The methylene blue test is used to determine the type of the blood disorder, methemoglobinemia.

The health care provider wraps a tourniquet or blood pressure cuff around your upper arm. This creates pressure that causes veins below the area to fill with blood.

After cleaning the area with a germ killer (antiseptic), the person performing the test will place a needle into your vein, usually near the inside of the elbow or back of the hand. A thin tube, called a catheter, is then placed into the vein. (It may be called an IV, which means intravenous.) While the tube stays in place, the needle and tourniquet are removed.

A dark green powder called methylene blue goes through the tube into your vein. The health care provider looks at how the powder turns a substance in the blood called methemoglobin into normal hemoglobin.

No special preparation is required for this test.

When the needle is inserted, you may feel moderate pain or a stinging sensation. Afterward, there may be some throbbing.

There are several types of blood. One of them is methemoglobin. Normal methemoglobin levels in blood are usually around 1%. At higher levels, it can cause illness because it cannot carry oxygen. Because of its lower oxygen content, methemoglobinemia blood looks brown, rather than red.

Methemoglobinemia has several causes, many of which are genetic. This test is used to tell the difference between methemoglobinemia caused by the lack of an enzyme in the blood called cytochrome b5 reductase and other types that are passed down through families (inherited). Methemoglobinemia can also be caused by exposure to certain drugs, and chemical substances such as nitrates in water.

This test helps your health care provider develop a plan of care.

Normally, methylene blue rapidly lowers the levels of methemoglobin in the blood.

Normal value ranges may vary slightly among different laboratories. Talk to your doctor about the meaning of your specific test results.

If methylene blue does not significantly lower blood levels of methemoglobin, the health care provider will suspect a rare form of inherited methemoglobinemia.

Veins and arteries vary in size from one patient to another and from one side of the body to the other. Inserting an IV may be more difficult for you or your child than for other people.

Other risks associated with this type of blood test are minor, but may include:

• Excessive bleeding
• Fainting or feeling light-headed
• Hematoma (blood accumulating under the skin)
• Infection (a slight risk any time the skin is broken, but the chances of infection increase the longer the IV remains in the vein)