In a patient suspected of having pertussis, what laboratory test would be the gold standard for diagnosis?

The gold standard for diagnosing pertussis, or whooping cough, is polymerase chain reaction (PCR) of nasopharyngeal swabs. This method is highly sensitive and specific, allowing for the detection of the Bordetella pertussis bacteria's DNA. Early in the course of the illness, PCR testing can yield positive results, which is crucial since treatment is most effective when administered early.

While throat culture can also identify the organism, it is less sensitive than PCR, particularly in the first two weeks of illness when the bacterium is more likely to be present. Complete blood counts and rapid antigen tests provide additional information but do not serve as definitive diagnostic tools for pertussis. PCR testing's rapid turnaround and increased accuracy make it the preferred method for confirming a suspected case of this respiratory infection.