Measurement Of Rheumatoid Factor Research Articles

Prevalence of Diabetic Hyperglycemia Among Patients With Rheumatoid Arthritis

Introduction: Rheumatoid arthritis (RA) is one of the most prevalent systemic inflammatory diseases worldwide. Cardiac complications present the most common mortality cause among RA patients. One of the most important comorbid conditions with RA is diabetic hyperglycemia mainly type 2 diabetes mellitus (T2DM). Aim of the study: The present study was conducted to assess prevalence of T2DM among patients diagnosed with RA from Iraq. Methodology: We included a randomly selected 100 rheumatoid arthritis. All included patients were subjected to anthropometric measurements, diabetic profile assessment and ESR, CRP and rheumatoid factor measurement. Results: Among the included RA patients, 28 patients were diagnosed with new-onset DM. Our results showed that RA female patients, having obesity, HTN or hyperlipidemia exhibited higher risk for diabetic hyperglycemia. Conclusion: This current study revealed a statistically significant association between diabetic hyperglycemia incidence and comorbid RA.

Testing for isotypes does not help differentiating rheumatoid arthritis from other rheumatoid factor positive diseases.

Rheumatoid factors (RFs) are useful for diagnosis and classification of rheumatoid arthritis (RA). Nephelometric and turbidimetric techniques, which detect total RF but do not reveal the antibody isotype, are common diagnostic methods in clinical routine. Given the recent development of isotype-specific immunoassays, the detection of IgG, IgM, and IgA RFs represents an interesting challenge. The aim of the study was to evaluate whether specific RF tests performed as a second step after traditional nephelometry could help differentiating RA from other RF-positive diseases. We tested 117 consecutive serum samples that were RF-positive at nephelometry (BNII nephelometric analyzer, Siemens) for IgA, IgG, and IgM RF isotypes by a fluoroimmunoenzymatic assay (FEIA) on the Phadia 250 instrument (ThermoFisher). Fifty-five subjects had RA and 62 presented non-RA diagnoses. Eighteen sera (15.4%) were positive only by nephelometry, two were positive only for IgA RF, and the remaining 97 sera were all positive for IgM RF isotype (with or without IgG and IgA RF). Positive findings did not correlate with RA or non-RA diagnosis. Spearman rho correlation coefficient between nephelometric total RF and IgM isotype was moderate (0.657), and weak between total RF and IgA (0.396) and IgG (0.360) isotypes. Despite its low specificity, measurement of total RF by nephelometry still seems to be the method that performs best. As IgM, IgA, and IgG RF isotypes showed only a moderate correlation with total RF measurement, their diagnostic use as a second level test remains controversial.

Impact of autoimmune serology test results on RA classification and diagnosis

Rheumatoid arthritis (RA) is the most common systemic autoimmune disease and also the most severe arthritic disorder. The measurement of rheumatoid factor (RF) and anti-citrullinated protein antibodies (ACPA) in serum supports the diagnosis of RA, which gained increasing significance over the last 65 years. However, a high variability between RF and ACPA methods has been described, impacting the diagnostic performance of the current ACR/EULAR RA classification criteria.The great number of commercially available assays, often lacking traceability to an international standard, is a major factor attributing to this in-between assay variability. The adoption of an international standard for ACPA, as is since long available for rheumatoid factor, is therefore highly desirable.Further harmonization in clinical interpretation of RF/ACPA assays could be obtained by harmonization of the cut-offs, for both the low and high antibody levels, based on predefined specificity in disease controls. Reporting test result specific likelihood ratios (LR) adds value in the interpretation of autoantibody tests. However, a good understanding of the control population used to define antibody test result interval-associated LRs is crucial in defining the diagnostic performance characteristics of antibody serology.Finally, specificity in RA classification can be improved by refining serological weight scoring taking into account the nature of the antibody, the antibody level and double RF + ACPA positivity.

