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 drawn from a vein, usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic. An elastic band is placed around the upper arm to apply pressure and cause the vein to swell with blood.

A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the band is removed to restore circulation. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop 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 a chemical that kills bacteria. In chronic granulomatous disease, this chemical is missing. The chemical that kills the bacteria is the same one that causes NBT to change from clear to deep blue. If the chemical is 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 chemical necessary to kill bacteria.

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

Risks associated with having blood drawn are slight:

• Excessive bleeding
• Fainting or feeling light-headed
• Hematoma (blood accumulating under the skin)
• Infection (a slight risk any time the skin is broken)
• Multiple punctures to locate veins