Nitroblue tetrazolium test measures the ability of certain cells in the immune system to convert a colorless chemical, nitroblue tetrazolium (NBT), to 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, NBT is added to a sample of white blood cells. The neutrophils (white blood cell in your immune system that ingest and kill bacteria) normally make a chemical that kills the bacteria. In chronic granulomatous disease (CGD), this chemical is absent. As a result, neutrophils are able to ingest bacteria, but not kill them.

The chemical that kills the bacteria is the same that turns NBT from clear to deep blue. If the chemical is absent when NBT is added to the sample, it will not change color. This can be seen by looking at the white blood cells under an ordinary microscope.

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 performed as a screen for CGD.

Normally, the sample and the neutrophils within it turn blue when NBT is added. This indicates that the neutrophils are producing the chemical necessary to kill bacteria.

If the sample does not change color when NBT is added, the neutrophils are missing the chemical necessary to kill bacteria. This may indicate CGD.

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