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Make the Diagnosis: Pertussis, Adult or Adolescent

Prior Probability

In the absence of known pertussis outbreaks, adolescents and adults with persistent cough for more than 3 weeks have a 12%-32% prior probability of pertussis.1 During outbreaks, the prior probability would be higher.

Patients in Whom Pertussis Should Be Considered

Pertussis should be considered in patients with prolonged cough. Thus, the case definitions for cough are useful: coughs of <3 weeks duration are classified as acute, subacute coughs have 3-8 weeks duration, and chronic coughs last longer than 8 weeks. Pertussis infections go through catarrhal, paroxysmal, and chronic phases. The catarrhal phase lasts 1-2 weeks and appears like any other viral illness. The paroxysmal phase can begin during the second week of illness, and the hallmark is paroxysmal cough, with the patient seemingly well between paroxysms. The cough may be severe and associated with a whoop during vigorous inspirations. Some patients have such severe paroxysms that they may develop posttussive emesis or even syncope. The paroxysmal phase can last 2-3 months, and then the symptoms gradually abate into a chronic phase before resolution of all symptoms.

Assessing the Likelihood of Pertussis

Pertussis should be considered in patients with persistent coughing; however, the classical findings from descriptions of patients have limited diagnostic accuracy when taken in isolation. While no studies have evaluated combinations of findings, most experts probably consider disease more likely as the number of findings increase and less likely with fewer findings. The clinical impression becomes important because the Centers for Disease Control and Prevention (CDC) recommends that the threshold for laboratory testing and treatment should be the same because the disease is contagious.2, 3

Reference Standard Tests

The CDC clinical case definition for pertussis—used during outbreak investigations–is a cough illness lasting 2 weeks or longer without other apparent cause with 1 or more of the following: paroxysms of coughing, inspiratory whoop, or posttussive vomiting. Unfortunately, during outbreaks, these criteria can lead to overdiagnosis since a large proportion of typical viral cough illness lasts >2 weeks.4 There are several laboratory tests for diagnosing pertussis including culture (from the posterior nasopharynx), polymerase chain reaction, and pertussis toxin serologies. However, the timing of the test in relation to the clinical course and the technique used to obtain specimens may affect the results of these reference standard tests. Culture has the lowest sensitivity of these tests, but combining it with another reference standard test has been shown to be useful in studies.

Table 81-1Common Findings Used for Diagnosing Pertussis

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