Published by the CDC
With the continuing resurgence of pertussis, health care professionals will see more patients with suspected pertussis. Polymerase Chain Reaction (PCR) is an important tool for timely diagnosis of pertussis and is increasingly available to clinicians. PCR is a molecular technique used to detect DNA sequences of the Bordetella pertussis bacterium and unlike culture, does not require viable (live) bacteria present in the specimen. Despite these advantages, PCR can give results that are falsely-negative or falsely-positive. The following compilation of best practices is intended to help health care professionals optimize the use of PCR testing for pertussis by avoiding some of the more common pitfalls leading to inaccurate results.
Testing Patients with Signs and Symptoms of Pertussis
Early signs and symptoms of pertussis are often non-specific, making it difficult to determine clinically who has pertussis in the earliest stages (http://www.cdc.gov/pertussis/clinical/features.html). However, only patients with signs and symptoms consistent with pertussis should be tested by PCR to confirm the diagnosis. Testing asymptomatic persons should be avoided as it increases the likelihood of obtaining falsely-positive results. Asymptomatic close contacts of confirmed cases should not be tested and testing of contacts should not be used for post-exposure prophylaxis decisions.
Optimal Timing for PCR Testing for Pertussis
PCR has optimal sensitivity during the first 3 weeks of cough when bacterial DNA is still present in the nasopharynx. After the fourth week of cough, the amount of bacterial DNA rapidly diminishes which increases the risk of obtaining falsely-negative results. For more information on diagnostic testing for pertussis, including the use of serology for late diagnosis, please reference: http://www.cdc.gov/pertussis/clinical/diagnostic.html.
PCR testing following antibiotic therapy also can result in falsely-negative findings. The exact duration of positivity following antibiotic use is not well understood, but PCR testing after 5 days of antibiotic use is unlikely to be of benefit and is generally not recommended.
[Note from SaneVax: This recent CDC notice to professionals was released because pertussis can be controlled by vaccination and early diagnosis. Although the CDC appears to recognize the need for a sensitive DNA test for the diagnosis of this infectious disease, they fail to mention Sanger DNA sequencing. If this technology were used to confirm the results of PCR tests and any pertussis DNA detected were compared with the GenBank sequence database, specificity of the PCR test would be assured. CDC officials should know this. The SaneVax Team wants to know why they do not mention it in their communications with health professionals. Is it too accurate? Is it possible that diagnostic testing can be too accurate?]