Joint fluid analysis

For a joint fluid analysis, the doctor uses a needle to draw a small amount of fluid out of your joint so it can be looked at under a microscope. The test can be done in your doctor's office, a clinic, an operating room, or the emergency room.

This test is done to find inflammation, infection, gout, or pseudogout. Removing some of the joint fluid may also relieve pain caused by the buildup of fluid in your joint.

• You will sit or lie down on an examining table.
• After the joint area is cleaned, a local anesthetic may be injected into your skin to numb the area.
• A long, thin needle is slowly inserted in the joint area. A syringe attached to the needle is used to remove a sample of joint fluid.
• A cortisone shot may be given into the joint before this needle is removed.
• A tight (pressure) bandage will be placed over the site to reduce swelling and bruising. An elastic bandage may also be wrapped around your joint, such as your knee, to reduce swelling.

• In general, you don't need to prepare before having this test. Your doctor may give you some specific instructions.
• If you take a medicine that prevents blood clots, your doctor may tell you to stop taking it before your test. Or your doctor may tell you to keep taking it. (These medicines include aspirin and other blood thinners.) Make sure that you understand exactly what your doctor wants you to do.

The results of a joint fluid analysis are usually ready the same day. The results from a culture usually take a few days.

Abnormal values

Color and clarity.

Slightly cloudy fluid may be caused by inflammation, gout, or pseudogout. A deep, dark red color may be caused by bleeding in the joint. Milky white may be caused by infection or inflammation or a condition such as gout.

Blood cell count.

Large numbers of red blood cells may be caused by bleeding in the joint from injury, inflammation, or abnormal clotting of the blood. Large numbers of white blood cells may be caused by gout, pseudogout, other types of arthritis (such as rheumatoid arthritis), psoriatic arthritis, injury, or infection.

Presence of crystals.

Uric acid crystals in the joint mean you have gout. Calcium pyrophosphate crystals mean you have pseudogout.

Gram stain and culture.

Bacteria in the joint fluid that are causing an infection may be seen under a microscope after being colored with a Gram stain (a special dye). Joint fluid added to a substance that promotes the growth of germs (such as bacteria or a fungus) may show an infection. This is called a culture.