Pertussis laboratory findings
Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] ; Associate Editor(s)-in-Chief: Aditya Govindavarjhulla, M.B.B.S. [2]; Rim Halaby, M.D. [3]; Luke Rusowicz-Orazem, B.S.
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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.
PCR
- PCR is a rapid test with excellent sensitivity; however, PCR tests can vary in specificity and it is therefore recommended to obtaine 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.
- 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.[2]
Serology
- The CDC and FDA have developed a serologic assay that has been extremely useful for confirming pertussis diagnosis, especially during suspected outbreaks.[2]
- Many State Public Health Labs have include this assay as part of their testing regimen.
- Serologic tests are generally more useful for diagnosis in later phases of the disease:
- 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.
- Serology is often used to determine whether antibody against pertussis toxin or another component of Bordetella pertussis is present at high levels in the blood of the patient in the case of adolescents and adults that do not often seek medical care until several weeks into the illness.[2]
Depicted below is an image of the optimal timing for the different tests used for the diagnosis of pertussis.
