The emergency joint: arthrocentesis and synovial fluid analysis.

Arthrocentesis and the subsequent evaluation of synovial fluid is often the definitive diagnostic procedure for the patient presenting with a joint effusion or intrasynovial hemorrhage. The difficulty of performing arthrocentesis varies with the joint in question, but those joints most frequently involved are easily entered. The indications and contraindications for this procedure are discussed. Effusion-producing pathologic processes often yield fluids of a characteristic nature permitting their classifications into categories of noninflammatory (Group I), inflammatory (Group II), septic (Group III), and hemorrhagic. This categorization of the effusion may permit specific diagnosis or the narrowing of the differential diagnosis. Criteria are established on the basis of joint fluid features to differentiate septic arthritis, which requires inpatient treatment, from those entities for which the patient may reasonably be treated as an outpatient.