Feeding foals with poor quality colostrum predisposes them to failure of passive transfer (FPT). FPT is a major risk factor for neonatal infections.

To assess the optimal cut‐offs for the optical (OR) and digital (DR) refractometer and determine their accuracy for poor quality colostrum diagnosis.

A diagnostic validation study.

Eighty‐one colostrum samples and sera were collected from broodmares and their neonatal foals, respectively. Colostral and serum IgG concentrations were measured by radial immunodiffusion (RID), DR and OR. Correlation coefficients were calculated. ROC curves were generated to identify optimal cut‐offs for the refractometers and their diagnostic characteristics were evaluated.

The optimal cut‐offs for DR and OR were ≤23.75% and 23.9%, respectively. The sensitivity and specificity of the DR were 93.3% (95% CI: 66.0‐99.7) and 87.9% (95% CI: 77.0‐94.3) to detect colostral IgG <60 g/L, respectively. The sensitivity and specificity of the OR were 93.3% (95% CI: 66.0‐99.7) and 81.8% (95% CI: 70.0‐89.9), respectively. DR and OR had negative predictive values of 98.3% (95% CI: 89.7‐99.9) and 98.2% (95% CI: 89.0‐99.9), respectively, whilst positive predictive values were lower. No maternal variable, including breed, significantly influenced colostral IgG concentrations. Fifteen out of 81 colostrum samples had IgG <60 g/L. FPT and PFPT were diagnosed in 4/81 and 10/81 foals, respectively. Nine out of 14 animals with FPT/PFPT suckled colostrum with IgG <60 g/L. A moderate correlation (r_s 0.542; P = .01) was observed between IgG concentrations measured by RID in sera and colostrum.

A smaller number of samples than the size requirement based on a priori estimate of specificity and the low prevalence of poor quality colostrum.

The method has the potential to reliably differentiate between good and poor quality colostrum. Assessing colostrum quality by refractometry may be an indicator of passive transfer of immunity.