BACKGROUND: Serology testing is increasingly used to diagnose pertussis, despite the lack of a US Food and Drug Administration (FDA)-approved, commercially available assay. Positive results are not confirmatory for pertussis nor included in the Council for State and Territorial Epidemiologists (CSTE) pertussis case definition, and therefore do not prompt investigation in many states. We examined the clinical characteristics of patients with positive pertussis serology reported to the Georgia Department of Public Health (DPH) to determine how many met the clinical case definition necessary to be considered a probable pertussis case.
METHODS: All suspect cases with positive serology (pertussis IgM, IgA, or IgG antibodies) and cough onset between January 1, 2014 and September 1, 2016 were identified in surveillance data. Clinical characteristics, demographics, and vaccination histories were captured via patient and physician interviews. Patients that met the CSTE pertussis clinical case definition and were serologically positive, but not culture or PCR positive, were classified as probable; serology positive patients that did not meet the clinical case definition were considered not-a-case. Chi-square test was used to compare proportions; p <0.05 was considered significant.
RESULTS: During the study period, 214 serology positive patients were reported to DPH; 127 (59.3%) met the pertussis clinical case definition and were classified as probable cases. Of these, 71 (55.9%) were IgM positive, 65 (51.2%) IgG positive and 50 (39.4%) IgA positive – with some having multiple serology results. Most cases were among adults, with 110 (86.6%) ≥ 20 years of age. Thirty-eight (29.9%) cases reported whoop, 112 (88.2%) reported paroxysms, and 51 (40.2%) reported post-tussive vomiting. The proportion that reported paroxysms was significant among IgG positive cases compared to IgM positive cases (p=0.03). IgM positive cases were more likely to report a whoop compared to IgA positive cases (p=0.01). There was no significant difference in symptomology between IgG and IgA positive cases. Of the 127 cases, 32 (25.2%) had serum collected within 2 weeks of cough onset; 38 (29.9%) 2-4 weeks after cough onset; and 57 (44.9%) > 4 weeks after cough onset.
CONCLUSIONS: Physicians may be unaware of the appropriate laboratory tests required to confirm pertussis, especially in adults. However, the use of serology testing indicates that pertussis was a suspected etiology. Our findings suggest that positive serology results for pertussis may be useful in the identification of patients warranting public health investigation and deserves further study.