What Is It?
Uric acid is a waste product that is removed from the body by the kidneys. However, blood levels of uric acid may rise when too much uric acid is produced in the body, too little is removed by the kidneys, or because of a combination of both.
Diseases Associated with an Elevated UricAcid
An elevated uric acid (also called hyperuricemia) is associated with gout, a form of arthritis that has afflicted humans at least since the ancient Greeks. It develops because uric acid crystals deposit in the joint causing inflammation. For uncertain reasons, women do not get gout until 10 to 15 years after menopause. Men of any age can get gout though it is more common among older persons.
Gout is marked by episodes of sudden, severe arthritis (joint inflammation) lasting a few days or up to a week or more. The first toe is most commonly involved, but it is also common in other toes, the mid-foot, ankle or knee. In fact, almost any joint can be affected, including the hands. Sometimes lumps of gout crystals form, causing small white nodules below the skin (called tophi), most commonly on the outside of the ear, fingers, or near joints in the foot. Most people with gout have a high uric acid, but sometimes it is normal (though usually at the upper range of normal). It is especially rare for a person with gout to have a low uric acid level in the blood.
Uric acid kidney stones (also called nephrolithiasis) can form when uric acid crystals deposit in the kidney or in the ureters, the tubes connecting the kidneys to the bladder. Symptoms include severe pain in the mid or lower back, fever, shooting pain into the groin and blood in the urine.
Diseases or conditions associated with an elevated uric acid include:
Although a high blood uric acid level is common in people with gout or kidney stones, many (or even most) people who have an elevated uric acid do not develop either condition. A low level is rare in the setting of gout, but a normal or high level cannot establish the diagnosis. The best way to be sure about the diagnosis of gout is to remove a sample of the joint fluid (or other involved area such as a bursa or a suspected lump of gout under the skin) and examine it under a special microscope (called a polarizing microscope). Similarly, if a person has a kidney stone, catching the stone and submitting it to the laboratory for chemical analysis is the best way to determine whether uric acid is the culprit.
Why Is It Ordered?
Doctors usually order a blood uric acid level when they are concerned about gout or kidney stones. For example, if you had sudden, severe arthritis in the first toe, your symptoms and examination might suggest gout to your doctor and checking your uric acid might be appropriate. On the other hand, if your symptoms and examination are not highly suggestive of gout, the uric acid results might not be particularly helpful—most of the time, uric acid levels are normal or high and either result is compatible with gout.
A blood test for uric acid may be ordered as a prognostic test to assess the risk of future attacks of gout or to see whether medicines that lower uric acid are working adequately to prevent another attack.
If you develop a uric acid kidney stone, blood and urine measures of uric acid are common to confirm the adequacy of treatment and risk of future stones.
Obstetricians often check the uric acid during normal pregnancy because it tends to be quite low; a higher than expected uric acid level during pregnancy could indicate a complication, such as pre-eclampsia.
What Does the Result Mean?
How Uric Acid Is Reported
The uric acid is reported as a number of milligrams (mg) in a deciliter (dl)of blood. For example, a normal blood uric acid might be reported as 4.3 to 8.0 mg/dl.
Interpreting the Result
The meaning of any uric acid level depends on why it was ordered. For example, a healthy person who feels well may have a high, low or normal uric acid without any particular significance; even high values are not typically treated or cause for concern if there is no associated disease or condition. For this reason, measuring uric acid in healthy people is not recommended.
If a person has possible gout and a high (or even high-normal) value of uric acid, it supports the diagnosis but does not confirm it. For someone with known gout, the level of the uric acid may predict the chances of recurrent attacks – the higher the value, the more likely it is that additional attacks of gout will follow. On the other hand, low to low-normal levels of uric acid (for example, less than 4.0 mg/dl) make gout rather unlikely. That is also true for people with gout who take medicines to lower the uric acid – if it is low enough, repeated attacks are very unlikely.
If it was ordered to monitor the treatment to lower the uric acid (such as allopurinol), a low result means that the medication is working well at the current dose; if the uric acid is high (or at the higher end of normal) despite urate-lowering therapy, it means the medicine is not working well enough; consideration should be given to increasing the dose or adding or changing treatment.
For people with sudden attacks of arthritis with joint pain and swelling, the best way to be sure about the diagnosis is to sample the fluid and to analyze it; measuring uric acid in the blood is a more indirect way to evaluate the situation and the results may not be helpful.
Finally, for people with kidney stones, catching the stone and getting a chemical analysis is a better way to figure out the cause than measuring blood uric acid. However, the appearance of the stone on x-rays and other blood and urine tests can be helpful, regardless of the uric acid level. Sometimes it is impossible to determine the type of kidney stone; in that case, the blood and urine levels of uric acid can be useful to argue for or against a uric acid stone as the cause.
American College of Rheumatology
Last Updated October 2004
Copyright 2004, Harvard Medical School, Harvard University.