Rocky Mountain Spotted Fever - Diagnosis

What tests or methods were used to detect and diagnose your case of RMSF?

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How is Rocky Mountain spotted fever diagnosed?

Physicians should consider RMSF in a patient who has fever, headache, and other compatible symptoms, especially if the person has recently visited an area where the disease is known to occur. Because the disease is caused by ticks, it is seasonal and should be considered in months when ticks are active. A history of tick exposure is helpful, but it is important to remember that most ticks do not carry R. rickettsii and that many tick bites go unnoticed. The rash may appear late in the course of disease or be very mild, so it is important to think of the disease even if the rash is not present. If the facts of the case make the diagnosis appear to be likely, treatment should not be delayed to wait for the rash or results of tests.

RMSF is diagnosed through several methods. Using immunohistochemistry, a small piece of skin can be mixed with a special stain that allows the organism to show up under the microscope. Because the presence of the organism in skin may be patchy, a negative biopsy does not rule out the disease.

Blood can be tested for antibodies to the organism using techniques known as immunofluorescence (IFA) or enzyme immunoassays (ELISA or EIA). If a high concentration of antibody is found, it can be presumed that disease is present. It may take a week for antibodies to appear, so a negative titer does not completely rule out the disease. Blood can also be tested for antibodies at the onset of illness and again several weeks later to see if there is a substantial change in the titer. Other tests, such as the polymerase chain reaction (PCR) to detect the genetic material of the bacteria are available in research laboratories or from the U.S. Centers for Disease Control and Prevention (CDC) although they have not been standardized for general use.

R. rickettsii is difficult to culture and requires living host cells to grow. It is also dangerous to grow, because there have been cases of laboratory technicians getting sick while performing the culture. Only laboratories that are certified to handle this level of biohazard should attempt to culture the organism.

Most other laboratory findings are nonspecific, meaning that they cannot be used alone to make the diagnosis. White blood cell counts may be normal or increased slightly, or anemia may be present. Platelet counts may be decreased, especially in severe cases. Serum sodium levels are low in about half of patients, and liver enzyme tests in the blood may be elevated. If kidney failure occurs, the serum creatinine will be increased. If neurological symptoms occur, the spinal fluid may show increased numbers of white cells or an increased concentration of proteins.

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