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Relationship between quantitative real-time PCR cycle threshold and culture for detection of Streptococcus equi
J Scott Weese1, Matt Saab1, Allison Moore1
1Department of Pathobiology and Centre for Public Health and Zoonoses, Ontario Veterinary College, University of Guelph, 50 Stone Road East, Guelph, Ontario NIG 2W1 (Weese); Diagnostic Services (Saab) and Department of Health Management (Saab, McClure), Atlantic Veterinary College, University of Prince Edward Island, 550 University Avenue, Charlottetown, Prince Edward Island C1A 4P3; Animal Health and Welfare Branch, Ontario Ministry of Agriculture, Food and Rural Affairs, 5th Floor NW, 1 Stone Road West, Guelph, Ontario N1G 4Y2 (Moore); Animal Health Laboratory, University of Guelph, 419 Gordon Street, Guelph, Ontario N1G 2W1 (Cai).
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.
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