Rheumatoid factor as predictor of response to treatment with anti-TNF alpha drugs in patients with rheumatoid arthritis

We determined whether rheumatoid factor (RF) and anti-cyclic citrullinated peptide antibody (ACPA) can predict remission or severe disability in rheumatoid arthritis (RA) patients treated with anti-tumor necrosis factor (TNF) alpha drugs.We performed a cohort study based on the clinical data from a referral center for the treatment of RA in Bogotá, Colombia, were included patients aged ≥18 years with diagnosis of RA with an active disease and for whom a treatment scheme was begun with anti-TNF alpha medication, with a minimum follow-up time of 12 months. Disease activity of Rheumatoid Arthritis was assessed through measurement of RF, ACPA, disease activity score (DAS28), and health assessment questionnaire (HAQ). We calculated the incidence rates (IRs) for remission and severe disability. We also calculated the incidence rate ratio (IRR) for each outcome by adjusting for possible confounders using the Poisson regression method. The hypothesis was tested with a P value of <.05. Statistical analysis was performed in Stata 15.We included 400 patients receiving an anti-TNF alpha agent. Median age was 60 years, and 322 patients were women (80.5%). RF was positive in 357 patients (89%), ACPA in 348 patients (87%), and co-positivity in 324 patients (81%). Median follow-up was 41 months (range, 12–79 months). The IR for remission was 23 per 100 person-years in RF-negative patients and 16 per 100 person-years in RF-positive patients. The adjusted IRR (age sex, treatment, and ACPA) was 1.51 (95%CI, 1.05–2.18). The IR for severe disability was 10.8 per 100 person-years in the RF-positive cohort and 2.3 per 100 person-years in the RF-negative cohort. The IRR adjusted for these factors was 4.37 (95%CI, 1.6–12). Co-positivity had a similar behavior to RF. No differences were recorded in the rates of remission or disability in ACPA-positive and ACPA-negative patients.Our findings suggest that remission is less frequent and severe disability more frequent in RF-positive patients treated with anti-TNF alpha agents than in RF-negative patients.

リウマトイド因子測定試薬「オートRF・BML」の基礎的検討

リウマトイド因子測定試薬「オートRF・BML」の基礎的検討

Predictive Value of Autoantibody Testing for Validating Self-reported Diagnoses of Rheumatoid Arthritis in the Women's Health Initiative

Rheumatoid arthritis (RA) research using large databases is limited by insufficient case validity. Of 161,808 postmenopausal women in the Women's Health Initiative, 15,691 (10.2%) reported having RA, far higher than the expected 1% population prevalence. Since chart review for confirmation of an RA diagnosis is impractical in large cohort studies, the current study (2009-2011) tested the ability of baseline serum measurements of rheumatoid factor and anti-cyclic citrullinated peptide antibodies, second-generation assay (anti-CCP2), to identify physician-validated RA among the chart-review study participants with self-reported RA (n = 286). Anti-CCP2 positivity had the highest positive predictive value (PPV) (80.0%), and rheumatoid factor positivity the lowest (44.6%). Together, use of disease-modifying antirheumatic drugs and anti-CCP2 positivity increased PPV to 100% but excluded all seronegative cases (approximately 15% of all RA cases). Case definitions inclusive of seronegative cases had PPVs between 59.6% and 63.6%. False-negative results were minimized in these test definitions, as evidenced by negative predictive values of approximately 90%. Serological measurements, particularly measurement of anti-CCP2, improved the test characteristics of RA case definitions in the Women's Health Initiative.

Autoanticorpos na artrite reumatoide inicial: coorte Brasília - resultados de uma análise seriada de três anos

The diagnostic and prognostic value of the serial measurement of antibodies, such as rheumatoid factor (RF), anti-cyclic citrullinated peptide (anti-CCP), and anti-citrullinated vimentin (anti-Sa) antibodies, has not been defined in early rheumatoid arthritis (ERA). To prospectively assess the presence of RF, anti-CCP, and anti-Sa in ERA patients. Forty ERA (less than 12 months) patients of the Brasília cohort were evaluated and followed up for three years. Both clinical and demographic data were recorded, in addition to the results (ELISA) of RF (IgM, IgG, and IgA), anti-CCP (CCP2, CCP3, and CCP3.1), and anti-Sa at the baseline assessment and after 3, 6, 12, 18, 24 and 36 months of follow-up. The results were compared by use of Student t test and paired t test. The patients' mean age was 45 years, and 90% of them were female. At the time of diagnosis, RF was identified in 50% of the patients (RF IgA, 42%; RF IgG, 30%; and RF IgM, 50%), anti-CCP in 52.5% (no difference between CCP2, CCP3, and CCP3.1), and anti-Sa in 10%. After three years, no difference was observed in RF and anti-CCP prevalence, but anti-Sa increased to 17.5% (P = 0.001). Repeated RF and anti-CCP measurement, including different isotypes, during three years of follow-up showed no significant changes. The third generation of anti-CCP assays did not increase the diagnostic value of the second-generation assays.

