Which of the following laboratory tests is considered the gold standard for diagnosing pertussis?

The gold standard for diagnosing pertussis is nasopharyngeal PCR (polymerase chain reaction). This test is highly sensitive and specific for detecting the presence of Bordetella pertussis DNA in the nasopharyngeal secretions of patients suspected of having whooping cough. PCR allows for the rapid and accurate identification of the bacteria, especially during the early stages of the illness when culture results might not be available, and the patient may have already begun antibiotic treatment.

In contrast, the other listed options lack the specificity and sensitivity required for an accurate diagnosis of pertussis. A complete blood count can provide supportive information, such as lymphocytosis, which is often seen in pertussis but is not diagnostic. A Tzanck smear is utilized for herpes virus infections and has no relevance to pertussis. An X-ray of the chest may reveal complications or associated conditions in severe cases of pertussis but does not confirm the presence of Bordetella pertussis. Thus, nasopharyngeal PCR effectively meets the criteria for a gold standard diagnostic test in this context.