasing thereafter. Albumin and transthyretin levels were higher during midlife among males as compared to females; the maximum being at 25 years for albumin (5%) and 35 years for transthyretin (16%). In contrast, above the age of 10 years, transferrin levels were increasingly higher among females (7% at 20 years). When values were expressed as multiples of the age- and gender-specific median levels, the resulting distributions fitted a log-Gaussian distribution. When patient data are normalized in this manner, the distribution parameters can be used to assign a corresponding centile to an individual's measurement simplifying interpretation. The ultimate interpretation of an individual's measurement relies upon the clinical setting.

Serum triglyceride: a possible risk factor for ruptured abdominal aortic aneurysm

BACKGROUND

We aimed to determine the relationship between ruptured abdominal aortic aneurysm (AAA) and serum concentrations of lipids and apolipoproteins.

METHODS

A cohort of 21 520 men, aged 35-64 years, was recruited from men attending the British United Provident Association (BUPA) clinic in London for a routine medical examination in 1975-1982. Smoking habits, weight, height and blood pressure were recorded at entry. Lipids and apolipoproteins were measured in stored serum samples from the 30 men who subsequently died of ruptured AAA and 150 matched controls.

RESULTS

Triglyceride was strongly related to risk of ruptured AAA. In univariate analyses the risk in men on the 90th centile of the distribution relative to the risk in men on the 10th (RO10-90) was 12 (95% confidence interval [CI] : 3.8-37) for triglyceride, 5.5 (95% CI: 1.8-17) for apolipoprotein B (apoB) (the protein component of low density lipoprotein [LDL]), 0.15 (95% CI : 0.04-0.56) for apo A1 (the protein component of high density lipoprotein [HDL]), 3.7 (95% CI: 1.4-9.4) for body mass index and 3.0 (95% CI: 1.1-8.5) for systolic blood pressure. Lipoprotein (a) (Lp(a)) was not a significant risk factor (RO10-90 = 1.6, 95% CI: 0.6-3.0). In multivariate analysis triglyceride retained its strong association.

CONCLUSION

Triglyceride appears to be a strong risk factor for ruptured AAA, although further studies are required to clarify this. If this and other associations are cause and effect, then changing the distribution of risk factors in the population (by many people stopping smoking and adopting a lower saturated fat diet and by lowering blood pressure) could achieve an important reduction in mortality from ruptured AAA.

Analytical evaluation of particle-enhanced immunonephelometric assays for C-reactive protein, serum amyloid A and mannose-binding protein in human serum

Against a background of growing interest in more sensitive assays for quantifying various acute phase proteins, we evaluated the performance of recently developed tests for C-reactive protein (CRP), serum amyloid A (SAA) and mannose-binding protein (MBP) on the Behring nephelometer II (BNII). Sample results outside the calibration ranges of 3.5 to 220 mg/L for CRP, 3.3 to 215 mg/L for SAA and 0.09 to 5.6 mg/L for MBP were automatically re-measured at another dilution. The lower limits of detection were 0.01, 0.7 and 0.01 mg/L for CRP, SAA and MBP, respectively. The coefficients of variation (CV) for intra- (n > or = 20) and inter- (n > or = 15) assay precision were < 5.2% and < 8.5%, respectively, for the three proteins at concentrations representing low, normal and high. Linearity for each method was within 5% of the expected values throughout the calibration range. We observed no significant interference from bilirubin (up to 300 mg/L) or haemoglobin (up to 10 g/ L) for the three tests. Method comparison studies performed for CRP and SAA yielded the following results: y (CRP on BNII) = 0.75x (ELISA, Hemagen) -0.25 mg/L (r = 0.981, Sy/x = 2.1 mg/L; y (SAA on BNII) = 1.44x (ELISA, Hemagen) -9.9 mg/L (r = 0.972, Sy/x = 6.9 mg/L), where ELISA is enzyme-linked immunosorbent assay. Reference intervals established in 261 adult blood donors (aged 36.2 +/- 9.0 years) were found to be log-normal with 2.5th, 50th, and 97.5th centiles of < 0.17, 1.00 and 10.1 mg/L for CRP, < 0.84, 2.10 and 9.70 mg/L for SAA; and 0.30, 1.28 and 4.10 mg/L for MBP. We observed no relationship with CRP concentration and age; however, SAA levels increased with age while MBP levels decreased. The BNII provides a simple, rapid and sensitive system for measuring CRP, SAA and MBP in human serum.

