The clarity of the fluid is assessed by experienced clinicians in the syringe but more reliably can be done by expressing a small amount of fluid out of the plastic syringe into a glass tube. (Plastic tubes have a slight opacity that may confuse the results; see Plate 5-14). Cloudy fluid is not typical of uncomplicated osteoarthritis and suggests an inflammatory process. An opaque, pasty fluid is most often due to pus, thus indicating the presence of infection, but a thick, purulent-appearing fluid occasionally results from massive numbers of crystals, amyloid, or, in rheumatoid arthritis, from degenerated synovial villi (rice bodies). Most infections do not produce pus; cloudy nonopaque fluids can also be due to infection.

Bloody fluid in the joint (hemarthrosis) suggests numerous diagnostic possibilities, including trauma (with or without fracture), pigmented villonodular synovitis, tumors in or near the joint, hemangioma, severe joint destruction (i.e., neuropathic), hemophilia, and, rarely, other bleeding disorders.

MICROBIOLOGIC CULTURES AND LABORATORY STUDIES

If joint infection is suspected, the aspirated fluid should be promptly transported to the microbiology laboratory. Some studies have suggested increased yield with the use of blood culture vials, but different laboratories have different collection guidelines. Unless there is specific clinical concern for mycobacterial, anaerobic, or fungal infection, it is appropriate and cost effective to send the initial fluid only for routine bacterial cultures. There is no purpose in sending the fluid for protein or glucose. Determination of lactate dehydrogenase in the fluid is not usually of value. Polymerase chain reaction may be beneficial when specific infections are suspected.

MICROSCOPIC EXAMINATION

Crystal-induced arthritis is definitively diagnosed only from examination of joint fluid for the presence of intracellular or extracellular crystals (2 to 20 µm). Occasionally, crystals are also found in tissue or tophi. One or two drops of fluid are expressed onto a clean slide, which is promptly covered with a coverslip. Fluid on a slide can be preserved for a few hours, or a fresh drop preparation can be taken for each examination from fluid kept in a tube. The fluid should ideally be examined promptly because small numbers of calcium pyrophosphate dihydrate (CPPD) crystals may dissolve overnight and, with time, artifactual urate-like structures can form from degenerating cells; but this is not likely a common problem. Anticoagulants in collecting tubes are generally avoided because they may also produce confusing artifacts.