Evaluation of the immunocrit method to detect failure of passively acquired immunity in dairy calves

OBJECTIVE To evaluate the usefulness of serum immunocrit measurement to detect failure of passively acquired immunity (FPI) in dairy calves. DESIGN Diagnostic test evaluation. ANIMALS 249 female dairy calves (age, 2 to 6 days). PROCEDURES A blood sample was collected from each calf, and serum was harvested. Immunocrit was measured in serum samples by use of 55% ammonium sulfate solution and the standard technique. Serum IgG concentration was measured by means of radial immunodiffusion (reference standard), with FPI defined as a result < 1,000 mg/dL. The immunocrit value (cutpoint) that maximized both sensitivity and specificity of the method for detection of FPI was determined by construction of receiver operating characteristic curves, and likelihood ratios for positive and negative test results were calculated. RESULTS Immunocrit values were significantly correlated (ρ = 0.71) with serum IgG concentration as measured by radial immunodiffusion. An immunocrit cutpoint of 11% was optimal for detection of FPI in the calves. Sensitivity and specificity of the immunocrit method at this cutpoint were 0.88 (95% confidence interval [CI], 0.79 to 0.97) and 0.51 (95% CI, 0.44 to 0.58), respectively. Likelihood ratios for positive and negative test results were 1.80 (95% CI, 1.51 to 2.14) and 0.23 (95% CI, 0.11 to 0.51), respectively. CONCLUSIONS AND CLINICAL RELEVANCE The immunocrit method was useful for identifying dairy calves with FPI and was simple and could be quickly performed. Because precipitation of immunoglobulins by ammonium sulfate is not species specific, the immunocrit method should be evaluated for detection of FPI in other veterinary species as well.