Reference distributions for immunoglobulins A, G, and M: a practical, simple, and clinically relevant approach in a large cohort

Serum immunoglobulins are measured millions of times each year, yet clinical interpretations remain hampered by inadequate age- and gender-specific reference limits. In order to provide more reliable and comprehensive reference distributions for IgA, IgG, and IgM measurements, we analyzed automated immunoassay values from 115,017 serum samples from northern New England patients (99% Caucasian) who were tested in our laboratory between 1986 and 1995. Measurements were standardized to reference material, CRM 470 (RPPHS). A simple, practical, and clinically relevant approach was used to determine reference distributions for the immunoglobulins over a wide range of ages for males and females. Levels of IgA and IgM varied considerably by age, and by gender for IgM. For each of the analytes, the observed 5th and 95th centiles were symmetric about the median and approximately constant over the entire age range. When immunoglobulin reference values are expressed as multiples of the age- and gender-specific regressed medians, the resulting distributions fit a log-Gaussian distribution well. This finding enables interpretation of serum immunoglobulin measurements using a common unit (multiples of the median) that is independent of age or gender. Insights gained from this study can help improve and simplify the interpretation of immunoglobulin measurements.

New laboratory guidelines for serologic diagnosis of Lyme disease: evaluation of the two-test protocol

Recent guidelines established by the Association of State and Territorial Public Health Laboratory Directors (ASTPHLD) and the U.S. Centers for Disease Control and Prevention (CDC) recommend the use of a two-test protocol for the serologic diagnosis of Lyme disease (LD). The two-test protocol relies on a sensitive screening test, which is followed by specific immunoglobulin M (IgM) and/or IgG immunoblotting (IB), depending on the date of disease onset, of all samples with equivocal and positive screening test results. We evaluated a commercially available IgM-IgG enzyme-linked immunosorbent assay (ELISA) and separate IB tests for IgM and IgG antibodies to Borrelia burgdorferi as candidate assays for the two-test protocol. Serum samples obtained from healthy controls (n = 29), from patients with diagnoses or laboratory findings associated with serologic cross-reactivity to LD (n = 24), and from patients with well-documented early- and late-stage LD provided by the CDC and the College of American Pathologists (n = 53) were examined to determine each assay's individual sensitivity and specificity. No false-positive results were detected among the healthy controls by either ELISA or IB, whereas four false-positive ELISA results were recorded within the cross-reactive group. None of these sera, however, were positive for either IgM or IgG reactivity according to IB band criteria. With regard to the patients with LD, we determined the sensitivity and specificity of the ELISA to be 96 and 100%, respectively, compared with the reference data provided for these specimens. When we compared our IB results with data from CDC, the assay sensitivity and specificity were 80 and 96.2%, respectively, for IgM and 81.8 and 95.8%, respectively, for IgG. Pursuant to this evaluation we assessed the suitability of the two-test protocol by performing a retrospective analysis using clinical history to define samples as positive or negative for LD. We determined clinical sensitivity and specificity for all study subjects (n = 112) to be 50 and 100%, respectively. A reduction in the clinical sensitivity of the two-test protocol was associated with a lack of antibody response or seroconversion in LD patients treated with antibiotics. We conclude that the CDC-ASTPHLD guidelines provide useful criteria for test performance and interpretation aimed at standardizing the serologic diagnosis of LD.

Anti-oxidized LDL antibodies and antiphospholipid antibodies in healthy subjects: relationship with lipoprotein- and oxidation-related analytes

IgG autoantibodies against malondialdehyde-modified LDL (alpha oxLDL), antiphospholipid antibodies (APA) and oxidation- and lipoprotein-related analytes were assayed in sera from healthy subjects (51 males, 115 females, aged 22-63 years). alpha OxLDL levels were associated (P < 0.03) with IgG alpha cardiolipin (r = 0.18), IgM alpha cardiolipin (r = 0.17) and IgM alpha phosphatidyl-serine (r = 0.16) but not with age, cholesterol, triglyceride, apolipoproteins B and AI, lipoprotein(a), lipid peroxides, ceruloplasmin, copper, ferritin, transferrin or iron. APA levels were inversely associated with levels of both oxidation- and lipoprotein-related analytes. Ferritin (3.5%) and alpha oxLDL (1.4%) contributed independently to variation in IgG alpha cardiolipin levels, and apo B (2%) to variation in IgM alpha cardiolipin levels. These associations are small, indicating that there are no major biological associations between the measured variables. The lack of association between alpha oxLDL and lipoprotein- or oxidation-related analytes suggests that the relevant antigen is not in serum.

