Culture - joint fluid

A sample of joint fluid is needed. This may be done in a doctor's office using a needle, or during an operating room procedure. Removing the sample is called joint fluid aspiration.

The fluid sample is sent to a laboratory. There, it is placed in a special dish and watched to see if bacteria, fungi, or viruses grow. This is called a culture.

If these germs are detected, other tests may be done to further identify the infection-causing substance and determine the best treatment.

Your health care provider will tell you how to prepare for the procedure. No special preparation is needed. But, tell your provider if you're taking a blood thinner, such as aspirin, warfarin (Coumadin) or clopidogrel (Plavix). These medicines can affect test results or your ability to take the test.

Sometimes, the provider will first inject numbing medicine into the skin with a small needle, which will sting. A larger needle is then used to draw out the synovial fluid.

This test may also cause some discomfort if the tip of the needle touches bone. The procedure usually lasts less than 1 to 2 minutes.

Your provider may order this test if you have unexplained pain and inflammation of a joint or a suspected joint infection.

The test result is considered normal if no organisms (bacteria, fungi, or viruses) grow in the laboratory dish.

Abnormal results are a sign of infection in the joint. Infections may include:

• Bacterial arthritis
• Fungal arthritis
• Gonococcal arthritis
• Turberculous arthritis

Risks of this test include:

• Infection of the joint -- unusual, but more common with repeated aspirations
• Bleeding into the joint space

El-Gabalawy HS. Synovial fluid analyses, synovial biopsy, and synovial pathology. In: Firestein GS, Budd RC, Gabriel SE, McInnes IB, O'Dell JR, eds. Kelly and Firestein's Textbook of Rheumatology. 10th ed. Philadelphia, PA: Elsevier; 2017:chap 53.

Karcher DS, McPherson RA. Cerebrospinal, synovial, serous body fluids, and alternative specimens. In: McPherson RA, Pincus MR, eds. Henry's Clinical Diagnosis and Management by Laboratory Methods. 23rd ed. St Louis, MO: Elsevier; 2017:chap 29.