A 57-year-old man presents with a swollen right knee

Test the fluid for bacterial cultures.

Educational Objective:

Characterize appropriate strategies for joint fluid analysis.

Key Point:

Joint fluid analysis is a critical step in analyzing possible septic arthritis, and even a minimal amount of joint fluid can provide suitable culture media.

Explanation:

Culturing of joint fluid is the definitive test to diagnose bacterial arthritis. In situations in which only a few drops of fluid are obtained, it is imperative to establish priorities for diagnostic testing. The most important diagnostic test is a bacterial culture and should be the top priority. If there is an adequate amount of fluid, then a Gram stain and smear, cell count, and crystal analysis would be useful. Even a scant amount should be sent for culture. Reattempting the aspiration after it was properly performed will probably not produce a better yield. Empiric antibiotic coverage should not be delayed while waiting for test results.

References:

Kellerman RD, et al, eds. Conn’s Current Therapy. 2018.

Walls R, et al. Rosen’s Emergency Medicine: Concepts and Clinical Practice. 9th ed., 2018.