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 (C-T) was <= 30, 7/30 (23%) when the C-T was 30.1 to 35, and 1/119 (0.8%) when the C-T was 35.1 to 40. The highest C-T sample from a sample that yielded a positive culture was 36.9. The optimal Youden's J value was at a C-T 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 X that of a false positive. Conclusions Viable S. equi was only detected in a minority of quantitative PCR (qPCR) positive samples. A qPCR C-T of 34.2 was a reasonable breakpoint for likelihood of the presence of culturable S. equi.