Oxidation-related analytes and lipid and lipoprotein concentrations in healthy subjects

The relations between oxidation-related analytes and lipoprotein risk factors for coronary heart disease are poorly understood. To address this issue, ceruloplasmin, copper, iron, ferritin, cotinine, lipid peroxides, cholesterol, triglyceride, apoB, apoA-I, and lipoprotein(a) levels were measured in sera from apparently healthy subjects (51 men and 115 women). Pairwise comparisons revealed strong positive associations (P < .001) of copper and ceruloplasmin with lipid peroxides, total cholesterol, triglycerides and apoB, of transferrin with apoA-I and cholesterol, and of ferritin with triglycerides. Serum levels of oxidation-related analytes did not differ between smokers and nonsmokers. In multivariate analysis, serum copper was the major independent determinant of serum lipid peroxide level, accounting for 15% of the variability in concentration (ferritin accounted for 1.6%). Copper and ceruloplasmin accounted for 20.5% of the variation in triglyceride levels; triglycerides and apoB accounted for 12% of the variability in ferritin levels; apoB and apoA-I accounted for 9% of the variability in transferrin levels. The data suggest that serum copper contributes to lipid peroxidation in vivo. There are significant associations between lipoprotein and transition metal-related analytes, and further work is needed to elucidate the physiological basis for these relations.

Apolipoproteins and ischaemic heart disease: implications for screening

Apolipoproteins and lipids are established risk factors of ischaemic heart disease (IHD) but their efficacy as screening tests is not known. We therefore examined the mortality from IHD and serum concentrations of lipids and apolipoproteins in a prospective study of 21,520 men aged 35-64 years. Serum apo B was the apolipoprotein most strongly associated with IHD risk; a decrease in apo B of 10% was associated with 22% lower risk of IHD. However, measurement of apo B alone detected only 17% of all IHD deaths at the cost of a 5% false-positive rate. Combining apo B with apo AI and apo (a) increased the detection rate to 19%. With systolic blood pressure, smoking, and family history of IHD the detection rate increased to 28%. We conclude that screening for IHD by measuring apo B alone or with apo AI and apo (a) is too poor to discriminate between recommending drug therapy or lifestyle change for some and not others. It is not advisable to screen for IHD by measuring any combination of cholesterol, apo B, apo AI, apo (a) and the other risk factors. The primary aim in prevention of ischaemic heart disease should be to lower the risk factors in the population.

Preventing manifestations of hereditary haemochromatosis through population based screening

OBJECTIVE

To evaluate the efficacy of identifying presymptomatic hereditary haemochromatosis through population based screening.

DESIGN

Review the hereditary pattern, prevalence, and clinical manifestations of haemochromatosis. Estimate the detection and false positive rates associated with available screening and diagnostic tests. Develop examples of screening protocols and other components that would be necessary for proper implementation. Identify potential barriers and objections.

CONCLUSIONS

Hereditary haemochromatosis, an autosomal recessive disorder with a prevalence of three to five per thousand in the general population, is associated with a wide variety of clinical manifestations, usually beginning in mid to late adult years. Identifying and treating this disorder after symptoms appear can arrest its progress but usually cannot reverse existing damage to joints, liver, pancreas, pituitary gland, and other organs. Measuring transferrin saturation in serum is now known to be a reliable screening test for haemochromatosis when applied to a general population of healthy adults, detecting about 80% of cases, with a 0.3% false positive rate. Liver biopsy with iron staining and total iron concentration is the recommended diagnostic test for subjects with positive screening tests. Treatment with phlebotomy can then prevent manifestations. Effective systematic identification and management of presymptomatic haemochromatosis in the general population is best accomplished within the framework of an organised screening programme. Potential barriers include accessibility of young adult populations and attitudes in the health community that severe clinical manifestations are relatively uncommon. It is recommended that pilot programmes be undertaken to determine the feasibility of introducing screening for haemochromatosis as part of routine health care.

The relationship between serum levels of lipoprotein(a) and proteins associated with the acute phase response

The association of serum lipoprotein(a) (Lp(a)) with inflammation was investigated in a primarily rheumatologic study group (n = 570; 202 males and 368 females) by studying the relationship between serum levels of Lp(a) and a panel of acute phase proteins (C-reactive protein (CRP), alpha 1-antitrypsin (AAT), alpha 1-acid glycoprotein (AGP), haptoglobin (HPT), complement components 3 and 4 (C3, C4), prealbumin (PAL), albumin (ALB) and transferrin (TRF)). Lp(a) data were adjusted for age and sex, but not clinical condition as no significant differences in Lp(a) levels were observed, using analysis of variance, among the 15 diagnostic categories in the study group. Univariate analyses revealed significant positive associations between Lp(a) levels and levels of C4, AGP, C3 and HPT. Multivariate analysis revealed that C4 and AGP (in descending order of significance) were significant independent predictors of Lp(a) concentration, together accounting for 2.9% of the variability in Lp(a) concentration in the present study group. The data indicate that confounding effects of an acute phase response should be considered in epidemiologic studies, if a high prevalence of inflammation is suspected.

