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.
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.
A normal result means no bacteria are present on the Gram stain.
Normal value ranges may vary slightly among different laboratories. Some labs use different measurements or test different samples. Talk to your doctor about the meaning of your specific test results.
What Abnormal Results Mean
Abnormal results mean bacteria were seen on the Gram stain. This may be a sign of a joint infection, for example, gonococcal arthritis or arthritis due to Staphylococcus aureus.
There is no risk to the person associated with a Gram stain. For information regarding risks related to the removal of joint fluid, see joint fluid aspiration.
Gram stain of joint fluid
Matteson EL, Osmon DR. Infections of bursae, joints, and bones. In: Goldman L, Schafer AI, eds. Goldman's Cecil Medicine. 24th ed. Philadelphia, PA: Elsevier Saunders; 2011:chap 280.
Ohl CA, Forster D. Infectious arthritis of native joints. In: Bennett JE, Dolin R, Blaser MJ, eds. Mandell, Douglass, and Bennett's Principles and Practice of Infectious Diseases. 8th ed. Philadelphia, PA: Elsevier Churchill Livingstone; 2014:chap 105.
Review Date 11/24/2014
Updated by: Daniel Levy, MD, PhD, Infectious Diseases, Lutherville Personal Physicians, Lutherville, MD. Review provided by VeriMed Healthcare Network. Also reviewed by David Zieve, MD, MHA, Isla Ogilvie, PhD, and the A.D.A.M. Editorial team.