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Letters to the Editor

What is the Significance of a High Cycle Threshold Positive IS481 PCR for Bordetella pertussis?

Papenburg, Jesse MD, FRCPC; Fontela, Patricia MD, MSc

Author Information
The Pediatric Infectious Disease Journal: December 2009 - Volume 28 - Issue 12 - p 1143
doi: 10.1097/INF.0b013e3181bd4e1f
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To the Editors:

We read with great interest the article by Waters et al regarding an outbreak of atypical pertussis diagnosed by polymerase chain reaction (PCR).1

It is well established that PCR targeting the IS481 gene, of which there are 50 to 200 copies per bacterial cell, is a highly sensitive method for detecting Bordetella pertussis.2 Indeed, with this assay, a positive result with a cycle threshold (CT) value >35 likely represents the detection of less than 1 organism per sample.2,3 Consequently, it remains unclear how to interpret a late-cycle positive PCR result. Does it represent detection of true pertussis disease (possibly modified by vaccination, partial natural immunity or antimicrobial therapy)? Or, does it represent transient colonization where B. pertussis was an innocent bystander with regards to the clinical syndrome that prompted diagnostic testing? Given that Walters et al used a CT of ≤40 to define a case, and given that their mean CT value was 38.4, further analysis is necessary to understand the clinical relevance of their results and to shed light on the dilemma of interpreting CT values >35. To do so, it would be invaluable to look for the existence of an association between higher CT values and the presence of coinfection with other respiratory pathogens (as tested by PCR on the study's pertussis-positive nasopharyngeal samples). Furthermore, a receiver operating characteristic curve would help elucidate how CT cut-off values affect the diagnostic assay's sensitivity and specificity when compared with a reference standard such as the clinical case definition of pertussis.

Jesse Papenburg, MD, FRCPC

Infectious Diseases Division, Department of Pediatrics and Department of Microbiology

Montreal Children's Hospital, McGill University Health Centre

Montreal, Canada

Infectious Disease Research Centre

Centre Hospitalier Université Laval

Quebec City, Canada

Patricia Fontela, MD, MSc

Department of Epidemiology and Biostatistics

McGill University

Montreal, Canada

REFERENCES

1.Waters V, Jamieson F, Richardson SE, et al. Outbreak of atypical pertussis detected by polymerase chain reaction in immunized preschool-aged children. Pediatr Infect Dis J. 2009;28:582–587.
2.Guthrie JL, Seah C, Brown S, et al. Use of Bordetella pertussis BP3385 to establish a cutoff value for an IS481-targeted real-time PCR assay. J Clin Microbiol. 2008;46:3798–3799.
3.Loeffelholz MJ, Thompson CJ, Long KS, et al. Comparison of PCR, culture, and direct fluorescent-antibody testing for detection of Bordetella pertussis. J Clin Microbiol. 1999;37:2872–2876.
© 2009 Lippincott Williams & Wilkins, Inc.