Background Neonatal foals with failure of transfer of passive immunity (FTPI) are at higher risk of morbidity and mortality. Successful treatment of FTPI is time‐dependent, thus rapid and accurate measurement of serum IgG concentration is important for the management and care of neonatal foals. Objectives To validate the use of digital and optical refractometers for assessing FTPI in neonatal foals and compare the diagnostic performance and level of agreement of the two refractometers to the reference standard radial immunodiffusion (RID) assay. Study design A retrospective validation study. Methods Serum samples (n = 253) were collected from 230 foals admitted to the Veterinary Teaching Hospital and Ambulatory Equine Service between 2012 and 2017. The serum IgG concentrations were measured by the reference RID assay, digital Brix and optical refractometers. The correlation between results of two refractometers and RID assay was assessed. A receiver operating characteristic curve was created and used to identify the optimal cut‐offs for evaluating sensitivity and specificity of the two refractometers to detect foals with complete and partial FTPI. Results The RID‐IgG concentrations were positively correlated with the Brix scores obtained from a digital refractometer (r = 0.73, P = 0.001) and serum total protein obtained from an optical refractometer (r = 0.72, P = 0.001). The sensitivity and specificity of the digital Brix refractometer at optimal cut‐off (≤7.8% Brix) were 88.1 (95% CI: 74.4–96.0) and 67.7% (95% CI: 60.6–74.3) to detect RID‐IgG<4 g/L and 79.0 (95% CI: 68.5–87.3) and 77.3% (95% CI: 69.8–83.8) to detect RID‐IgG≤8 g/L, respectively. The sensitivity and specificity of the optical refractometer at optimal cut‐off (≤42 g/L) were 86.1 (95% CI: 72.1–94.7) and 70.9% (95% CI: 63.9–77.3) to detect RID‐IgG<4 g/L and at cut‐off (≤44 g/L) were 82.9 (95% CI: 73.0–90.3) and 72.7% (95% CI: 64.8–79.6) to detect RID‐IgG≤8 g/L, respectively. Main limitations The number of diseased foals was small to investigate the validity of the selected cut‐off values for assessing FTPI in sick foals. Conclusions The two refractometers exhibit utility as rapid, inexpensive screening tests and have a good sensitivity for assessing FTPI in neonatal foals.