nitroblue tetrazolium test

The NBT test assesses the respiratory (oxidative) burst of phagocytes, which relies on the NADPH oxidase complex to generate superoxide (O2-) from oxygen using NADPH as an electron donor. This superoxide reduces nitroblue tetrazolium dye from a soluble yellow compound to an insoluble blue-black formazan precipitate, visible microscopically within activated neutrophils. In Chronic Granulomatous Disease (CGD), a genetic defect (most commonly X-linked, affecting the gp91phox subunit; autosomal recessive forms affect p22phox, p47phox, or p67phox) impairs NADPH oxidase activity, so phagocytes cannot generate superoxide. As a result, the NBT test shows no color change (negative test), indicating the diagnosis. Clinically, CGD patients suffer recurrent infections with catalase-positive organisms (e.g., Staphylococcus aureus, Serratia marcescens, Burkholderia cepacia, Aspergillus species) because these organisms neutralize their own hydrogen peroxide, which would otherwise be used by myeloperoxidase to kill them in the absence of a functional oxidative burst. Patients may also develop granulomas due to chronic inflammation from unresolved infections. A newer, more sensitive test for CGD is the dihydrorhodamine (DHR) flow cytometry assay, which has largely replaced the NBT test in many clinical settings due to better quantification and reproducibility. Understanding the NBT test is high-yield for Step 1 because it links innate immunity, phagocyte oxidative burst mechanisms, and the clinical presentation of CGD.