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

OBJECTIVE

To compare PCR and culture results for the detection of subspecies .

ANIMALS

Respiratory tract samples (N = 158) from horses being tested for

PROCEDURE

Bacterial culture was carried out on samples from which was 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 C was 30.1 to 35, and 1/119 (0.8%) when the C was 35.1 to 40. The highest C sample from a sample that yielded a positive culture was 36.9. The optimal Youden's J value was at a C of 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

Viable was only detected in a minority of quantitative PCR (qPCR) positive samples. A qPCR C of 34.2 was a reasonable breakpoint for likelihood of the presence of culturable

CLINICAL RELEVANCE

Evaluation of C values may be useful as a proxy to indicate the likelihood of cultivable being present and could be useful as part of risk assessments.