Comparison of nucleic acid amplification, serology, and microbiologic culture for diagnosis of Rhodococcus equi pneumonia in foals

Recently, a technique was described for amplification of Rhodococcus equi-s pecific chromosomal and vapA DNA from blood and tracheal wash fluids. It,va s hypothesized that this technique would be more sensitive than standard cu lture techniques or serology for diagnosis of R, equi pneumonia in foals. T racheal wash fluid, nasal swabs, whole blood samples, and serum samples fro m 56 foals with pneumonia were analyzed. Final clinical diagnosis was deter mined by the attending clinician on the basis of final interpretation of al l available information about each foal, including clinical presentation, d iagnostic test results, response to therapy, and outcome. Clinical diagnosi s was used as a final reference standard for calculation of sensitivity, sp ecificity, and predictive values for PCR, serology using an agar gel immuno diffusion test, and tracheal wash fluid culture, PCR of tracheal wash fluid using primers that recognized the,vapA virulence plasmid of R, equi had a diagnostic sensitivity of 100% and specificity of 90.6%. Sensitivity and sp ecificity were 57.1 and 93.8%, respectively, for standard microbiologic cul ture of tracheal wash quid and 62.5 and 75.9%, respectively, for serology, PCR of tracheal wash fluid is more sensitive and specific for diagnosis of R, Equi pneumonia than are other available diagnostic tests.