Should Anti-citrullinated Protein Antibody and Rheumatoid Factor Status Be Reassessed During the First Year of Followup in Recent-Onset Arthritis? A Longitudinal Study

Presence and levels of antibodies contribute to the classification of rheumatoid arthritis. We investigated the longitudinal course of anti-citrullinated protein antibodies (ACPA) and immunoglobin M (IgM) rheumatoid factor (RF) during the first year after arthritis onset in patients with very short disease duration. Patients (aged 18-75 years) with ≥ 1 swollen joint of ≤ 16 weeks' duration had assessments of ACPA (2nd generation anti-cyclic citrullinated peptide antibodies, anti-CCP2) and IgM RF at inclusion and after 3, 6, and 12 months. Frequencies of seroconversions (negative to positive and vice versa) and changes in antibody levels during followup were determined. A total of 281 early arthritis patients (median duration of joint swelling 32 days, 14.2% ACPA positives, 12.8% IgM RF positives) with 978 longitudinally collected serum samples were included. Only 5 patients (1.8%) negative for both antibodies at baseline turned antibody-positive during followup, while 9 antibody-positive patients (3.2%) turned antibody-negative. ACPA was more stable than RF regarding both status and levels. Antibody status (ACPA/RF) is a stable phenotype in very early arthritis, as seroconversion was only found in 5% of patients. Repeated measurement of ACPA or RF during the first year after onset of arthritis does not offer major additional information.

Rheumatoid factor isotypes in patients with erosive osteoarthritis of the hand

Dear Editor, Erosive osteoarthritis (EOA) is generally considered an uncommon subset of osteoarthritis (OA), targeting interphalangeal (IP) joints and characterized by an abrupt onset, marked pain, functional impairment, inflammatory symptoms and signs, including stiffness, soft tissue swelling, erythema, paraesthesiae and a worse clinical outcome than non-erosive OA of the hand (HOA).1 Laboratory tests are usually negative, despite clinical findings of inflammation, even though a slight increase of erythrocyte sedimentation rate (ESR) and high sensitive C-reactive protein (hsCRP), although at low degree, are noticeable.2 Furthermore, in a previous study, we showed the absence of anti-cyclic citrullinated peptide antibodies (anti-CCP), in blood samples of a group of EOA patients.3 Erosive osteoarthritis is defined radiographically by subchondral erosion, cortical destruction and subsequent reparative change, which may include bony ankylosis.1,4 Moreover, in the early stages of this disease, the differential diagnosis between EOA and other arthritis types, such as rheumatoid arthritis (RA) or psoriatic arthritis (PsA), may pose a challenge.5,6 Rheumatoid factor (RF) is a family of autoantibodies that recognizes the Fc part of IgG molecules and exists as IgA, IgG and IgM isotypes.7 The present study is aimed at detecting for the first time the prevalence of sera IgA, IgG and IgM RF in patients affected by EOA, compared to patients with nodal osteoarthritis (NOA), RA and healthy subjects. HOA diagnosis was made according to the American College of Rheumatology (ACR) criteria for HOA.8 EOA and NOA were defined by European League Against Rheumatism (EULAR) recommendations.1 RA is defined by the ACR criteria.9 Thirty-two consecutive patients with EOA (five men and 27 women, median age 59.2 years, range 49–75), attending the outpatient Rheumatology Unit of Siena Hospital, were enrolled in the study. Each patient showed the common EOA radiographic findings.1,4 The control group included 30 patients affected by NOA,1 32 RA patients and 30 healthy subjects, sex- and age-matched. For all the studied subjects, co-morbidities such as infections, cancer, renal and liver diseases, cryoglobulinemia, scleroderma, Sjogren syndrome, autoimmune thyroiditis, PsA, undifferentiated spondyloarthropathies, gout and pseudogout, were excluded. After informed consent was obtained, serum samples were stored at −20°C until tested to anti-CCP antibodies and IgA, IgG and IgM RF. Anti-CCP antibodies were tested by FEIA technique, by using EliA system (Phadia Diagnostics, Freiburg, Germany); IgA, IgG and IgM RF were assessed using enzyme-linked immunosorbent assay (ELISA) commercial kits (Orgentec Diagnostica, Mainz, Germany). We considered positive every IgA, IgG and IgM RF > 20 U/mL. All EOA patients, NOA patients and healthy controls were negative for anti-CCP, IgA-RF, IgG-RF and IgM-RF. In the RA group, 77% of the patients were found to be positive for anti-CCP, 35% for IgM and IgG RF and 52% for IgA RF. Although a relationship between EOA and RA was first suggested by Ehrlich,10 there have been few studies concerning the serological tests for RF in EOA. In our study the detection and quantitative measurement of RF in various immunoglobulin classes (IgA, IgG and IgM) was performed using ELISA, for its specificity and sensitivity. Using this technique, in the literature we found that the majority (up to 70–80%) of RA patients presented an increase of two or three RF isotypes.11 Our study suggests that the combined detection of the three isotypes of RF and of anti-CCP antibodies may be helpful in the differential diagnosis between EOA and RA, especially in equivocal cases. The absence of the three isotypes of RF and of anti-CCP antibodies in the serum of the EOA patients shows that EOA represents a subtype of HOA and not a specific inflammatory disease.

