Increasing Number of Pertussis Outbreaks Prompt CDC Health Alert

Document Helps Docs Avoid Inaccurate Polymerase Chain Reaction Results

February 23, 2011 04:15 pm News Staff

With outbreaks of pertussis ongoing in multiple parts of the country, the CDC issued a health alert(emergency.cdc.gov) on Feb. 16, reminding health care professionals of best practices for the use of polymerase chain reaction, or PCR, testing to diagnose the disease, which also is known as whooping cough.

The CDC said in its alert that health care professionals likely will see more patients with suspected cases of the resurgent disease. California had more than 8,300 reported cases of pertussis last year, and the California Department of Public Health said Jan. 7(www.cdph.ca.gov) that new cases still are being reported.

In Ohio, Columbus and Franklin counties had nearly 1,000 reported cases last year, and Columbus Public Health said Feb. 12 that the two counties had nearly 100 reported cases so far this year. Michigan had more than 1,500 reported cases(www.michigan.gov) last year, nearly five times more than in 2008.

According to the CDC's online pertussis resources(www.cdc.gov), culture is considered the "gold standard" for testing for pertussis, although serology also is acceptable. The agency noted, however, that PCR also is an important tool for timely diagnosis of pertussis, and it is increasingly available to clinicians.

PCR is a molecular technique used to detect DNA sequences of the Bordetella pertussis bacterium. Unlike culture, it does not require that live bacteria be present in the specimen. However, PCR can produce false-negative and false-positive results, according to the CDC.

The agency's best practices document is intended to help health care professionals optimize the use of PCR testing for pertussis and avoid common pitfalls that lead to inaccurate results.

Safety First, Cautions FP

Doug Campos-Outcalt, M.D., M.P.A., the AAFP's liaison to the CDC's Advisory Committee on Immunization Practices said polymerase chain reaction, or PCR, testing for pertussis is faster than culture, and some family physicians are unlikely to draw blood for serology when a respiratory sample can be used.

Campos-Outcalt, who is associate head of the department of family and community medicine at the University of Arizona College of Medicine, Phoenix, cautioned that physicians who use PCR should wear personal protective equipment, including gloves, mask and a face shield, because specimens are obtained by aspiration or swabbing the posterior nasopharynx. Each of those activities can produce sneezing and/or coughing, generating droplets and aerosolized particles.

"You're stimulating a cough," said Campos-Outcalt. "You have to protect yourself when you do this."

The CDC emphasized that proper testing criteria, timing of testing, specimen collection techniques, protocols for avoiding specimen contamination and appropriate interpretation of test results are all necessary to ensure that PCR reliably informs patient diagnosis. To that end, the agency offered the following recommendations:

  • Only patients with signs and symptoms(www.cdc.gov) of pertussis should be tested using PCR to confirm the diagnosis. Testing asymptomatic persons increases the likelihood of false-positive results. Asymptomatic close contacts of confirmed cases should not be tested, and testing of contacts should not be used for postexposure prophylaxis decisions.
  • Patients should be tested during the first three weeks of cough, if possible, when bacterial DNA is still present in the nasopharynx. After the fourth week of cough, the amount of bacterial DNA rapidly diminishes, increasing the risk of obtaining false-negative results.
  • PCR testing after five days of antibiotic use is unlikely to be of benefit because testing following antibiotic therapy also can result in false-negative findings.
  • Specimens should be obtained(www.cdc.gov) by aspiration or swabbing the posterior nasopharynx, rather than by throat swabs or anterior nasal swabs.
  • The use of liquid transport media likely contributes to false-positive results from contaminant DNA. Use of a semisolid or nonliquid transport media or transport of a dry swab without media should prevent contaminant DNA on the swab shaft from reaching the part of the specimen that is later extracted.
  • PCR assays for pertussis are not standardized across clinical laboratories. Testing methods, DNA targets used and result interpretation criteria vary, and laboratories do not use the same cutoffs for determining a positive result. Clinicians are encouraged to inquire about which PCR target or targets are used by their laboratories.
  • Clean gloves should be worn immediately before and during specimen collection, as well as during pertussis vaccine preparation and administration, and gloves should be disposed of immediately after these procedures. Clinic surfaces should be cleaned using a 10 percent bleach solution.

Some pertussis-containing vaccines -- Pentacel, Daptacel and Adacel -- have been found to contain PCR-detectable B. pertussis DNA, and environmental sampling has identified B. pertussis DNA from these vaccines in clinic environments. The CDC said accidental transfer of the DNA from environmental surfaces to a clinical specimen can result in specimen contamination and false-positive results.

However, the agency said there is no need to switch vaccines if clinicians prepare and administer vaccines in areas separate from pertussis specimen collection and take care to avoid contamination of surfaces when preparing and administering vaccines.


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