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Uric Acid (Synovial Fluid)

Does this test have other names?

Synovial fluid analysis 

What is this test?

The uric acid test measures levels of uric acid that can collect in joint fluid. Uric acid is a normal bodily waste product. It forms when chemicals called purines break down. Purines are a natural substance found in the body and are also found in many foods such as liver, shellfish, and alcohol. They can also be formed in the body when DNA is broken down. 

When purines are broken down  to uric acid in the blood, the body gets rid of  it when you urinate or have a bowel movement. But if your body makes too much uric acid, or if your kidneys aren't working properly, uric acid can build up in the blood. Uric acid levels can also increase when  you eat too many high-purine foods or take certain medications like diuretics, aspirin, and niacin. Then crystals of uric acid can form and collect in the joints, causing painful inflammation. This condition is called gout.

If you have gout, you may have crystals of uric acid in your synovial fluid, the substance that surrounds joints to help them move smoothly. 

Why do I need this test?

You may need this test if you have symptoms of gout:

  • Joint pain or tenderness

  • Swelling in a joint or reddened skin around a joint

  • Swelling and pain in the big toe, ankle, or knee

  • Joints that are hot to the touch

  • Swelling and pain that affects only one joint in the body

  • Skin that looks shiny and is red or purple

These symptoms often develop overnight. You may wake up in the morning with sudden gout symptoms. 

What other tests might I have along with this test?

Your doctor may also order blood and urine tests to measure uric acid levels. Higher than normal levels of uric acid in the blood or urine can suggest gout, but the only way your doctor can definitively diagnose the condition is by measuring the levels of uric acid in your synovial fluid.

What do my test results mean?

Many things may affect your lab test results. These include the method each lab uses to do the test. Even if your test results are different from the normal value, you may not have a problem. To learn what the results mean for you, talk with your health care provider.

If your synovial fluid sample shows uric acid crystals, you may have gout. But even if your sample doesn't show uric acid crystals, you still may have gout. Crystals don't always form in the synovial fluid during a gout attack. 

How is this test done?

This test requires a sample of synovial fluid. It's collected during a process called joint aspiration. To collect the fluid, your doctor inserts a needle into the skin near an inflamed joint and withdraws some of the surrounding fluid into a vial or tube.

Does this test pose any risks?

Joint aspiration has some minor risks. You may experience bleeding in the area around the joint. Although rare, an infection can develop in the joint from the test. 

What might affect my test results?

Certain medications may affect your test results, including:

  • Aspirin and other drugs that contain salicylate

  • Cyclosporine, a medication sometimes used for autoimmune diseases

  • Levodopa, a drug used to treat Parkinson's disease

  • Certain diuretic medications such as hydrochlorothiazide

  • Niacin, or vitamin B3

Other factors that may affect your test results include:

  • Vigorous exercise

  • Chemotherapy or radiation therapy to treat cancer

  • Foods high in purines, including organ meats, mushrooms, some types of fish and seafood, and dried peas and beans 

How do I get ready for this test?

Ask your doctor about what to do before having this test. You may need to avoid eating, drinking, or taking certain medications on the day of the test. Be sure your doctor knows about all medicines, herbs, vitamins, and supplements you are taking. This includes medicines that don't need a prescription and any illicit drugs you may use.