Evaluation of Optical Refractometer for Assessing Failure of Transfer of Passive Immunity in Foals.
Authors: Sobral Gilvannya Gonçalves de, Neto Oswaldo Christiano Gomes, da Silva Avaniel Marinho, Carneiro Gustavo Ferrer
Journal: Journal of equine veterinary science
Summary
# Editorial Summary: Optical Refractometry for Detecting Failure of Passive Immunity in Foals Failure of passive transfer of immunity (FPTI) in neonatal foals remains a significant clinical concern, yet field-based diagnostic tools remain limited. Researchers compared serum total protein measurements using optical refractometry against radial immunodiffusion (RID)—the gold-standard laboratory assay for immunoglobulin G quantification—in 30 foals sampled at 12 hours post-partum, also measuring colostral IgG concentration in their dams to assess transfer efficiency. Total protein concentration showed strong positive correlation with IgG-RID values, with receiver operating characteristic (ROC) curve analysis identifying a total protein threshold of ≤5.7 g/dL as optimal for FPTI diagnosis, yielding 100% sensitivity and 73.3% specificity (area under curve: 0.931). Notably, colostral IgG concentration demonstrated only moderate correlation with foal plasma IgG levels, highlighting variable transfer efficiency between mares and offspring. For equine practitioners, this work validates optical refractometry as a practical, cost-effective screening tool deployable at the stable or field with exceptional diagnostic sensitivity—meaning no FPTI cases would be missed—though the moderate specificity suggests occasional false positives requiring confirmatory laboratory testing when clinical doubt exists.
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Practical Takeaways
- •An optical refractometer reading of ≤5.7 g/dL total protein in 12-hour-old foals reliably identifies failure of passive transfer with 100% sensitivity—use this as a quick field screening tool
- •This portable, inexpensive method eliminates the need for laboratory IgG testing in most cases, enabling rapid diagnosis and treatment decisions at the farm
- •While specificity is moderate (73.3%), the high sensitivity means few truly affected foals will be missed; borderline cases can be confirmed with RID testing if needed
Key Findings
- •Optical refractometer total protein measurement was positively correlated with IgG concentration measured by radial immunodiffusion (RID)
- •ROC curve analysis identified ≤5.7 g/dL total protein as optimal cutoff point with 100% sensitivity and 73.3% specificity (AUC = 0.931)
- •Colostrum IgG concentration showed moderate correlation with foal plasma IgG concentration at 12 hours post-birth
- •Optical refractometer demonstrated effective utility for diagnosing FPTI in foals with high sensitivity, low cost, and field applicability