American Board of Internal Medicine (ABIM) Certification Practice Exam

In joint fluid analysis, what leukocyte count typically indicates a bacterial infection?

1. 200/µL

2. >50,000/µL

3. 2000/µL

4. Less than 200/µL

The correct answer is: >50,000/µL

A leukocyte count greater than 50,000/µL in joint fluid analysis is a strong indicator of bacterial infection. In cases of septic arthritis, the synovial fluid typically shows a marked increase in the number of white blood cells, predominantly neutrophils, which reflects the body's inflammatory response to the bacterial pathogens present in the joint. This high leukocyte count is critical for differentiating bacterial infections from other conditions, such as viral infections or crystalline arthropathies, which generally present with lower leukocyte counts. In conditions other than bacterial infection, such as gout or pseudogout, the leukocyte count is usually much lower, often in the range of a few thousand or even lower. Thus, while a count of 200/µL or 2000/µL can occur in inflammatory conditions or non-bacterial infections, it is the count exceeding 50,000/µL that reliably signals a bacterial presence, guiding the clinician towards an urgent intervention and appropriate antibiotic therapy.