Rheumatoid factor measurement—continuing problems 70 years after discovery

There have been long-standing unpublished differences in rheumatoid factor (RF) results, based on the Royal College of Pathologists of Australasia external quality assurance program. This study compared RF results between two commonly used commercial instruments. Serum samples were exchanged between a laboratory using a Beckman Immage immunonephelometer and another using a Roche Modular immunoturbidimeter. The World Health Organization (WHO) reference standard for RF (W1066) was then used to compare the two methods. The Roche immunoturbidimeter appears to have superior sensitivity. Furthermore, there is significant bias between the two instruments as levels of RF increase. Results from the WHO RF reference preparation correlated most closely with the results from the Roche immunoturbidimeter. Standardization for RF measurement is not optimal and has not been achieved between these two commonly used instruments. This may have clinical implications for patient management.

Pericarditis

Pericarditis

The Combined Measurement of Total Serum IgG and IgG4 May Increase Diagnostic Sensitivity for Autoimmune Pancreatitis Without Sacrificing Specificity, Compared With IgG4 Alone

Until now, there was no international consensus on the diagnostic criteria for autoimmune pancreatitis (AIP). As for serologic criteria, the HISORt criteria use elevated immunoglobulin (Ig) G4 alone, whereas the Asian diagnostic criteria include elevations of total IgG or IgG4, or the presence of autoantibodies. This study was mainly aimed at determining whether the combined measurement of total IgG and IgG4 could increase the diagnostic sensitivity for AIP while maintaining specificity, compared with IgG4 alone. Another aim was to determine the utility of autoantibodies to diagnose AIP. We prospectively measured total serum IgG and IgG4 together in 82 consecutive patients with AIP, and seropositivity was defined as elevation of either total IgG or IgG4. To evaluate specificity in the differentiation of AIP from pancreatic cancer, total serum IgG and IgG4 were prospectively measured in 110 patients with pancreatic cancer. Also, the detection rates of antinuclear antibody (ANA) and rheumatoid factor (RF) were retrospectively reviewed in patients with AIP. In patients with AIP, the sensitivity of IgG4 (> or = 135 mg/dl) was 52.5% (43/82), significantly higher than that (46.3%, 38/82) of total IgG (> or = 1,800 mg/dl) (P<0.05). The sensitivity of combined measurement of total IgG and IgG4 for AIP was 68.3% (56/82), significantly higher than that of IgG4 alone (P<0.05). The specificity of total IgG and IgG4 in the differentiation of AIP from pancreatic cancer was 96.4 and 99.1%, respectively. The specificity of combined measurement of total IgG and IgG4 was 95.5%, and it was not significantly different from that of IgG4 alone (P=0.125). The sensitivity of ANA (> or = 1:80) and RF was 24.4% (19/78) and 20.3% (13/64), respectively. All but one patient who had positive results for ANA or RF also showed elevations of either total IgG or IgG4, respectively. The combined measurement of total serum IgG and IgG4 may increase diagnostic sensitivity without sacrificing specificity, compared with IgG4 alone. However, the measurement of ANA or RF may show no additional benefit when combined with total serum IgG and IgG4.

