Evaluation of a turbidimetric immunoassay for measurement of plasma IgG concentration in foals

Abstract

To validate a turbidimetric immunoassay (TIA) for measurement of plasma IgG concentrations in foals. 36 foals. Blood samples were collected from foals before suckling and at 12 and 24 to 36 hours after birth. Plasma IgG concentrations were determined via a commercial single radial immunodiffusion (RID) assay. By use of goat anti-equine IgG antiserum and a spectrophotometer, a TIA was developed to measure plasma and serum IgG concentrations; the percentage light transmission was calibrated against RID assay-determined IgG concentrations. Assay repeatability and effects of serial dilution, sample type, and ambient temperature on assay results were evaluated. Serial dilution of plasma samples from foals 12 and 24 to 36 hours of age with presuckle plasma yielded percentage light transmission results that were highly inversely correlated (r = -0.95) with IgG concentrations determined via RID assay. Measurements of IgG in plasma and serum samples via TIA did not differ. When samples were assayed multiple times, the coefficient of variation was < 5.0%. Ambient temperature did not affect TIA results. At IgG concentrations of 400 and 800 mg/dL, TIA sensitivity was > 90%; specificity was 99.1% and 70.5%, respectively; and positive and negative predictive values were 98.1% and 71.5%, respectively, and 96.4% and 91.1%, respectively. Plasma IgG concentrations in foals determined via the TIA and RID assay were highly correlated. The TIA rapidly yielded quantitative results and would be useful in clinical situations where intervention decisions are time dependent.