NBT test

Alternative names
Nitroblue tetrazolium test

Definition
This is a test that measures the ability of certain cells in the immune system to convert a colorless chemical, nitroblue tetrazolium (NBT), to a deep blue color.

How the test is performed

Blood is drawn from a vein, usually on the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and an elastic band is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the band to fill 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 tourniquet 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 (a type of white blood cell in your immune system that kill bacteria by ingesting them) 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.

How to prepare for the test
If your child is to have this test performed it may be helpful to explain how the test will feel, and even practice or demonstrate on a doll. The more familiar your child is with what will happen to them, and the purpose for the procedure, the less anxiety they will feel.

How the test will feel
When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.

Why the test is performed
This test is performed as a screen for CGD.

Normal Values

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.

What abnormal results mean

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.

What the risks are

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

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