Rocky Mountain spotted fever laboratory tests

Overview
A diagnosis of Rocky Mountain spotted fever is based on a combination of clinical signs and symptoms and specialized confirmatory laboratory tests. Other common laboratory findings suggestive of Rocky Mountain spotted fever include thrombocytopenia, hyponatremia, and elevated liver enzyme levels.

Laboratory Findings


Although it is technically feasible, specific rapid laboratory confirmation of early Rocky Mountain spotted fever is rarely done. Therefore, treatment decisions should be based on epidemiologic and clinical clues, and should never be delayed while waiting for confirmation by laboratory results. Fundamental understanding of the signs, symptoms, and epidemiology of the disease is crucial in guiding requests for tests for Rocky Mountain spotted fever, sample collection and submission, and interpretation of laboratory results.

Routine clinical laboratory findings suggestive of Rocky Mountain spotted fever may include abnormal white blood cell count, thrombocytopenia, hyponatremia, or elevated liver enzyme levels. Serologic assays are the most widely available and frequently used methods for confirming cases of Rocky Mountain spotted fever. The indirect immunofluorescence assay (IFA) is generally considered the reference standard in Rocky Mountain spotted fever serology and is the test currently used by CDC and most state public health laboratories, but other well validated assays including ELISA, latex agglutination, and dot immunoassays can be used.

IFA can be used to detect either IgG or IgM antibodies. Blood samples taken early (acute) and late (convalescent) in the disease are the preferred specimens for evaluation. Most patients demonstrate increased IgM titers by the end of the first week of illness. Diagnostic levels of IgG antibody generally do not appear until 7-10 days after the onset of illness. It is important to consider the amount of time it takes for antibodies to appear when ordering laboratory tests, especially because most patients visit their physician relatively early in the course of the illness, before diagnostic antibody levels may be present. The value of testing two sequential serum or plasma samples together to show a rising antibody level is very important in confirming acute infection with rickettsial agents because antibody titers may persist in some individuals for years after the original exposure to any of a number rickettsial agents. IgG antibodies are more specific and reliable since other bacterial infections can also cause elevations in riskettsial IgM antibody titers. The most rapid and specific diagnostic assays for Rocky Mountain spotted fever rely on molecular methods like PCR which can detect DNA present in 5-10 rickettsiae in a sample. While organisms can be detected in whole blood samples obtained at the acute onset of illness in a few hours, rickettsial DNA is most readily detected in fresh skin biopsies like those used in immunostaining procedures. PCR can also be done on the fixed tissues used in immunostaining, but it is less sensitive than with unfixed tissues. PCR methods can be R. rickettsii-specific but are usually confirmed by DNA sequencing of the amplified gene fragments. Consequently, this procedure is more specific than antibody-based methods which are often only genus or spotted fever group-specific. Specified diagnosis can also be confirmed by isolation of R. rickettsii from clinical samples like whole blood and biopsies. Materials can be shipped unfrozen or frozen and on dry ice to ensure optimal chances of isolation at the CDC. Isolation may require several weeks, but isolates are very important for investigating differences in the pathogenic properties and antimicrobial resistance of rickettsiae which cause disease in different parts of the United States.

Another approach to Rocky Mountain spotted fever diagnostics is immunostaining. This method is used by taking a skin biopsy of the rash from an infected patient prior to therapy or within the first 48 hours after antibiotic therapy has been started. Because rickettsiae are focally distributed in lesions of Rocky Mountain spotted fever, this test may not always detect the agent. Even in laboratories with expertise in performing this test, the sensitivity is only about 70% on biopsied tissues because of the scarcity of organisms in some samples. This assay may also be used to test tissues obtained at autopsy and has been used to confirm Rocky Mountain spotted fever in otherwise unexplained deaths (see figure). Immunostaining for spotted fever group rickettsiae is offered by the CDC, a few state health departments, and some university-based hospitals and commercial laboratories in the United States.