An ANA test detects antinuclear antibodies (ANA) in your blood. Your immune system normally makes antibodies to help you fight infection. In contrast, antinuclear antibodies often attack your body's own tissues — specifically targeting each cell's nucleus.

In most cases, a positive ANA test indicates that your immune system has launched a misdirected attack on your own tissue — in other words, an autoimmune reaction. But some people have positive ANA tests even when they're healthy.

Your doctor is likely to order an ANA test for a suspected autoimmune disease such as lupus, rheumatoid arthritis or scleroderma.

Many rheumatic diseases have similar signs and symptoms — joint pain, fatigue and fever. While an ANA test can't confirm a specific diagnosis, it can rule out some diseases. And if the ANA test is positive, your blood can be tested for the presence of particular antinuclear antibodies, some of which are specific to certain diseases.

An ANA test requires a sample of your blood. If your sample is being used only for an ANA test, you can eat and drink normally before the test. If your blood sample will be used for additional tests, you might need to fast for a time before the test. Your doctor will give you instructions.

Certain drugs affect the accuracy of the test, so bring your doctor a list of the medications you take.

For an ANA test, a member of your health care team takes a sample of blood by inserting a needle into a vein in your arm. The blood sample is sent to a lab for analysis. You can return to your usual activities immediately.

The presence of antinuclear antibodies is a positive test result. But having a positive result doesn't mean you have a disease. Many people with no disease have positive ANA tests — particularly women older than 65.

Some infectious diseases and cancers have been associated with the development of antinuclear antibodies, as have certain drugs.

If your doctor suspects you have an autoimmune disease, he or she is likely to order a number of tests. The result of your ANA test is one piece of information your doctor can use to help determine the cause of your signs and symptoms.