Ehrlichial infections pose difficult diagnostic challenges to both clinicians and laboratorians, and the availability of confirmatory assays is limited. Therefore, treatment decisions should be based on epidemiologic and clinical clues, and should never be delayed while waiting for confirmation. Similarly, test results should be interpreted in the context of the patient’s illness and the epidemiologic setting. Problems arise from overuse of specialized tests for patients with a low probability of the disease and in areas with a low prevalence of disease. Fundamental understanding of the signs, symptoms, and epidemiology of the disease is crucial in guiding requests for tests for ehrlichiosis and interpretation of testing results.  Routine clinical laboratory tests indicative of ehrlichiosis include low white blood cell count, low platelet count, and elevated liver enzymes.  The organisms can be demonstrated in blood smears by staining with Diff-Quik or Giemsa stains (Figure 18).

Laboratory confirmation of ehrlichiosis requires serologic, molecular, or culture-based methods. Serologic evaluations are conducted by using the indirect immunofluorescence assay (IFA). Antibodies in the serum bind to the organisms on a slide and are detected by a fluorescein-labeled conjugate (Figure 17). Although IFA remains the principal diagnostic tool for the detection of ehrlichial infection, there is no standardized antigen, conjugate, or agreement on what constitutes a positive result among the various laboratories providing these tests. Individual laboratories should be consulted as to their test threshold levels.  Blood specimens taken early (acute) and late (convalescent) in the disease course represent the preferred specimens for evaluation. Relatively few data describe the kinetics of IFA-detectable antibodies for the types of ehrlichiosis. Most patients demonstrate increased IgM or IgG titers by the second week of the illness. However, patients may lack diagnostic IgG antibody titers in the first 7 days of illness. This is an important consideration, because patients seek health care at a median of 3½ days after onset of the illness. The period for which anti-ehrlichial antibodies persist is also poorly defined. In some persons, high titers have been observed for longer than 2½ years after the acute illness.  Most reference laboratories conduct testing for IgG; however, IgM-specific testing may provide helpful evidence of ehrlichial infection.

Antibodies reactive with one Ehrlichia species can be cross-reactive with other species of Ehrlichia. The diagnosis of HGE is based on seroreactivity to any of several strains of granulocytic ehrlichiae. However, serologic cross-reactivity between different genogroups is also observed and may hamper epidemiologic distinction between the ehrlichial infections. It is possible that some serologically confirmed cases of infection thought to be caused by E. chaffeensis or the HGE agent may in fact represent infections with the other agent or with another, antigenically related ehrlichial species. Although cross-reaction with E. canis antigen was used to identify the initial cases of E. chaffeensis infection, this antigen is no longer appropriate for serodiagnosis.

After serologic methods, amplification of the ehrlichial DNA by polymerase chain reaction (PCR) is the next most frequently used method for detecting infection. This test is available through CDC and some state health laboratories, as well as a number of research and commercial laboratories.  PCR tests remain unstandardized, and analytical and diagnostic sensitivity and specificity may vary among individual assays. In HGE, the organism has been detected by PCR from the blood of clinically ill patients 3-5 weeks following the onset of symptoms. In persons infected with E. chaffeensis, ehrlichial DNA has been detected by PCR from febrile, untreated patients as long as 7 weeks into the illness. 

Direct isolation of the organism remains the gold standard for confirmatory diagnosis, but is the most difficult and time-consuming approach. Both E. chaffeensis and the HGE agent have been recovered from the blood of acutely ill patients by using a variety of cell lines, most frequently canine DH82 (Figure 18) and human HL-60 cells, respectively. Ehrlichia chaffeensis has typically been observed within 7-36 days in culture; the HGE agent may be seen within 7-12 days after inoculation of cells with patient blood. EDTA-treated blood obtained prior to the administration of antibiotic therapy is the most common specimen for isolation attempts, as well as the most frequent sample used for PCR analysis. Samples should be processed as quickly as possible, although successful recovery of viable ehrlichiae may result from samples refrigerated at 4^oC for as long as 1 week.

Figure 18.  Diff-Quik Stain of Ehrlichia chaffeensis in DH82 cells, 1000X

New techniques, including enzyme immunoassays using recombinant ehrlichial antigens and fluorometric PCR, are currently under investigation, and these tests may eventually have broader application in public health laboratories.