American Board of Internal Medicine (ABIM) Certification Practice Exam

What is the best method for diagnosing Rocky Mountain spotted fever at the time of acute infection?

1. Culturing Rickettsia rickettsii

2. Immunohistochemistry or PCR of a skin biopsy

3. Serological testing for antibodies

4. Diagnosis based on clinical features alone

The correct answer is: Immunohistochemistry or PCR of a skin biopsy

The best method for diagnosing Rocky Mountain spotted fever (RMSF) at the time of acute infection is through immunohistochemistry or PCR of a skin biopsy. This approach is advantageous because it allows for the direct detection of the causative agent, Rickettsia rickettsii, in the tissue. During the acute phase of the illness, the presence of the bacteria can often be confirmed via these techniques, particularly when skin lesions are present, as the bacteria replicate and are found within endothelial cells of small blood vessels. Culturing Rickettsia rickettsii is challenging due to the fastidious nature of the organism and the risk of contamination; therefore, it is not typically used in acute diagnosis. Serological testing for antibodies, while useful for determining a past infection, may not provide reliable results during the acute phase because antibody levels take time to rise after infection. Additionally, relying on clinical features alone can lead to misdiagnosis, as symptoms can overlap with other conditions. Thus, immunohistochemistry or PCR offers the most immediate and specific diagnostic yield during acute infection.