Evaluation of a lateral flow immunoassay for the quantitative measurement of immunoglobulin G in foals compared with radial immunodiffusion.

BACKGROUND: Reliable point-of-care diagnostic tests are essential for diagnosing failure of transfer of passive immunity (FTPI) in foals. Quantitative lateral flow immunoassays (LFIAs) could reduce subjectivity but have not been well validated in foals. HYPOTHESIS/OBJECTIVES: To validate the performance and diagnostic accuracy of a LFIA for quantifying immunoglobulin G (IgG) concentrations in foals, using radial immunodiffusion (RID) as the reference standard. ANIMALS: Forty-two neonatal foals were sampled within 24 h of birth. METHODS: This prospective exploratory diagnostic accuracy study evaluated the diagnostic accuracy of an LFIA for neonatal foal IgG measurement in accordance with the STARD guidelines. Blood samples were analyzed by LFIA, RID, and a SNAP-based ELISA. Primary analyses used serum samples. Diagnostic performance was evaluated for serum, plasma, and whole blood at predefined RID thresholds (<400 and ≤ 800 mg/dL) using receiver operating characteristic analysis, sensitivity, specificity, and Bland-Altman analysis. RESULTS: Lateral flow immunoassays results were strongly correlated with RID (R2 = 0.91) within the analytical range (100-1000 mg/dL). The assay demonstrated good linearity, precision, and accuracy. Receiver operating characteristic analysis showed high point estimates for diagnostic performance across sample matrices, with AUCs ranging from 0.988 to 1.000, although estimates were imprecise because of the limited number of FTPI-positive foals. CONCLUSIONS AND CLINICAL IMPORTANCE: Lateral flow immunoassays demonstrated good agreement with RID for FTPI classification and could be suitable as a point-of-care screening tool, but is not interchangeable with RID for quantitative measurement. Although the limited number of FTPI-positive foals warrants cautious interpretation, these findings support LFIA as a screening tool for FTPI classification.