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

Objective To compare PCR and culture results for the detection of Streptococcus equi subspecies equi (S. equi). Animals Respiratory tract samples (N = 158) from horses being tested for S. equi. Procedure Bacterial culture was carried out on samples from which S. equi was detected by quantitative real-time PCR. Results S. equi was isolated from 12 (7.6%) samples: 4/9 (44%) samples when the PCR cycle threshold (CT) was ≤ 30, 7/30 (23%) when the CT was 30.1 to 35, and 1/119 (0.8%) when the CT was 35.1 to 40. The highest CT sample from a sample that yielded a positive culture was 36.9. The optimal Youden's J value was at a CT 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 S. equi was only detected in a minority of quantitative PCR (qPCR) positive samples. A qPCR CT of 34.2 was a reasonable breakpoint for likelihood of the presence of culturable S. equi. Clinical relevance Evaluation of CT values may be useful as a proxy to indicate the likelihood of cultivable S. equi being present and could be useful as part of risk assessments.