Definition of Joint fluid Gram stain:

Joint fluid Gram stain is a laboratory test to identify bacteria in a sample of joint fluid using a special series of stains (colors). The Gram stain method is one of the most commonly used methods to rapidly diagnose bacterial infections.

How the test is performed:

A sample of joint fluid is needed. For information on how this procedure is done, see joint fluid aspiration.

The fluid sample is sent to a lab where a small drop is placed in a very thin layer onto a microscope slide. This is called a smear. Several different colored stains are applied to the sample. The laboratory personnel 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.

How to prepare for the test:

For information on how to prepare for the removal of joint fluid, see joint fluid aspiration.

How the test will feel:

For information on how it will feel when the joint fluid is removed, see joint fluid aspiration.

Why the test is performed:

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