Pertussis laboratory findings
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] ; Associate Editor(s)-in-Chief: Serge Korjian M.D.; Aditya Govindavarjhulla, M.B.B.S. [2]; Rim Halaby, M.D. [3]; Luke Rusowicz-Orazem, B.S.; Yazan Daaboul, M.D.
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Overview
Several laboratory tests may be used to diagnose pertussis. Culture, obtained by nasopharyngeal swab, is considered the gold standard for diagnosis. Other tests that can be performed include polymerase chain reaction (PCR) and serology.
Laboratory Findings
Nasopharyngeal Swab
- Whenever possible, a nasopharyngeal swab or aspirate should be obtained from all suspected pertussis patients.[1]
- The specimen obtained via the nasopharyngeal swab can be used both for culture and polymerase chain reaction (PCR).
- If a culture is planned:
- The nasopharyngeal swab should be plated directly or placed into transport media.[1]
- It should be dispensed and plated within 24 hours of collection.
Culture
- Since cultures have high specificity, they particularly useful in confirming the pertussis diagnosis when an outbreak is suspected.[2]
- Obtaining isolates from a culture allows for strain identification of Bordetella pertussis and antimicrobial resistance testing.
- Culture is best done from nasopharyngeal specimens collected during the first 2 weeks of cough when viable bacteria are still present in the nasopharynx.
- The bacteria can be recovered from the patient only during the first three weeks of illness, rendering culturing useless after this period.
- Despite high specificity, the sensitivity of a culture is low and the risk of false-negatives increases after the first 2 weeks.
Polymerase Chain Reaction (PCR)
- PCR is a rapid test with excellent sensitivity; however, PCR tests can vary in specificity. It is therefore recommended to obtain culture confirmation for at least one case for any suspicion of a pertussis outbreak.
- PCR should be tested from nasopharyngeal 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 false-negative results.
- PCR assay protocols that include multiple target sequences allow for speciation of 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.[2]
Serology
- Serology tests for high concentrations of antibodies against the pertussis toxin or other components of Bordetella pertussis
- It is more useful among patients who do not seek medical care until several weeks into the illness.[2]
- The CDC and FDA have developed a serologic assay that has been extremely useful for confirming pertussis diagnosis, especially during suspected outbreaks.[2]
- Several state public health labs have included this assay in their testing regimen.
- Serologic tests are generally 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.
- Serology may still be performed on specimens collected up to 12 weeks following cough onset.