TRFIA enhances low-titer anti-PLA2R antibody detection, reducing biopsy need in suspected PMN cases. Consider TRFIA for ambiguous ELISA results.
Accurate detection of circulating anti PLA2R antibodies is central to diagnosing primary membranous nephropathy. ELISA is widely used, but sensitivity drops when antibody levels fall below 20 RU/ml. Many patients in this range still have biopsy proven PMN, which creates diagnostic uncertainty and often leads to an invasive biopsy.
Time resolved fluoroimmunoassay offers higher analytical sensitivity. This study evaluated its performance for PLA2R IgG and IgG4 in patients with serum anti PLA2R antibody between 2 and 20 RU/ml.
This study analyzed 175 patients who had renal biopsies and serum anti PLA2R antibody levels between 2 and 20 RU/ml from January 2018 to December 2024. Among these, 121 patients had stored serum samples available for TRFIA testing and were included in the diagnostic comparison.
Anti PLA2R IgG and IgG4 were measured with TRFIA. Anti PLA2R IgG was also measured with ELISA. Biopsy results served as the diagnostic gold standard. ROC curves established optimal cutoff values for each assay.
Of 175 patients, 52 percent (91 patients) had biopsy confirmed PMN. The remaining patients had IgA nephropathy (26 percent), minimal change disease (5 percent), diabetic kidney disease (5 percent), or other renal diseases (12 percent).
Patients with PMN were older and had higher 24 hour urinary protein, LDL, total cholesterol, hemoglobin, eGFR, and D dimer. Albumin, IgG, IgA, BUN, and serum creatinine were lower in PMN.
Using manufacturer recommended cutoffs:
PLA2R IgG TRFIA
Sensitivity 14.3 percent
Specificity 100 percent
PLA2R IgG4 TRFIA
Sensitivity 34.1 percent
Specificity 100 percent
ELISA
Sensitivity 0 percent for values <20 RU/ml
Specificity 100 percent
TRFIA identified more patients across all low titer subgroups, especially patients in the 2 to 7 RU/ml range. PLA2R IgG4 TRFIA outperformed IgG TRFIA in every subgroup.
Among eight PMN patients who later became ELISA positive during follow up, three had been positive on PLA2R IgG4 TRFIA at baseline and one on PLA2R IgG TRFIA.
ROC analysis showed:
AUC 0.829 for PLA2R IgG TRFIA
AUC 0.814 for PLA2R IgG4 TRFIA
AUC 0.712 for ELISA
Optimal cutoffs were:
PLA2R IgG TRFIA: 0.44 RU/ml
Sensitivity 83.3 percent
Specificity 82.4 percent
PLA2R IgG4 TRFIA: 25.07 ng/ml
Sensitivity 83.3 percent
Specificity 76.9 percent
ELISA: 2.84 RU/ml
Sensitivity 50 percent
Specificity 88 percent
These adjusted cutoffs improved sensitivity at the cost of some specificity, but TRFIA maintained stronger discriminative power than ELISA.
Multivariate logistic regression confirmed PLA2R IgG TRFIA as an independent diagnostic factor for PMN in patients with low anti PLA2R levels.
PLA2R IgG and IgG4 TRFIA correlated moderately with ELISA. IgG and IgG4 TRFIA strongly correlated with each other. PLA2R IgG4 TRFIA showed a weak negative correlation with albumin. No other clinical variables correlated meaningfully with any assay.
PMN patients positive on PLA2R IgG4 TRFIA were older and had lower albumin, bilirubin, and C3.
Patients with anti PLA2R antibody levels between 2 and 20 RU/ml pose a diagnostic challenge. Many have true PMN but test negative on ELISA. TRFIA closes this sensitivity gap.
Key observations:
PLA2R IgG4 TRFIA was the most sensitive assay for low titer PMN.
TRFIA detected early seropositivity in patients who later became positive on ELISA.
AUC values confirm the stronger diagnostic performance of TRFIA.
A subset of PMN patients may avoid biopsy when TRFIA is used with standard cutoffs. In this study, 34.1 percent of biopsy proven PMN patients in the 2 to 20 RU/ml zone could have been diagnosed noninvasively with PLA2R IgG4 TRFIA.
Continuous antibody monitoring also identified delayed seropositivity in 15.4 percent of antibody negative PMN patients.
In patients with suspected PMN and ELISA values below 20 RU/ml, TRFIA should be considered for improved sensitivity.
PLA2R IgG4 TRFIA is especially useful in patients with very low titers and strong clinical suspicion.
Serial antibody monitoring remains important for patients who initially test negative.
This was a single center retrospective study. The TRFIA sample size was modest. Multicenter validation and prospective follow up are needed to confirm optimal diagnostic thresholds and prognostic utility.
TRFIA enhances the detection of anti PLA2R antibodies in patients with low ELISA titers. Both IgG and IgG4 TRFIA outperform ELISA, with IgG4 TRFIA offering the highest sensitivity. TRFIA provides a practical, noninvasive tool that may reduce the need for renal biopsy and support earlier diagnosis of primary membranous nephropathy.
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