Rheumatology

Rheumatology

Genome-wide single-nucleotide polymorphism linkage analyses of quantitative rheumatoid arthritis phenotypes in Caucasian NARAC families

We applied nonparametric quantitative trait linkage analysis to two rheumatoid arthritis quantitative phenotypes, IgM rheumatoid factor (RF) and anti-cyclic citrullinated peptide autoantibody titer measurements, using 5700 genome-wide Illumina single-nucleotide polymorphism genotypes on 658 Caucasian North American Rheumatoid Arthritis Consortium families. Peak LOD scores for both quantitative traits were located in the human leukocyte antigen region 6p21 (15.8 and 13.8 for RF and anti-cyclic citrullinated peptide, respectively) followed by 11p12 (3.2 and 3.6). In addition, there were LOD scores of 3.2 on 2q32 for RF and 3.6 on 4q24 for anti-cyclic citrullinated peptide. The resulting linkage signals for both phenotypes are very similar to previous results for rheumatoid arthritis as a qualitative variable, with rheumatoid factor measurements being most closely aligned. Interestingly, anti-cyclic citrullinated peptide exhibits a stronger linkage peak on 2p14 than rheumatoid factor and rheumatoid arthritis, and stronger linkage on 4q24. Finally, we used ordered subset analyses to determine if sub-ranges of these two traits increased rheumatoid arthritis linkage signals; however, our analyses did not reveal significant effects of the quantitative traits on rheumatoid arthritis linkage signals in this population.

IgM, IgG, and IgA rheumatoid factors in pigeon hypersensitivity pneumonitis

The association of rheumatoid factor (RF) and lung disease in several immunologically mediated conditions has suggested that it may be physiopathologically relevant. Since previous reports in hypersensitivity pneumonitis (HP) have dealt mainly with the immunoglobulin M (IgM) RF measurement, we studied such antibody activity in other immunoglobulins, to determine the IgG and IgA RF levels in pigeon-HP, and in asymptomatic breeders (AB) and rheumatoid arthritis (RA) as controls. RFs were measured in 35 HP patients, 41 AB, 31 RA controls, and 55 healthy donors by enzyme-linked immunosorbent assay (ELISA) using human or rabbit immunoglobulin G (IgG), anti-IgM, F(ab')2 of IgG, and IgA F(ab')2 conjugates. An affinity chromatography, fragment crystallizable (Fc) preparations of IgG, pepsin digestion, and Western blots were used to confirm RF specificity. We also evaluated anti-avian antibodies (AA) and cross-reacting antibodies. The HP group revealed positive IgM (51.4%), IgG (31.4%), and immunoglobulin A (IgA) (34.2%) RF tests, and these antibody values exceeded the AB reference levels (P<0.02). HP and RA showed a similar frequency and distribution of RFs. Possible immunoassay interferences were excluded. As in other immunologically mediated diseases, IgG and IgA RFs may play a pathogenic role in HP, amplifying the inflammatory reaction, immune-complex formation, and complement activation. IgM-RF producing cells that have been implicated in the presentation of self and foreign antigens, and T-cell activation might induce the isotype switching of RFs.