Apolipoprotein (a): a comparison of isoforms identified by sodium dodecyl sulfate-polyacrylamide gel electrophoresis or by sodium dodecyl sulfate-agarose gel electrophoresis

Lipoprotein(a) resembles low density lipoprotein in structure, except that a unique apolipoprotein (apo), apo(a), is linked to apo B-100. Variations in the number of sequence repeats in the apo(a) gene give rise to a range of isoforms. Depending on the method used, 6-30 apo(a) isoforms have been observed; however, the correspondence of these different isoforms has not been reported, making between-study comparisons difficult. In the present study we address this question by characterizing the apo(a) phenotypes of 48 sera using two previously reported separation methods, sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE, 3-12% gels) and SDS-agarose gel electrophoresis. In addition, the molecular weight of each isoform was estimated using haptoglobin 2-2 polymers as molecular weight standards. Among the 48 sera, 15 distinct apo(a) isoforms were separated by SDS-PAGE and 28 by SDS-agarose gel electrophoresis. There was excellent correlation between the two nomenclature systems (r = -0.97, p < 0.001, by rank correlation), and the ranges were totally overlapping, with the same two isoforms being identified as the largest and smallest by either method. The apparent molecular mass range for the isoforms was 294-624 kDa, which is in close agreement with the theoretical molecular mass range of 238-643 kDa, calculated from the sequence and carbohydrate content of recombinant apo(a). The disparity in number of isoforms between methods was expected, due to the poorer separation of apo(a) by SDS-PAGE; 3.1 +/- 1.7 (median, 2.0) SDS-agarose isoforms were combined for each SDS-PAGE isoform.(ABSTRACT TRUNCATED AT 250 WORDS)

Background staining in immunoblot assays. Reduction of signal caused by cross-reactivity with blocking agents

Certain serum samples produce high background in Western and direct immunoblot assays that detect human serum IgG against specific antigens. We determined that this was due to a reaction between endogenous IgG and the membrane blocking agent (we refer to this as blocking-specific background). Using milk as blocking agent, we screened 107 sera by Western immunoblot or checkerboard immunoblot assays, and found that 6.5% of sera had background intensities sufficient to interfere with the interpretation of final results. Blocking-specific background was also observed using bovine serum albumin and other animal protein-based blocking agents. As the primary antibody in these immunoblot assays was human IgG, we investigated human serum albumin as a blocking agent; this approach eliminated the problem of blocking-specific background.

Application of checkerboard immunoblotting (CBIB) to the detection of anti-viral IgG in human serum

In the present study, we have begun to investigate the possibility of using checkerboard immunoblotting (CBIB) as a semi-quantitative screening tool for detecting human serum IgG against specific viral antigens. The viral antigens studied were Epstein-Barr, herpes simplex I and II, cytomegalovirus, varicella zoster, rubella, rubeola, and mumps. Western immunoblotting experiments using these partially purified preparations demonstrated that there were apparently no interactions between IgG from non-immune sera and the respective viral antigen preparations. The CBIB assay was evaluated using sera of known positive or negative immune status for the viral antigens. There was excellent agreement between the results of CBIB and the results of alternative methods for evaluating immune status: all discrepancies (1/18 sera for mumps, 3/18 sera for rubeola, and 1/28 sera for rubella) involved sera with borderline results, either by CBIB or by the alternative method. Therefore, although further work is required to define the method in terms of sensitivity and clinical specificity, and to refine positive/negative cutpoint criteria for certain antigen components, our preliminary experience suggests that CBIB has considerable potential in the efficient and inexpensive screening of sera for the presence of IgG against a panel of viral antigens, so as to identify subjects at risk for infection.

