Completion of Paired Serological Testing Algorithms for Spotted Fever Rickettsiosis and Ehrlichiosis, North Carolina: 2017-2020.

Abstract

Ehrlichia and Rickettsia are tickborne pathogens capable of causing severe disease. Paired serological testing, involving both acute and convalescent samples, remains the primary method of diagnostic confirmation and source of surveillance data. Yet, few patients complete recommended testing algorithms. We examined the demographic, clinical, and geographic factors associated with obtainment of convalescent samples for patients with suspected ehrlichiosis and spotted fever rickettsiosis using results from a large academic center in North Carolina between 2017 and 2020. More than 4400 patients underwent serological testing of an acute sample for Rickettsia (N = 4224) and Ehrlichia (N = 2339); however, only 15.0% (662/4415) had testing performed on a convalescent sample. Over the study period, the proportion of convalescent testing completed increased from 4% to 23% for Ehrlichia, 7% to 11% for Rickettsia, and 12% to 28% for both. A reactive test on the acute sample, undergoing testing for both pathogens, and proximity to a health facility were significantly associated with obtainment and testing of a convalescent sample. The presence of a reactive acute titer for Ehrlichia and Rickettsia had 8.3 (95% confidence interval, 6.3-10.9) and 8.2 (95% confidence interval, 6.5-10.3) times the probability of obtainment of a convalescent sample compared to nonreactive results, respectively. Our findings suggest that clinicians' knowledge of tickborne disease testing practices, in addition to patient distance to health facilities, contribute to poor performance of testing completion. Moreover, these results highlight the need for more investment in public health surveillance and, ultimately, assays that are not dependent on convalescent testing.