Background: Failure of transfer of passive immunity (FTPI) is the most important cause of sepsis and mortality in neonatal foals. Objectives: This study aimed to evaluate the performance of the digital Brix refractometer for the diagnosis of FTPI in 12-h-old foals. Study design: Cross-sectional, diagnostic accuracy study. Methods: Jugular blood samples were collected from 124 foals at 12 ± 0.5 h after birth. IgG concentrations were estimated using a digital Brix refractometer and measured using the radial immunodiffusion test (RID). Correlation coefficients were calculated between serum Brix percentages and IgG concentrations. A receiver operating curve (ROC) was created to identify the Brix thresholds for diagnosing FTPI and to calculate the sensitivity (Se) and specificity (Sp) of the tests. Optimum thresholds were selected based on the Youden index. The area under the ROC curve (AUC) was calculated to evaluate the discrimination ability of the Brix refractometer. Results: There was a high correlation (r = 0.72) between the Brix percentages and RID IgG concentrations of the foals at 12 h after birth. The optimum threshold of the digital Brix refractometer to diagnose complete FTPI (IgG of <400 mg*dL−1) was ≤7.5% with 77.8% Se and 76.4% Sp. The optimum Brix threshold to diagnose complete/partial FTPI (IgG of <800 mg*dL−1) was ≤7.8% with 79.5% Se and 78.8% Sp. The Brix refractometer was moderately accurate in differentiating between foals with or without complete/partial FTPI (AUC ranged from 0.7 to 0.9). Alternatively, when FTPI was diagnosed with >90% sensitivity, the Brix threshold was identified as ≤8.4% for both IgG concentrations (Se 92.9% at <400 mg*dL−1; Se 90.0% at <800 mg*dL−1). Conclusion: The digital Brix refractometer detected complete and partial FTPI in 12-h-old foals with a sensitivity below 80%. This constitutes a handicap for the use of Brix refractometry in the horse industry. Future studies should compare the diagnostic accuracy of on-site tests, directly measuring IgG concentrations and refractometry in newborn foals and conduct a cost-benefit analysis of the combined use of these methods.