ABSTRACT. The object of this study was to evaluate the usefulness of measuring the differences in the values of the serum total protein (DVSTP) concentration of foals and the refractometry index (DVRI) of the milk of dams before and after nursing of the colostrum for assessing failure of passive transfer (FPT) in foals. Serum samples from 31 foals were collected before the first nursing and other 1 to 6 times between 4 and 24 hr after birth. Paired colostrum and milk samples were collected from 14 of their dams at the same time. Serum samples were analyzed for IgG concentration using a single radial immunodiffusion (SRID) test (98 samples) and total protein concentration using a temperature-compensating refractometer (98 samples). Colostrum and milk samples were analyzed for refractometry index (RI) using a Brix refractometer (71 samples). DVSTP concentration and DVRI were significantly correlated with serum IgG concentration. The negative predictive values (NPVs) of DVSTP concentration for detecting serum IgG concentrations<400 mg/dl and<800 mg/dl were 98.2% and 91.3% when the cutoff value is set to 0.4 mg/dl and 0.8 mg/dl, respectively. Furthermore, the NPVs of DVRI for detecting serum IgG concentrations<400 mg/dl and<800 mg/dl were 97.3% and 96.3% when the cutoff value is set to 6% and 10%, respectively. The results suggest that measurement of DVRI is useful in assessing FPT as an initial "stall-side" screening test, because it is easy, inexpensive to perform and allows for rapid interpretation. Foals have low concentrations of circulating immunoglobulins at birth, because the diffuse epitheliochorial placenta does not allow antibody transfer during pregnancy [16,19,30,43]. Foals are entirely dependent on maternal immunoglobulins (primarily IgG) absorbed through ingestion of mare's colostrum in the first 24 hr of life. Successful absorption of immunoglobulins prevents systemic infection during the lag time between exposure to pathogens and development of protective immune response [19,20]. Transfer of maternal antibodies is critical for the well-being of foals. Failure of passive transfer (FPT) significantly increases the risk of infections and death [10,31,32]. A foal greater than 24 hr of age is considered to have FPT, if the serum IgG concentration is less than 400 mg/dl. A serum concentration of 400 to 800 mg/dl over 24 hr after birth is considered partial FPT [16,31], and a serum concentration greater than 800 mg/dl is considered adequate for protection [16,25], although many normal foals have much higher concentrations [31]. The incidence of FPT in foals ranges from 3-24%, and that of partial FPT ranges from 14-31% [2,24,27,29,31,35,37,44].Early testing for serum IgG concentration in newborn foals can identify potential cases of FPT. Several screening tests have been developed over the years for evaluating the IgG concentration in foals. The single radial immunodiffusion (SRID) test is considered to be the most accurate test for quantitative measurement of IgG concentration in foals [41]. However, resul...