Humoral immune system role in the pathogenesis of vitiligo

Background and purpose Vitiligo is a common acquired skin disease that is due to the absence or destruction of melanocytes. The role of humoral immune system including presence of anti-melanocyte antibody (AMA), rheumatoid factor (RF), complement 3 and 4 (C3 and C4) and antinuclear antibody (ANA) were assessed to determine the etiology of this recently mentioned disease. Methods This was a study on serum values of 55 patients with vitiligo and 60 healthy person as control. There were performed by measuring the AMA values with indirect immunoflurescent test and C3 and C4 with single radial immuno-diffusion (SRID) plus that determination and measurement of RF was also performed with latex and enzyme-linked immunoabsorbant assay (ELISA). Results ANA test was positive in 4 (7.3%) patients and in 1 (1.7%) healthy control. In addition to that, there was no reasonable relationship recognized between ANA and the disease. C3 and C4 values already decreased in 14 (25.5%) patients (P<0.005) that there was reasonable relationship between the decreased level of C3 and C4 and vitiligo. RF was positive in 6 (10.8%) patients performed with ELISA whereas it was negative in all of the control group, but it was negative in both of them (the patient and control groups) with latex test. AMA was positive in 17 (30.9%) patients and was negative in all of the control group and there was also a relationship detected between the presence of AMA and vitiligo (P<0.001). Conclusion These results show that the important role of the humoral immune system in pathogenesis of vitiligo and it seems that this is an autoimmune process that needs several more studies to show the role of the other immune factors in pathogenesis of vitiligo.

Matrix effects in clinical immunoassays and the effect of preheating and cooling analytical samples.

Immunological reactions are influenced by various factors including antigens, antibodies and other variables. We focused on two items: i) matrix effects, especially of detergents and ii) temperature effects: preheating sera, especially effects on rheumatoid factor (RF) measurement and false-positive reactions in ELISAs, and cold storage of sera, especially effects on complement. Among various additives, detergents affected the agglutination reaction for fecal hemoglobin and hepatitis B surface (HBs) antigen. Some of the detergents examined abolished these antigenicities, however, polyethylenglycols enhanced the reactions. Heat-inactivation of sera at 56 degrees C for 30 min was employed in serological testing. However, in RF measurement, 10 min of preheating was sufficient to abolish C1q (subcomponent of C1), which could participate in the agglutination reaction. In ELISA for antibodies, false-positive reactions were caused by preheating sera. By the analyses of assays for antibodies to hepatitis C virus (HCV) and cardiolipin, it was found that they were induced by immunoglobulin G (IgG) modified by preheating. Cold storage induced activation of complement (cold activation) in anti-HCV antibody positive sera. CH50 titers in the sera were lowered by one cycle of freezing at -20 degrees C and thawing, and the decrease was affected by the containers.

A quantitative ELISA for measurement of rheumatoid factor associated cross-reactive idiotypes in serum from patients with rheumatic diseases.

A sensitive ELISA for the quantitation of RF associated cross-reactive idiotypes (CRI) has been developed. CRI bearing IgM was captured by immobilized F(ab')2 fragments of monoclonal anti-CRI antibodies and revealed with isotype specific conjugates. Factors affecting sensitivity, specificity and reproducibility of the assay were assessed. The sensitivity limit was less than 10 ng/ml and the linear assay range for the CRI standard curve was from 80 to 500 ng/ml. Using this ELISA system we have quantitated serum levels of the CRI in different groups of patients with RA, early synovitis and normal individuals. Our results indicate a significant increase in the levels of the CRI in the patient groups compared to the normal subjects tested, and suggest involvement of a diverse repertoire of Ig variable region germline genes and/or somatic mutation in production of polyclonal RF in RA.

Contribution of inherited factors to rheumatoid arthritis.

A total of 231 sibships of the same sex (186 female, 45 male), in which the proband had classical or definite rheumatoid arthritis (RA) have been selected from rheumatology clinics. Each sibship member was questioned about symptomatic joints, which were then examined. Hospital records, radiographs, and rheumatoid factor measurements allowed each sibling to be classified as having classical, definite, probable, or no RA. Each sibling was typed for HLA-A and B and was classified as sharing two, one, or zero HLA haplotypes with the proband. Concordance rates for classical and definite RA were three times greater in sibships of women than of men (9.3 v 3.0%). Concordance rates in HLA identical sibships were twice those in hemi- and non-identical sibships (15.5, 7.1, and 5.2%, respectively). Probable RA was more common in male and HLA hemi- and non-identical sibships. These results suggest that female sex and the two inherited HLA haplotypes are important for the presence and expression of RA. Although environmental factors may be shared more in twins than siblings, a concordance rate of 20.5% in seropositive HLA identical sibships of the same sex compared with 30% in monozygotic twins suggests that sex and HLA type account for about two thirds of the inherited risk of RA.

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).