What is the role of a nitroblue tetrazolium (NBT) test in the workup of pediatric chronic granulomatous disease (CGD)?

Updated: Aug 07, 2019
  • Author: Lawrence C Wolfe, MD; Chief Editor: Cameron K Tebbi, MD  more...
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Answer

The following tests are indicated in chronic granulomatous disease (CGD):

  • Nitroblue tetrazolium (NBT) test

    • The standard assay for phagocytic oxidase activity is the NBT test. The colorless compound NBT is reduced to blue formazan by the activity of the phox enzyme system. Several versions of the test exist; each has advantages and disadvantages.

    • The most efficient and informative version is the NBT slide test, in which a drop of whole blood is placed on a microscope slide coated with an activating agent, such as lipopolysaccharide or phorbol ester. Phagocytes adhering to the slide are activated and develop blue inclusions on incubation with NBT. The number of NBT-positive cells is scored under a microscope. This test is often preferred because of the small amount of blood required and the lack of a need for specialized equipment. Although the result is nonquantitative, an experienced technologist can differentiate normal phagocytes reliably from low-level phox activity observed in some cases of p47 deficiency.

    • The NBT test can be useful in identifying X-linked carrier female individuals when peripheral phagocytes consist of 2 cell populations: one that reduces NBT to formazan and one that does not.

    • The NBT is limited by its subjectivity, need for experienced technician, and false-negative results that cause the diagnosis of chronic granulomatous disease to be missed. False-negative findings occur when formazan accumulates in cells with low levels of active adenine dinucleotide phosphate (NADPH) oxidase. These patients clinically have the disease, but their NBT test results are negative.

    • In an alternative technique, leukocytes are isolated from blood and incubated with NBT in a test tube. Formazan is solubilized by addition of an organic solvent, and the blue color intensity is read by a spectrophotometer.


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