Evaluation of five commercially available assays and measurement of serum total protein concentration via refractometry for the diagnosis of failure of passive transfer of immunity in foals

Objective-To determine and compare sensitivity, specificity, accuracy, and predictive values of measurement of serum total protein concentration by refractometry as well as 5 commercially available kits for the diagnosis of failure of passive transfer (FPT) of immunity in foals. Design-Prospective study. Animals-65 foals with various medical problems and 35 clinically normal foals. Procedure-IgG concentration in serum was assessed by use of zinc sulfate turbidity (assay C), glutaraldehyde coagulation (assay D), 2 semiquantitative immunoassays (assays F and G), and a quantitative immunoassay (assay H). Serum total protein concentration was assessed by refractometry. Radial immunodiffusion (assays A and B) was used as the reference method. Results-For detection of IgG < 400 mg/dL, sensitivity of assay H (100%) was not significantly different from that of assays C, E, and G (88.9%). Specificity of assays H (96.0%) and G (95.8%) was significantly higher than that of assays C (79.4%) and E (78.1%). For detection of IgG < 800 mg/dL, sensitivities of assays H (976%), D (92.9%), C (81.0%), and G (81.0%) were significantly higher than that of assay F (52.4%). Specificity of assays F (100%), G (94.7%), and H (82.8%) was significantly higher than that of assays C (56.9%) and D (58.6%). Serum total protein concentration <= 4.5 g/dL was suggestive of FPT, whereas values >= 6.0 g/dL indicated adequate IgG concentrations. Conclusions and Clinical Relevance-Most assays were adequate as initial screening tests. However, their use as a definitive test would result in unnecessary treatment of foals with adequate IgG concentrations.