Culture - joint fluid (aspirate)

Alternative names
Joint fluid culture

Joint fluid culture is a laboratory test to isolate and identify organisms that cause infection, performed on a specimen of fluid obtained from fluid around a joint. See also Synovial fluid analysis.

How the test is performed

A sample of joint aspirate or fluid is removed from the joint with a needle or in the operating room, then is placed on culture media in containers for the purpose of growing microorganisms (bacteria, fungus, or viruses) in the laboratory.

The microbiologist in the laboratory inspects the cultures daily for growth. If microorganisms are detected, other tests may be initiated to identify the infectious agent and determine the sensitivity of the organisms to medications. Antibiotic therapy can then be determined based on these results.

How to prepare for the test
A physician will withdraw fluid from a joint.

Infants and children:
The preparation you can provide for this test depends on your child’s age, previous experiences, and level of trust. For general information regarding how you can prepare your child, see the following topics:

How the test will feel

The sample may be obtained in the health care provider’s office or in the hospital. The skin over the joint is cleansed with antibacterial soap. A local anesthetic may be injected into the skin over the joint. The injection may burn or sting.

After the area has become numb, a larger needle is inserted into the joint, and fluid is withdrawn. There may be pressure in the joint. Some minor soreness may be present for several days.

Why the test is performed
The test may be performed when there is unexplained pain and inflammation of a joint, or suspected infection in a joint.

Normal Values
The presence of no organisms in the culture (a sterile culture) is normal.

What abnormal results mean
Abnormal results indicate an infection in the joint. The infection may be bacterial, mycobacterial, or fungal. The causative organism and an effective medication to treat the organism are often indicated in the final results.

Other associated conditions:

  • Fungal arthritis  
  • Non-gonococcal (septic) bacterial arthritis  
  • Gonococcal arthritis

What the risks are
The risks are introducing an infection or bleeding in the joint.

Special considerations
Not applicable.

Johns Hopkins patient information

Last revised: December 2, 2012
by Arthur A. Poghosian, M.D.

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