Figure

Figure. Flowchart describing use of testing to diagnosis ehrlichiosis within the University of North Carolina health system, March 24, 2022–April 14, 2023. CSTE criteria are described in (18). CSTE,...

A total of 945 patient samples were tested for ehrlichiosis during the ≈12-month period observed, 5 of which underwent Ehrlichia PCR testing. Among all patients tested, the most frequently recorded symptoms at the time of testing were myalgia (33.9%, n = 320) and headache (31.3%, n = 296) (Table 1). Of those, only 273 (28.9%) were classified as appropriately tested; the most common reasons for exclusion were the absence of documented fever in the EMR (97.9%, n = 658), patient complaint including only 1 symptom (15.8%, n = 106), a lack of any symptoms, or failure to document any symptoms in the EMR (10.4%, n = 70), or a combination. Of the patients who underwent PCR testing, 4 of 5 were not classified as appropriately tested because of the absence of fever. Of note, among the patients excluded, 30.5% reported a tick bite in the 2 weeks before the visit. Most of the appropriately tested patients (93.8%, n = 256) had not received doxycycline at the time of sample collection and therefore would have been eligible for a PCR test (Figure). However, only 1 patient meeting clinical criteria (0.4%) underwent PCR, whereas the remaining had serologic testing performed.

Among the 256 patients who were eligible for PCR, 69 (27.0%) had an acute titer result of >1:64. Only 47 (18.4%) patients completed convalescent testing. Among those who underwent convalescent testing, 6 (12.8%) demonstrated seroconversion and 12 (17.4%) who were positive at acute testing showed a 4-fold or greater titer increase. In addition, 5 (10.6%) reverted from a positive to a negative titer (Table 2). Of note, 106 (38.8%) of 256 patients had a negative acute Ehrlichia IgG titer and were started on antimicrobial drugs but did not return for convalescent blood testing. Similarly, 58 (22.7%) of 256 patients had a negative acute titer and neither received antimicrobial drugs nor returned for a convalescent test. Of the 47 patients who completed acute and convalescent Ehrlichia IgG testing, the median number of days between tests was 22 (interquartile range 17–32).

Of the 5 patients who had whole blood samples collected and tested by PCR, 1 test was positive. Attending physicians in the emergency department were responsible for ordering 3 of 5 PCR tests. In all 5 cases, serologic testing for Ehrlichia was also conducted. Three of the 5 had an acute but not a convalescent Ehrlichia IgG test, whereas 4 of 5 were classified as not appropriately tested because no fever was reported in the EMR (all had >1 other symptom in the CSTE criteria). However, the single positive PCR sample was from a patient whose clinical manifestations included a fever and several other consistent symptoms. The send-out PCR test took an average of 6.2 days to receive a result from the reference laboratory.

Overall, 12 (4.7%) of 256 patients who were eligible for PCR were classified as confirmed ehrlichiosis cases (11 by serology and 1 by PCR and serology); and 75 (29.3%) were classified as probable cases. However, on further review of cases that we defined as not appropriately tested, 228 (33.9%) of 673 patients also had 1 positive Ehrlichia spp. test, including 193 (28.7%) who had a positive acute IgG test and 90 (13.4%) who had a convalescent IgG test. Among those who returned for a convalescent test, 35 patients showed seroconversion. Of probable cases, 7 exhibited a 4-fold IgG increase from acute to convalescent titers, raising the total number of laboratory-confirmed cases to 48.

Our study of routine diagnostic testing practices demonstrated that most patients suspected of having ehrlichiosis did not undergo PCR testing when it was available only as a send-out test, despite the multiple advantages of this testing method. Instead, providers continued to rely on serologic testing, even though rates of diagnostic algorithm completion were low (18.4%). Because the lone star tick (which does not transmit Rickettsia rickettsii, the causative bacteria of Rocky Mountain spotted fever) is the primary vector in the state (19), it is very likely that ehrlichiosis, along with R. parkeri, accounts for most symptomatic, tickborne illness episodes (2). The limited understanding of the diagnosis and management of ehrlichiosis and the underuse of Ehrlichia PCR as a confirmatory assay is particularly concerning. Those issues underscore the need to educate providers at all levels of training and across specialties on the diagnosis and management of ehrlichiosis. In addition, improved uptake of PCR can increase knowledge of the true rate of ehrlichiosis cases in the US, specifically through a higher rate of confirmation and additional information regarding infecting species.