Effect of Sample Storage on the Assay of Lipoprotein(a) by Commercially Available Radial Immunodiffusion and Enzyme-Linked Immunosorbent Assay Kits

Lipoprotein(a) [Lp(a)] was measured by both a radial immunodiffusion (RID) kit from Immuno AG (Zurich, Switzerland) and a Tint Elize enzyme-linked immunosorbent assay (ELISA) kit from CytRx Biopool Ltd. (Umeå, Sweden) in serum samples that had been stored at -20 and -70 degrees C for six months. Storage temperature had no significant effect on the Lp(a) concentrations obtained by either method. After six months, mean Lp(a) degradation was 46% (95% confidence interval, 34-58%) with the RID kit; the ELISA data could not be compared between time points. In fresh sera, Lp(a) concentrations obtained by RID were 41% higher than by ELISA (because of differences in assay calibration materials), but in paired measurements of a set of 215 samples stored at -40 degrees C for an average of 10 years, Lp(a) concentrations were 62% lower by RID. This suggests that RID is more sensitive to the effects of long-term storage than is ELISA.

Effect of sample storage on the assay of lipoprotein(a) by commercially available radial immunodiffusion and enzyme-linked immunosorbent assay kits

Lipoprotein(a) [Lp(a)] was measured by both a radial immunodiffusion (RID) kit from Immuno AG (Zurich, Switzerland) and a Tint Elize enzyme-linked immunosorbent assay (ELISA) kit from CytRx Biopool Ltd. (Umeå, Sweden) in serum samples that had been stored at -20 and -70 degrees C for six months. Storage temperature had no significant effect on the Lp(a) concentrations obtained by either method. After six months, mean Lp(a) degradation was 46% (95% confidence interval, 34-58%) with the RID kit; the ELISA data could not be compared between time points. In fresh sera, Lp(a) concentrations obtained by RID were 41% higher than by ELISA (because of differences in assay calibration materials), but in paired measurements of a set of 215 samples stored at -40 degrees C for an average of 10 years, Lp(a) concentrations were 62% lower by RID. This suggests that RID is more sensitive to the effects of long-term storage than is ELISA.

A flexible, efficient, checkerboard immunoblot system for the detection and semiquantitation of specific antinuclear antibodies

In this paper an uncomplicated method for the simultaneous detection and semiquantitation of 11 of the 12 commonly studied antinuclear antibodies (ANA) in a single run is described. This new application of checkerboard immunoblotting (CBIB) is based upon available technology and employs purified antigens which can be either purchased or produced in-house. CBIB requires no electronic instrument, can be formatted to meet the needs of the user, is rapidly performed, and has acceptable labor and materials costs. Data on the use of the method to examine available reference antisera is presented. CBIB has also proven practical for the clinical study of 18 sera, at two dilutions per membrane, for each set of specific antinuclear antibodies, also at two or more dilutions.

International Federation of Clinical Chemistry standardization project for measurements of apolipoproteins A-I and B

To minimize differences in apolipoprotein measurements among laboratories and methods, a standardization program involving common suitable reference material is needed. The Committee on Apolipoproteins of the International Federation of Clinical Chemistry initiated a collaborative study for the standardization of test systems for measuring apolipoproteins (apo) A-I and B, with 25 company laboratories and three research laboratories involved in apolipoprotein analysis to: (a) evaluate calibration differences among the test systems; (b) evaluate whether comparability of the data can be achieved with the use of frozen serum pools to recalibrate the different systems; and (c) evaluate and select suitable candidate reference material. We used 26 test systems for apo A-I and 28 for apo B. Relatively modest differences were found in calibration for apo A-I, but very wide differences were observed for apo B methods. After uniform calibration, the overall among-laboratory CV for apo B decreased from 19% to 6%. Three lyophilized serum preparations for apo A-I and three liquid-stabilized serum preparations for apo B were selected for further evaluation as candidate international reference materials.

International Federation of Clinical Chemistry standardization project for measurements of apolipoproteins A-I and B

To minimize differences in apolipoprotein measurements among laboratories and methods, a standardization program involving common suitable reference material is needed. The Committee on Apolipoproteins of the International Federation of Clinical Chemistry initiated a collaborative study for the standardization of test systems for measuring apolipoproteins (apo) A-I and B, with 25 company laboratories and three research laboratories involved in apolipoprotein analysis to: (a) evaluate calibration differences among the test systems; (b) evaluate whether comparability of the data can be achieved with the use of frozen serum pools to recalibrate the different systems; and (c) evaluate and select suitable candidate reference material. We used 26 test systems for apo A-I and 28 for apo B. Relatively modest differences were found in calibration for apo A-I, but very wide differences were observed for apo B methods. After uniform calibration, the overall among-laboratory CV for apo B decreased from 19% to 6%. Three lyophilized serum preparations for apo A-I and three liquid-stabilized serum preparations for apo B were selected for further evaluation as candidate international reference materials.