From the Guidelines
The most effective way to test for Rocky Mountain spotted fever (RMSF) is through a combination of serologic tests, including the indirect immunofluorescence antibody (IFA) test, and molecular tests such as PCR, with treatment initiated immediately if RMSF is suspected, without waiting for laboratory confirmation. The IFA test is considered the gold standard for diagnosing RMSF, but it requires paired serum samples taken 2-4 weeks apart to show a fourfold rise in antibody titers 1. PCR testing of whole blood or skin biopsy specimens can detect rickettsial DNA during early infection, making it useful in the first week of illness 1. Immunohistochemical staining of skin biopsies from rash sites can also directly visualize the bacteria in tissue samples.
Key Diagnostic Tests for RMSF
- Indirect immunofluorescence antibody (IFA) test: Requires paired serum samples for confirmation
- PCR testing of whole blood or skin biopsy specimens: Useful in early infection
- Immunohistochemical staining of skin biopsies: Directly visualizes bacteria in tissue samples
- Complete blood count: May show thrombocytopenia, hyponatremia, and elevated liver enzymes, supporting diagnosis but not specific
It's crucial to understand that treatment should never be delayed while waiting for test results, as RMSF can rapidly become life-threatening 1. Doxycycline should be started immediately when RMSF is suspected, even before test confirmation, and continued for at least 3 days after fever resolves, with a minimum course of 5-7 days. The clinical features of RMSF, including fever, rash, and reported tick bite, are rarely all present when patients first seek care, emphasizing the need for early empiric therapy based on suspicion rather than waiting for diagnostic confirmation 1.
Clinical Considerations
- Early empiric therapy is crucial to prevent RMSF progression and morbidity
- Delay in recognition and treatment is a significant factor associated with risk for death from RMSF
- Clinical features like thrombocytopenia, increased immature neutrophils, elevations in hepatic transaminase levels, and hyponatremia may be present but are not specific for RMSF 1.
From the Research
Diagnostic Tests for Rocky Mountain Spotted Fever
The diagnostic tests for Rocky Mountain spotted fever (RMSF) include:
- Laboratory evaluation to demonstrate hyponatremia, anemia, thrombocytopenia, abnormal liver enzymes, and elevated coagulation tests 2
- Antibody testing, which can be helpful but results are not typically available to the emergency clinician 2
- Immunofluorescent demonstration of Rickettsia in cutaneous lesions, which can provide an immediate, positive laboratory diagnosis 3
- Weil-Felix test, hemagglutination, complement fixation, and skin biopsy immunofluorescence, with skin biopsy being the most sensitive and specific method 4
- Serodiagnosis using IgM and IgG enzyme-linked immunosorbent assays (ELISA) and indirect fluorescent antibody test, which can detect antibody increases in paired sera and seroconversions in vaccinees 5
- Empirical administration of doxycycline, which can be started based on suspicion of RMSF due to symptoms and laboratory values 6
Laboratory Evaluation
Laboratory evaluation is important in the diagnosis of RMSF, with tests including:
- Complete blood count to demonstrate anemia and thrombocytopenia
- Electrolyte panel to demonstrate hyponatremia
- Liver function tests to demonstrate abnormal liver enzymes
- Coagulation tests to demonstrate elevated coagulation times
Immunofluorescent Demonstration
Immunofluorescent demonstration of Rickettsia in cutaneous lesions is a sensitive and specific method for diagnosing RMSF, with a sensitivity of 70% and specificity of 100% 4
Serodiagnosis
Serodiagnosis using IgM and IgG ELISA and indirect fluorescent antibody test can detect antibody increases in paired sera and seroconversions in vaccinees, with a diagnostic agreement between ELISA and IFA test of 76% and 52% for IgG and IgM antibody, respectively 5