Immunoassays for the detection of IgA antibodies to tissue transglutaminase: significance of multiples of the upper limit of normal and inter-assay correlations.

BACKGROUND: The presence of IgA antibodies to tissue transglutaminase (anti-tTg) is associated with variable risk for celiac disease. The use of common multiples of the upper limit of normal (ULN) has been suggested to optimize diagnostic pathways as well as improve harmonization between assays. METHODS: The characteristics of four anti-tTG IgA assays relative to endomysial IgA (EMA) by indirect immunofluorescence assay (IFA) as reference test were assessed. Commutability between anti-tTG immunoassays and/or EMA based on manufacturer's recommended cut-off values and three common multiples of ULN (3×, 5× and 10×) was also investigated. Sera from 200 patients and 100 healthy individuals were analyzed. RESULTS: At manufacturer's cut-off; the sensitivities for the tTG assays ranged from 72.5% to 98.6% and specificities from 60.3% to 99.2%. The percent positive agreements between any anti-tTG and EMA or any two anti-tTG immunoassays varied from 56.7% to 98.0% and 46.7% to 100.0%, respectively. At 3×, 5× or 10× ULNs, the inter-rater reliability as measured by Cohen κ between any two anti-tTG assays were quite variable and ranged from 0.28 to 0.96, 0.26 to 0.89 or 0.13 to 0.78, respectively. Furthermore, the percent positive agreements between any two anti-tTg IgA immunoassays ranged from 83.1% to 98.2%, 92.0% to 100%, or 100%, at 3×, 5× or 10×, respectively. CONCLUSIONS: Commutability between tTG IgA immunoassays or tTG IgA and EMA is kit-dependent and common multiples of the ULN are not sufficient to correct for inter-assay variations. Many factors influence the performance of anti-tTG IgA assays which limit their commutability.

Comparative analysis of neutrophil gelatinase-associated lipocalin and other laboratory markers for lupus nephritis: a cross-sectional investigation.

Background Lupus nephritis is one of the most serious complications of systemic lupus erythematosus. This study evaluates the prevalence and correlation between neutrophil gelatinase-associated lipocalin and other biomarkers associated with renal involvement in systemic lupus erythematosus. Methods Paired serum and urine specimens from 50 suspected systemic lupus erythematosus patients, characterized by antinuclear antibodies detected by indirect immunofluorescence assay and varying positive concentrations of anti-double stranded DNA antibodies by Crithidia luciliae immunofluorescence assay, were investigated. Of these 50 patients, 18 were identified with renal involvement based upon laboratory serology. Patients and healthy control serum samples ( n = 50) were also evaluated for high avidity double stranded DNA IgG antibodies, anti-C1q IgG antibodies, and serum creatinine. The prevalence and relationship between biomarkers were evaluated using statistical methods. Results Serum and urine neutrophil gelatinase-associated lipocalin concentrations were significantly elevated in patients compared to controls, with a prevalence of 24% and 36%, respectively. These concentrations were also more markedly increased in systemic lupus erythematosus patients with renal involvement than those without. Spearman's correlations between neutrophil gelatinase-associated lipocalin and other biomarkers tested ranged from 0.06 to 0.66 in all patients. Combined concordance as determined by Cronbach alpha coefficient between biomarkers was reduced from 0.71 to 0.58 (serum) and 0.62 (urine) when neutrophil gelatinase-associated lipocalin was removed. Conclusions Neutrophil gelatinase-associated lipocalin concentrations are elevated and demonstrate variable associated with other laboratory markers for renal involvement in systemic lupus erythematosus. Prospective longitudinal studies are needed to determine the optimal biomarker combinations for use in routine management of systemic lupus erythematosus patients at-risk for lupus nephritis.

APhL antibody ELISA as an alternative to anticardiolipin test for the diagnosis of antiphospholipid syndrome.

