Usefulness of digital and optical refractometers for the diagnosis of failure of transfer of passive immunity in neonatal foals.
Authors: Elsohaby I, Riley C B, McClure J T
Journal: Equine veterinary journal
Summary
# Editorial Summary Failure of transfer of passive immunity (FTPI) in neonatal foals demands rapid diagnosis because treatment efficacy is time-dependent, yet the gold-standard radial immunodiffusion assay takes days to produce results. Elsohaby and colleagues validated digital Brix refractometers and optical refractometers as practical point-of-care alternatives by measuring serum IgG concentration in 253 samples from 230 foals, comparing results against the RID assay reference standard. Both devices showed good sensitivity for detecting critical FTPI cases: the digital refractometer achieved 88.1% sensitivity at a cut-off of ≤7.8% Brix for severe FTPI (IgG <4 g/L), whilst the optical refractometer achieved 86.1% sensitivity at ≤42 g/L total protein for the same threshold. Specificity was moderate (67.7–70.9% for severe FTPI), meaning false positives are likely—a limitation worth noting when deciding whether to treat—though both instruments performed better at detecting partial FTPI thresholds (sensitivity 79–83%, specificity 72–77% at IgG ≤8 g/L). For field practitioners seeking rapid screening to identify at-risk foals requiring laboratory confirmation or empirical treatment, either refractometer offers acceptable diagnostic utility at low cost, though neither should replace definitive IgG measurement in borderline cases where clinical confidence is critical.
Read the full abstract on PubMed
Practical Takeaways
- •Both digital and optical refractometers are rapid, inexpensive point-of-care screening tools suitable for field diagnosis of FTPI in neonatal foals, offering good sensitivity for detecting at-risk foals
- •Digital Brix refractometer at ≤7.8% and optical refractometer at ≤42 g/L total protein are practical cut-off values for identifying complete FTPI (IgG <4 g/L) requiring immediate intervention
- •These refractometers should not replace laboratory confirmation (RID assay) when definitive IgG quantification is critical for treatment decisions, but are valuable for rapid triage and screening
Key Findings
- •Digital Brix refractometer showed 88.1% sensitivity and 67.7% specificity at ≤7.8% Brix cut-off for detecting complete FTPI (IgG <4 g/L)
- •Optical refractometer showed 86.1% sensitivity and 70.9% specificity at ≤42 g/L total protein cut-off for detecting complete FTPI
- •Both refractometers demonstrated good positive correlation with reference RID assay (r=0.73 and r=0.72 respectively, P=0.001)
- •Both refractometers show improved sensitivity and specificity for partial FTPI detection (79-82.9% sensitivity, 72.7-77.3% specificity)