The Canadian veterinary journal = La revue veterinaire canadienne · 2023
Relationship between quantitative real-time PCR cycle threshold and culture for detection of Streptococcus equi subspecies equi.
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Weese J Scott, Saab Matt, Moore Allison, Cai Hugh, McClure J Trenton
Аннотация
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.
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.