Small ruminants, such as newborn lambs, rely on timely colostrum intake to acquire passive immunity through the absorption of immunoglobulin (Ig). Evaluating Ig transfer is important for ensuring lamb health and survival. However, current methods such as enzyme-linked immunosorbent assay (ELISA) and radial immunodiffusion (RID) are widely used but remain costly and require specialized facilities. The immunocrit assay has been proposed as a lower-cost alternative for evaluating serum Ig concentrations. This study aimed to evaluate the immunocrit method in lambs by comparing it with ELISA, RID, and total serum protein. Serum was collected from 135 Katahdin-Easycare lambs 24–36 h after birth. Samples were analyzed using sheep immunoglobulin G ELISA, Sheep immunoglobulin G RID, serum protein, and the immunocrit method. Pearson’s correlation was used to assess linear relationships between the methods, and Receiver Operating Characteristics (ROC) analysis was used to evaluate test accuracy, with RID as the gold standard (15 mg/mL cutoff). The immunocrit showed a high correlation with RID (r = 0.870), moderate correlation with serum protein (r = 0.725), and good correlation with ELISA (r = 0.607). The ROC analysis showed that the immunocrit had a sensitivity of 100% at a cutoff of 4.34%. These results indicate that the immunocrit method provides comparable accuracy to RID and serum protein, and could serve as a reliable, practical, and inexpensive tool for on-farm evaluation of passive transfer in Katahdin-Easycare crossbred lambs between 24 and 36 h after birth.