Evaluation of white blood cell concentration,plasma fibrinogen concentration,and an agar gel immunodiffusion test for early identification of foals with Rhodococcus equi pneumonia

OBJECTIVE: To evaluate WBC concentration, plasma fibrinogen concentration, and an agar gel immunodiffusion (AGID) test for early identification of Rhodococcus equi-infected foals. DESIGN: Prospective study. ANIMALS: 162 foals from a farm with enzootic R equi infection. PROCEDURE: Blood samples were obtained from each foal at 4-week intervals for measurement of WBC and plasma fibrinogen concentrations and at 2-week intervals for detection of anti-R equi antibody by an AGID assay. Diagnostic performance of WBC and fibrinogen concentrations was assessed by use of receiver operating characteristic curve analysis. For each assay, sensitivity, specificity, and predictive values were calculated at various cutoff points; bacteriologic culture of R equi from a tracheobronchial aspirate was used as the reference standard test. RESULTS: Diagnostic performance of WBC concentration was significantly higher than that of fibrinogen concentration. Sensitivity and specificity of measurement of WBC concentration at a cutoff of 13,000 cells/microL were 95.2 and 61.2%, respectively; at a cutoff of 15,000 cells/microL, sensitivity was 78.6% and specificity was 90.8%. When a positive test result was used as the cutoff, sensitivity of the AGID assay was 62.5% and specificity was 53.8%. CONCLUSION AND CLINICAL RELEVANCE: Monitoring WBC concentration is a useful approach for early detection of infected foals on farms with a high prevalence of R equi pneumonia. In contrast, serologic surveillance by use of an AGID assay is of little benefit for that purpose.