American Board of Internal Medicine (ABIM) Certification Practice Exam

What is the most appropriate diagnostic test for West Nile neuroinvasive disease?

1. Serum viral load testing

2. Identification of IgM antibodies in cerebrospinal fluid

3. Antibody testing in serum

4. Brain imaging

The correct answer is: Identification of IgM antibodies in cerebrospinal fluid

The most appropriate diagnostic test for West Nile neuroinvasive disease is the identification of IgM antibodies in cerebrospinal fluid (CSF). This choice is particularly relevant because West Nile virus can cause meningitis and encephalitis, which are forms of neuroinvasive disease. In such cases, the presence of IgM antibodies in the CSF indicates a central nervous system infection with the virus. Testing CSF is critical because it helps to confirm the diagnosis in cases of suspected West Nile virus neuroinvasion, especially when the patient is significantly symptomatic or has neurological deficits. IgM antibodies typically appear early in the course of the infection and persist, aiding in identifying recent infections specifically affecting the central nervous system. While other diagnostic methods exist, they may not provide the specificity needed in the context of neuroinvasive disease. Serum viral load testing is also not typically performed due to the variable nature of the viral load and its correlation with the timing of the patient’s illness. Antibody testing in serum, although useful, is less sensitive in the acute phase of the disease compared to CSF analysis. Lastly, brain imaging can be helpful in assessing structural changes or complications due to the infection but does not provide a definitive diagnosis of the West Nile virus itself.