BACKGROUND: Persistent levels of antiphospholipid (aPL) antibodies [lupus anticoagulant (LA), anticardiolipin (aCL), anti-beta 2 glycoprotein I (aß(2)GPI) IgG and/or IgM] in association with clinical features of thrombosis and/or pregnancy associated morbidity are indicative of antiphospholipid syndrome (APS). Of the aPL antibodies, aCL is the most sensitive for APS, however, their lack of specificity constitute a laboratory and clinical challenge. IgG/IgM antibodies directed against APhL (a mixture of phospholipids) has been reported to predict APS more reliably than aCL tests. The main objective of this study was to evaluate the performance characteristics of the APhL IgG/IgM ELISA, relative to the aCL and aß(2)GPI tests. METHODS: Sixteen (16) clinically confirmed APS and 85 previously tested serum (PTS) samples for aCL and aß(2)GPI IgG/IgM antibodies were evaluated with the APhL IgG/IgM ELISA. Clinical specificity was determined in 100 serum samples (50 healthy and 50 infectious disease controls [parvo- and syphilis-IgG/IgM positive]). RESULTS: The IgG antibody prevalence for aCL and APhL in the APS and PST groups was comparable with marginal differences in clinical specificities. In contrast to the aCL IgM ELISA, the APhL test showed improved clinical specificities (72% aCL vs 94% APhL in the healthy controls; 38% aCL vs 78% APhL in the infectious disease controls) with implications for increased reliability in the diagnosis of APS. The overall agreement of the APhL with the aCL or aß(2)GPI for the IgG tests was 89% and 85% respectively, and that of the APhL IgM to the aCL or aß(2)GPI IgM tests was 72% and 86% respectively. CONCLUSION: Routine use of the APhL IgG/IgM ELISA may substantially reduce the high number of false positives associated with the aCL test without loss in sensitivity for APS.

Profiling anti-cyclic citrullinated peptide antibodies in patients with juvenile idiopathic arthritis.

BACKGROUND: Anti-citrullinated protein/peptide antibodies (ACPA), have high specificity for rheumatoid arthritis (RA). Some children with juvenile idiopathic arthritis (JIA), phenotypically resemble RA and test positive for rheumatoid factor (RF) a characteristic biomarker of RA. We investigated the prevalence of ACPA and its relationship to other serologic markers associated with RA in a well-characterized JIA cohort. METHODS: Cases were 334 children with JIA, 30 of whom had RF + polyarticular JIA. Sera from all cases and 50 healthy pediatric controls were investigated by ELISA at a single time point for anti-cyclic citrullinated peptide (anti-CCP) IgG, RF IgM, IgA and IgG, anti-RA33 IgG, and antinuclear antibodies (ANA). Comparisons between cases and controls were made using Chi-square or Fisher exact tests and T-tests. RESULTS: The prevalence of RF was 8% among controls, and 12% among cases (ns). The prevalence of ACPA was 2% in controls and 14.3% in cases (OR 8.2, p <0.01). Children who were ACPA-positive and RF-negative (n = 23) had a significantly earlier onset-age (4.6 years vs. 12.1 years, p <0.00001) and had fewer HLA-DRB1 shared epitope alleles than those positive for both RF and ACPA (n = 25). Prevalence of anti-RA33 was not different between cases and controls. CONCLUSIONS: ACPAs are detectable in 14% of children with JIA. Children with positive ACPA but negative RF are frequent, and may define a distinct subset of children with JIA. ACPA testing should be included in the classification of JIA.

Evaluation of a high avidity anti-dsDNA IgG enzyme-linked immunosorbent assay for the diagnosis of systemic lupus erythematosus.

The high avidity (HA) anti-dsDNA IgG ELISA is considered highly specific for the diagnosis of systemic lupus erythematosus (SLE). The main objective of this study was to determine the performance of this test with existing assays for detecting anti-dsDNA IgG antibodies as well as assess its analytical characteristics. For method comparison studies, we investigated the correlation between the HA ELISA with 8 other assays for the detection of dsDNA IgG antibodies namely; six anti-dsDNA IgG ELISA, the Crithidia luciliae immunofluorescence test (CLIFT) and an in-house developed Farr radioimmunoassay (RIA). Overall, 125 patient (100 ANA-positive, 25 CLIFT-tested) and 100 healthy control samples were tested. The assay was also evaluated for imprecision, lot-to-lot consistency and the effect of interfering substances using commercial quality control materials based on the manufacturer's claims unless otherwise stated. Of the 100 ANA positive samples, 18 were positive in the HA ELISA with significant levels of antibodies in the six ELISAs and CLIFT. The HA ELISA had a specificity of 100% with an overall agreement of 84% with the RIA. Intra - and inter-assay imprecision ranged from 13.9-16.5% and the reproducibility between lots based on qualitative interpretation was 100%. Hemoglobin, bilirubin and lipemia showed variable interference with assay performance based on the manufacturer's claims and our in-house protocol. Our data suggest that the HA ELISA although less sensitive than the other dsDNA IgG assays evaluated, is specific and predicts high levels of anti-dsDNA IgG antibodies.