Relationship between quantitative real-time PCR cycle threshold and culture for detection ofsubspecies.

Abstract

OBJECTIVE: To compare PCR and culture results for the detection ofsubspecies. ANIMALS: Respiratory tract samples (N = 158) from horses being tested for PROCEDURE: Bacterial culture was carried out on samples from whichwas detected by quantitative real-time PCR. RESULTS: was isolated from 12 (7.6%) samples: 4/9 (44%) samples when the PCR cycle threshold (C) was ≤ 30, 7/30 (23%) when the Cwas 30.1 to 35, and 1/119 (0.8%) when the Cwas 35.1 to 40. The highest Csample from a sample that yielded a positive culture was 36.9. The optimal Youden's J value was at a Cof 34.2, the same value as determined by number needed to misdiagnose when the cost of a false negative is deemed to be either 5 or 10 × that of a false positive. CONCLUSIONS: Viablewas only detected in a minority of quantitative PCR (qPCR) positive samples. A qPCR Cof 34.2 was a reasonable breakpoint for likelihood of the presence of culturable CLINICAL RELEVANCE: Evaluation of Cvalues may be useful as a proxy to indicate the likelihood of cultivablebeing present and could be useful as part of risk assessments.