Validation of a Point-of-Care Quantitative Equine IgG Turbidimetric Immunoassay and Comparison of IgG Concentrations Measured with Radial Immunodiffusion and a Point-of-Care IgG ELISA

Background: Point-of-care (POC) diagnostic tests with good sensitivity and specificity are needed for diagnosing failure of transfer of passive immunity (FTPI) in foals. Turbidimetric immunoassays (TIA) have these characteristics and provide quantitative results. A commercially available TIA-based POC test (POC-TIA) has not been validated in horses. Objective: To validate a POC-TIA and compare results of POC-TIA, a POC-ELISA, and radial immunodiffusion (RID). Animals: Heparinized blood samples (n = 127) from 48 hospitalized foals (< 12 hour to 48 days). Methods: Prospective validation study. IgG concentrations were measured using RID (gold standard), POC-TIA, and POC-ELISA. Agreement between assays was assessed using Bland-Altman analysis. Sensitivity and specificity were calculated using ROC curves. Inter-and intra-assay coefficients of variation (CVs) and linearity were evaluated for POC-TIA. Results: The mean bias (95% limits of agreement) between RID and POC-TIA was -4 (-185 to 176), 27 (- 201 to 255), and 308 (- 377 to 993) mg/dL for samples with IgG concentrations of < 400, 400-800, and > 800 mg/dL, respectively. Sensitivity and specificity at optimal cutoff were 94 and 100% for the POC-TIA and 94 and 100% for the POC-ELISA to detect IgG < 400 mg/dL, and 85 and 87% (POC-TIA) and 69 and 79% (POC-ELISA) to detect IgG = 800 mg/dL. Intra-and interassay CVs for POC-TIA ranged between 1.6-3.8 and 11.9-18.8%, respectively. Linearity of the dilution series was preserved (R-2 > 0.96). Conclusions and Clinical Importance: The POC-TIA provided unambiguous results and had sufficient sensitivity, specificity, accuracy, and precision to be used as an alternative to other POC tests to assess FTPI in foals.