Several factors may explain the underuse of PCR testing. First, few health facilities have the capability or resources to perform an Ehrlichia PCR test in their own laboratories. At the time of this study, Ehrlichia PCR was only available at UNC Health as a referral test, with samples sent to Mayo Clinical Laboratories. Many providers may not have been aware that an Ehrlichia PCR test was available because it did not appear in the standard order menus. Instead, when ordering an Ehrlichia PCR, providers were required to order a generic referral test, which then launched a separate screen where the requested test can be entered as a free text. Ordering the test this way requires knowledge of the Ehrlichia PCR, the required sample type, and the receiving laboratory. In contrast, serologic testing, which is performed in-house, can be ordered by a simple search term lookup. This difference in processes might have affected ordering patterns, especially among nonspecialists who are not familiar with the test options. In addition, send-out PCR results still took nearly a week to return to the ordering provider. This timeline might have been insufficient for providers to perceive a benefit to guide treatment decisions, especially when other infections were on the differential diagnosis. Therefore, for optimal implementation, testing likely needs to be performed in-house, and the PCR order should be more prominent and easier to locate within routine test menus.

Whereas empirical antimicrobial drug treatment is recommended when there is a reasonable pretest probability of ehrlichiosis, the relatively fast turnaround of PCR, at least compared with paired serologic testing, offers a timely and objective result to confirm the diagnosis, thereby providing valuable information to both the patient and provider regarding the cause of illness. In our cohort, the number of patients for whom testing was ordered but were not empirically treated with doxycycline is concerning. Because the providers believed there was sufficient evidence to order an ehrlichiosis test, they also should have provided empirical treatment to patients. Without a convalescent blood test, an acute titer result cannot be used to either confirm or rule out ehrlichiosis. Positive PCR results can offer timelier confirmation of ehrlichiosis confirmation and subsequently ensure an appropriate treatment plan. Whereas treatment is ideally prescribed with clinical suspicion, findings show that this is not always the case, and a positive PCR may prompt more immediate action if otherwise not taken. Moreover, because convalescent testing has a much higher positivity rate than acute testing (61.7% vs. 27.0%) but most patients who receive a negative acute IgG titer fail to return for a convalescent titer, there are likely patients with ehrlichiosis who go undetected (2). Differing titer values in more than half of paired acute and convalescent samples indicate the convalescent test is clearly needed to interpret serologic results accurately, consistent with current guidelines (6). The relatively large proportion of paired titers of the same value implies prior infection and reinforces the importance of paired serologic specimens in areas where baseline seroprevalence is high.

Surprisingly, many patients tested for ehrlichiosis were considered not appropriately tested according to the 2007 CSTE criteria, primarily because of the absence of a subjectively reported fever as documented in the medical record. Yet, the number of laboratory-positive cases and seroconversion rates among patients without clinical evidence was notably higher than that for patients with clinical evidence (36 vs. 12). Those findings suggest that fever is not always a symptom associated with ehrlichiosis. It is also possible that fever is underrecognized because of antipyretic drugs used for headaches and pain control. Largely because of a lack of speciation with serodiagnostic testing, it is possible that infections with non–E. chaffeensis species, such as E. ewingii and E. muris eaclairensis, may exhibit different clinical manifestations than E. chaffeensis, which historically is the prototypical pathogen. Previous studies have identified E. ewingii in ticks across North Carolina, frequently in higher prevalence than E. chaffeensis, which has also been observed in other states (19,20). Those observations drove changes to the CSTE case definition, which removed fever as a required symptom.

Our study strengths include conduct in an area with high incidence of ehrlichiosis and the use of a robust electronic database. The first limitation of the study is its retrospective nature and that it relied on routine clinical records to determine clinical symptoms associated with manifestation. Second, unknown data, such as the presence of fever not being entered into a patient’s chart, might have resulted in the misclassification of patients. Third, the occurrence of and reliance on single serologic tests has major drawbacks, as described in this article. Fourth, there is potential uncertainty about the true disease state of the patients described in this study. Fifth, the study took place during the COVID-19 pandemic, when care-seeking patterns and diagnostic testing algorithms were disrupted. Finally, we do not have long-term outcomes to assess if participants experienced adverse clinical outcomes because of delayed or lack of antimicrobial drug administration resulting from incomplete testing.

In conclusion, our investigation revealed major underuse of molecular testing for ehrlichiosis in a disease-endemic area, despite the well-established issues associated with serologic testing. Our findings highlight the potential gains that can be made with increased uptake through both provider education and implementation of local testing. Molecular testing could provide information on infecting species, which could help clarify the clinical spectrum, epidemiology, and geographic distribution of different Ehrlichia species, and ultimately improve surveillance for this emerging disease and more accurately identify patients at risk of infection.