Useful diagnostic information can be obtained from synovial fluid (joint fluid) removed from a joint using a needle and syringe. The procedure, called a joint aspiration, can be performed in a doctor's office. The procedure is also referred to as synovial fluid analysis, joint tap, or arthrocentesis and is used to find out what is causing joint pain or joint swelling.
Removing or draining the joint fluid may actually have a side benefit of relieving any pressure or pain it is causing at the joint. A joint aspiration can be performed at the same time as a joint injection.
Joint aspiration and injection is easier, and therefore more common, in some joints. Knees are easy to aspirate and inject. Shoulders, elbows, wrists, thumbs, small joints of the hands and feet, and ankles may also be aspirated and injected. Hip joints, which are less easy to access, may need to be done via a guided procedure using x-ray or ultrasound for that purpose.
The joint fluid specimen is examined microscopically for bacteria, crystals, and blood cells. Sometimes levels of glucose, protein, and LDH (lactic dehydrogenase) are measured. The color and clarity of the joint fluid is also observed and reported.
Parameters found on a normal joint-fluid report include a clear to light yellow appearance, no or few blood cells, no crystals present, and no bacteria seen microscopically or grown in culture. In a normal report, protein will be less than 3 g/dl, glucose will be greater than or equal to 40 mg/dl, and there will be an LDH level of 105–333 IU/L.
Abnormal joint-fluid results have a cloudy (indicative of infection or inflammation) — or bloody (indicative of bleeding) appearance. Abnormal results also have large numbers of blood cells (red blood cells would suggest bleeding in the joint while white blood cells suggest gout, pseudogout, inflammatory forms of arthritis, injury, or infection). A large number of white cells would go against injury, while a large number of red cells would favor an injury. Crystals will also be present (uric-acid crystals are indicative of gout; calcium pyrophosphate dihydrate crystals are indicative of pseudogout) and bacteria will be seen microscopically or grown in culture (indicative of infection). A protein level of greater than or equal to 3 g/dl (which suggests inflammation or infection) represents abnormal results, as does a glucose level of less than 40 mg/dl (suggests inflammation or infection). The LDH level will be greater than 333 IU/L (rheumatoid arthritis, gout, or infection are suspected with high LDH in joint fluid, but normal LDH level in blood).
Differential white blood-cell counts can be done on joint fluid. The predominance of certain types of white cells can help distinguish a septic joint or an inflammatory type of arthritis from a crystal-induced arthropathy.