The nitroblue tetrazolium test looks to see if certain immune system cells can change a colorless chemical called nitroblue tetrazolium (NBT) into a deep blue color.

Blood is typically drawn from a vein, usually from the inside of the elbow or the back of the hand. The site is cleaned with germ-killing medicine (antiseptic). The health care provider wraps an elastic band around the upper arm to apply pressure to the area and make the vein swell with blood.

Next, the health care provider gently inserts a needle into the vein. The blood collects into an airtight vial or tube attached to the needle. The elastic band is removed from your arm.

Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

In infants or young children, a sharp tool called a lancet may be used to puncture the skin and make it bleed. The blood collects into a small glass tube called a pipette, or onto a slide or test strip. A bandage may be placed over the area if there is any bleeding.

In the laboratory, the chemical NBT is added to a sample of white blood cells. The laboratory specialist looks at the cells under a microscope and determines if the NBT made them turn blue.

If your child is to have this test performed, it may be helpful to explain how the test will feel, and even demonstrate on a doll. Explain the reason for the test. Knowing the "how and why" may reduce the anxiety your child feels.

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

This test is done to screen for chronic granulomatous disease.

Normally, white blood cells called neutrophils make special oxygen compounds that kills bacteria. In chronic granulomatous disease, these compounds are missing. These compounds causes NBT to change from clear to deep blue. If they are missing, the white blood cells will not change color when NBT is added.

Normally, the white blood cells turn blue when NBT is added. This means that the cells are producing the special oxygen compounds needed to kill bacteria.

Talk to your doctor about the meaning of your specific test results.

If the sample does not change color when NBT is added, the white blood cells are missing the substance necessary to kill bacteria. This may be due to chronic granulomatous disease.

There is very little risk involved with having your blood taken. Veins and arteries vary in size from one patient to another and from one side of the body to the other. Taking blood from some people may be more difficult than from others.

Other risks associated with having blood drawn are slight 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)