If only a small sample is obtained, it is most important to send for Gram stain and culture to rule out infectious arthritis

    About 1 to 2 mL is enough for Gram stain, culture, and wet prep for crystals

    Mucin clot and string test are physical tests of viscosity and inflammation, which are less reliable than laboratory analyses and are therefore not widely used

    For septic arthritis, a review by Carpenter et al¹ calls into question the long-standing practice of measuring fluid glucose and lactate dehydrogenase (LDH). Joint fluid lactate may be useful; however, the data is not conclusive.

    Although leukocyte count and differential will generally distinguish noninflammatory, inflammatory, and septic arthritides, leukocyte counts as low as 5,000 have been associated with early septic joints

    Rarely, crystal arthritis and septic arthritis can occur concomitantly

    Patients with sickle cell disease who are being evaluated for the possibility of joint infection are at increased risk for Salmonella species being the causative organism

    Patients who are active intravenous drug abusers are at risk for Pseudomonas species being the causative organism

    Patients with negative cultures and synovial fluid suspicious for inflammatory arthritis should be evaluated for Lyme disease, especially if they have a potential exposure within 1 year and a history of asymmetric, recurrent, and remitting joint pains (TABLE 61.2)