Usefulness of a commercial equine IgG test and serum protein concentration as indicators of failure of transfer of passive immunity in hospitalized foals.
Authors: Metzger Nadine, Hinchcliff Kenneth W, Hardy Joanne, Schwarzwald Colin C, Wittum Thomas
Journal: Journal of veterinary internal medicine
Summary
# Editorial Summary: Commercial IgG Testing for Failure of Transfer of Passive Immunity in Hospitalised Foals Early detection of failure of transfer of passive immunity (FTPI) significantly impacts survival and morbidity in neonatal foals, yet readily available diagnostic tools remain limited in clinical settings. Metzger and colleagues evaluated the SNAP Foal IgG Test Kit against single radial immunodiffusion (the reference standard) in 67 hospitalised foals under 19 days old, alongside measuring serum total protein and globulin concentrations as potential surrogate indicators of FTPI. The SNAP test demonstrated high sensitivity for detecting IgG concentrations ≤400 mg/dl (90%) and ≤800 mg/dl (95%), but specificity was problematically low at 79% and 52% respectively—a limitation worsened in septic foals and those with bacteraemia, where false positives increased substantially. Serum total protein proved similarly unreliable as a standalone indicator, with 94% sensitivity but only 47% specificity for IgG ≤800 mg/dl. For practitioners, this means the SNAP test remains clinically useful for ruling out FTPI when negative in non-septic foals, but positive results warrant confirmation through radial immunodiffusion or clinical correlation, particularly in hospitalised cases where infection confounds interpretation; serum protein and globulin measurements are best employed as supporting evidence rather than diagnostic anchors.
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Practical Takeaways
- •The SNAP IgG test is useful for ruling out FTPI in hospitalized foals due to high sensitivity, but positive results should be confirmed with other indicators like total protein or clinical signs, particularly in septic foals
- •Do not rely on serum total protein or globulin concentration alone to diagnose FTPI in sick foals—use these tests only as supporting evidence alongside clinical assessment
- •Be aware that test accuracy is reduced in systemically ill foals with sepsis or bacteremia; results require careful interpretation in context of the individual foal's clinical presentation and other diagnostic findings
Key Findings
- •SNAP IgG test showed 90% sensitivity and 79% specificity for detecting IgG ≤400 mg/dL, but specificity dropped to 52% for IgG ≤800 mg/dL threshold
- •Test specificity was significantly impaired in foals with sepsis or bacteremia (25-50%) compared to unaffected foals (62-100%)
- •Serum total protein ≤5.0 g/dL had 94% sensitivity but only 47% specificity for detecting IgG ≤800 mg/dL
- •SNAP test reliability was moderate (kappa 0.64) with repeat testing on 37 samples showing identical results in only 65% of cases