A sample of joint fluid is needed. This may be done in your health care provider'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 lab where a drop of fluid is spread in a very thin layer onto a microscope slide. This is called a smear. Several different colored stains are applied to the sample. The lab team member will look at the stained smear under a microscope to see if bacteria are present. The color, size, and shape of the cells help identify the bacteria.

Your 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, your provider will first inject numbing medicine into the skin with a small needle, which may 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.

The test is performed when there is unexplained swelling, joint pain, and inflammation of a joint, or to check for suspected joint infection.

A normal result means no bacteria are present on the Gram stain. Other tests may be done to help diagnose the problem.

Abnormal results mean bacteria were seen on the Gram stain. This may be a sign of a joint infection, for example, gonococcal arthritis due to bacteria called Neisseria gonorrhoeae or arthritis due to bacteria called Staphylococcus aureus.

Risks of this test include:

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

Gram stain of joint fluid

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. 24th ed. Philadelphia, PA: Elsevier; 2022:chap 30.

O'Neil L, Tanner S, El-Gabalawy HS. Synovial fluid analyses, synovial biopsy, and synovial pathology. In: Firestein GS, McInnes IB, Koretzky GA, Mikuls TR, Neogi T, O'Dell JR, eds. Firestein & Kelley's Textbook of Rheumatology. 12th ed. Philadelphia, PA: Elsevier; 2025:chap 53.

Updated by: Frank D. Brodkey, MD, FCCM, Associate Professor, Section of Pulmonary and Critical Care Medicine, University of Wisconsin School of Medicine and Public Health, Madison, WI. Also reviewed by David C. Dugdale, MD, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.