ESR and C-reactive protein tend to elevate and fever often occurs with severe inflammatory arthritis, but these may also be caused by an inflammatory process elsewhere in the body. Soft-tissue swelling tends to favor an inflammatory process, but aspiration of an effusion is essential to determine its nature. Osteoarthritis of the knee, although primarily degenerative, often causes knee effusions. Preparation for handling the fluid obtained is critical. It is not necessary to perform all tests on each fluid.
Effusions can also be hemorrhagic.
So-called noninflammatory effusions are actually mildly inflammatory but tend to suggest diseases with less inflammatory mechanisms. If infection is suspected, a portion of the synovial fluid sample should be sent to the laboratory for bacteriologic assessment. Use of an inexpensive polarizer over the light source and another between the specimen and the examiner's eye will visualize crystals with a shiny white birefringence. Compensated polarized light is provided by inserting a first-order red plate, as is found in commercially available microscopes. The effects of a compensator can be reproduced by placing two strips of clear adhesive tape on a glass slide and placing this slide over the lower polarizer.