Measurement of serum IgG in foals by radial immunodiffusion and automated turbidimetric immunoassay.
Authors: Davis Deborah G, Schaefer Deanna M W, Hinchcliff Kenneth W, Wellman Maxey L, Willet V Ellen, Fletcher Jana M
Journal: Journal of veterinary internal medicine
Summary
# Failure of passive immunity transfer (FTPI) remains a significant threat to neonatal foal survival, making rapid and accurate quantification of serum immunoglobulin G essential for clinical decision-making; radial immunodiffusion (RID) has traditionally served as the diagnostic gold standard, but its lengthy turnaround time can delay intervention in critically vulnerable foals. Davis and colleagues compared RID with automated turbidimetric immunoassay (TIA) across 84 neonatal foals, evaluating whether TIA could serve as a viable alternative for measuring serum IgG concentrations. Although TIA showed good overall correlation with RID (R² = 0.59) and excellent precision (2.5–3% coefficients of variation), its diagnostic performance varied considerably depending on the IgG threshold: sensitivity and specificity were 81% and 86% respectively for the standard FTPI cutoff of ≤800 mg/dL, but sensitivity dropped to 63% at the more severe threshold of ≤400 mg/dL, despite specificity remaining robust at 92%. The practical significance lies in TIA's substantially faster turnaround time and reliable intermediate-range performance, making it suitable for screening purposes in most clinical settings, though practitioners should recognise its reduced sensitivity at lower IgG concentrations and ensure samples are analysed promptly, as freezing at −20°C for extended periods produced mean IgG reductions of 17.6%. This work supports incorporating TIA as a pragmatic diagnostic tool for FTPI assessment alongside—or potentially ahead of—RID, particularly where rapid decision-making regarding passive immunisation or antimicrobial therapy is required.
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Practical Takeaways
- •TIA is a viable alternative to RID for diagnosing FTPI in foals with faster turnaround time and acceptable sensitivity/specificity for clinical decision-making
- •If using TIA, be cautious with the lower IgG threshold (≤400 mg/dL) where sensitivity drops to 0.63, and consider confirmatory testing for borderline results
- •For archived foal serum samples, avoid long-term freezing at -20°C as IgG concentrations degrade significantly; use fresh samples or shorter-term storage protocols
Key Findings
- •Turbidimetric immunoassay (TIA) showed sensitivity of 0.81 and specificity of 0.86 for diagnosing FTPI at IgG ≤800 mg/dL threshold
- •A significant linear relationship exists between IgG measured by TIA and RID (R² = 0.59, P < 0.0001), with TIA = 0.9511×RID + 8.4354
- •TIA demonstrated good precision with between-run and within-run coefficients of variation of 2.5% and 3% respectively
- •Long-term storage at -20°C causes a 17.6% reduction in measured serum IgG concentration over 10-12 months, limiting sample preservation utility