Several types of laboratory tests are commonly used for the diagnosis of Bordetella pertussis. Culture is considered the gold standard because it is the only 100% specific method for identification. Other tests that can be performed include polymerase chain reaction (PCR) and serology.
Since culture has excellent specificity, it is particularly useful for confirming pertussis diagnosis when an outbreak is suspected. Many other respiratory pathogens have similar clinical symptoms to pertussis and co-infections do occur. Furthermore, obtaining isolates from culture allows for strain identification and antimicrobial resistance testing. Identifying which strains of B. pertussis are causing disease is of public health importance. Culture is best done from nasopharyngeal (NP) specimens collected during the first 2 weeks of cough when viable bacteria are still present in the nasopharynx. After the first 2 weeks, sensitivity is decreased and the risk of false-negatives increases.
PCR is a rapid test and has excellent sensitivity. PCR tests vary in specificity, so obtaining culture confirmation of pertussis for at least one suspicious case is recommended any time there is suspicion of a pertussis outbreak. Results should be interpreted along with the clinical symptoms and epidemiological information. PCR should be tested from NP specimens taken at 0-3 weeks following cough onset, but may provide accurate results for up to 4 weeks. After the fourth week of cough, the amount of bacterial DNA rapidly diminishes, which increases the risk of obtaining falsely-negative results. PCR assay protocols that include multiple target sequences allow for speciation among Bordetella species. The high sensitivity of PCR increases the risk of false-positivity, but following some simple best practices can reduce the risk of obtaining inaccurate results.
CDC and FDA have developed a serologic assay that has been extremely useful for confirming diagnosis, especially during suspected outbreaks. Many State Public Health Labs have included this assay as part of their testing regimen for pertussis. Commercially, there are several different serologic tests used in United States with unproven or unknown clinical accuracy. CDC is actively engaged in better understanding the usefulness of these commercially available assays. This website will be updated as we learn more about these commercial assays. Generally, serologic tests are more useful for diagnosis in later phases of the disease. For the CDC single point serology, the optimal timing for specimen collection is 2 to 8 weeks following cough onset, when the antibody titers are at their highest; however, serology may be performed on specimens collected up to 12 weeks following cough onset.
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