Joint fluid analysis can be done in your doctor's office, clinic, operating room, or emergency room. Depending on which joint will be examined, you may be asked to undress and put on a hospital gown. You will sit or lie down on an examining table.

Your doctor will examine the joint to find out where the needle should be inserted. Your doctor may use ultrasound to guide the needle placement. The skin over the joint area will be cleaned with antiseptic solution. A local anesthetic is often injected into the skin over the joint. For young children, a sedative may also be given.

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. Samples of the fluid may be put in special tubes or containers and sent to the lab. A cortisone shot may be given into the joint before the needle is removed. It can help keep fluid from building up again.

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.

How long the test takes

• The test will take about 20 minutes.

You will feel a prick or sting when the anesthetic is given. You may feel tingling, pressure, pain, or fullness in your joint as the fluid is removed.

There is very little chance of having a problem after a joint fluid analysis. Infection, bleeding, or damage to the tendon, nerve, or joint is rare.

Sometimes the doctor may not be able to draw any fluid out. The joint space may be too small, or you may have scar tissue in the joint space. Or there may not be any fluid in the joint.

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

Colour and clarity.

Slightly cloudy fluid may be caused by inflammation, gout, or pseudogout. A deep, dark red colour 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 coloured 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.