Joint fluid gram stain is a method of identifying bacteria using a special series of stains. In this test, a specimen of joint fluid is stained and examined under the microscope.

The gram stain method can be applied to almost any specimen and is one of the most commonly used techniques for the rapid diagnosis of bacterial infections.

The skin around the joint is cleaned, a needle is inserted into the joint, and fluid is aspirated (drawn out).

A drop of the joint fluid is then applied in a very thin layer to a microscope slide -- this is called a smear. A series of stains called a gram stain is applied to the specimen. It is first stained with crystal violet stain, then iodine, then decolorized, then stained with safranin.

The stained smear is then examined under the microscope for the presence of bacteria. The color, size, and morphologic appearance (shape) of the cells help identify the infecting organism.

This test may be performed in the health care provider's office.

The joint is cleansed with an anti-bacterial soap. An anesthetic may be injected into the joint, and a stinging sensation may be felt. A needle is then inserted into the joint, and a small amount of fluid is withdrawn into a syringe. Pressure and pain in the joint may be present.

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

No organisms are present.

Abnormal results -- bacteria on the gram stain -- may indicate infection of the joint (for example, gonococcal arthritis). The type of bacteria seen can guide the doctor's choice of antibiotics.

There is a small risk of infection or bleeding.

Other tests may be performed on the joint fluid (see synovial fluid analysis).