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Background: The current pertussis case classification system may underestimate the true burden of disease. Cases testing positive for pertussis by PCR but not meeting clinical case definition cannot be classified as either confirmed or probable and are therefore not reported to the CDC. Many such individuals likely represent true pertussis infections with clinical illness but are misclassified and therefore missing from state and national surveillance data.
Objectives: To compare select characteristics of PCR-positive “suspect” cases with those of confirmed cases.
Methods: We compared clinical symptoms, treatment, and outcomes of PCR-positive suspect cases to confirmed cases reported in Washington (WA) State from 2007-2011. Univariate analysis was done on the following variables available in the case report: chronic lung disease, pneumonia, acute encephalopathy, overnight hospitalization, ICU admission, death, two-week cough, and receipt of appropriate antibiotic.
Results: From 2007-2011, 2,254 confirmed cases and 309 suspect cases were reported. Of the suspect cases, 151 (49%) were PCR-positive. The respective proportions of confirmed and PCR-positive suspect cases with the following outcomes were: chronic lung disease, 8% vs. 4%; pneumonia, 5% vs. 3%; acute encephalopathy, 0.05% vs. 0%; overnight hospitalization, 7% vs. 7%; ICU admission, 1.7% vs. 1.6%; and case fatality, 0.2% vs. 0%. None of these differences were statistically significant. Nearly all confirmed and PCR-positive suspect cases were treated with an appropriate antibiotic for pertussis (94% vs. 99%, respectively). PCR-positive suspect cases received antibiotics sooner than confirmed cases (9 vs. 16 days, p<0.0001).
Conclusions: The confirmed and PCR-positive suspect cases reported in WA State during the past 5 years have similar clinical outcomes. PCR-positive suspect cases were treated with antibiotics significantly earlier than confirmed cases cases, which may have contributed to shorter cough duration. These findings suggest that the current case definition may result in an underestimation of the true burden of pertussis.