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Spontaneous NST test
Last reviewed: 04.07.2025
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A spontaneous test with NBT (nitroblue tetrazolium) allows one to evaluate the state of the oxygen-dependent bactericidal mechanism of blood phagocytes (granulocytes) in vitro.
Normally, in adults, the number of NBT-positive neutrophils is up to 10%.
It characterizes the state and degree of activation of the intracellular NADPH-oxidase antibacterial system. The principle of the method is based on the restoration of the soluble dye NBT absorbed by the phagocyte into insoluble diformazan under the influence of superoxide anion (intended for intracellular destruction of the infectious agent after its absorption), formed in the NADPH-oxidase reaction. The NBT test indicators increase in the initial period of acute bacterial infections, while they decrease in the subacute and chronic course of the infectious process. Sanitation of the body from the pathogen is accompanied by normalization of the indicator. A sharp decrease indicates decompensation of anti-infective protection and is considered a prognostically unfavorable sign.
The NBT test plays an important role in the diagnosis of chronic granulomatous diseases, which are characterized by the presence of defects in the NADPH oxidase complex. Patients with chronic granulomatous diseases are characterized by the presence of recurrent infections (pneumonia, lymphadenitis, lung, liver, and skin abscesses) caused by Staphylococcus aureus, Klebsiella spp., Candida albicans, Salmonella spp., Escherichia coli, Aspergillus spp., Pseudomonas cepacia, Mycobacterium spp. and Pneumocystis carinii.
Neutrophils in patients with chronic granulomatous diseases have normal phagocytic function, but due to a defect in the NADPH oxidase complex, they are unable to destroy microorganisms. Hereditary defects of the NADPH oxidase complex are in most cases linked to chromosome X, and less often autosomal recessive.
A decrease in the spontaneous test with NST is characteristic of chronic inflammation, congenital defects of the phagocytic system, secondary and primary immunodeficiencies, HIV infection, malignant neoplasms, severe burns, injuries, stress, malnutrition, treatment with cytostatics and immunosuppressants, and exposure to ionizing radiation.
An increase in the spontaneous test with NST is noted in case of antigenic irritation due to bacterial inflammation (prodromal period, period of acute manifestation of infection with normal phagocytosis activity), chronic granulomatosis, leukocytosis, increased antibody-dependent cytotoxicity of phagocytes, autoallergic diseases, and allergies.
Activated test with NST
Normally, in adults, the number of NBT-positive neutrophils is 40-80%.
The activated NBT test allows to evaluate the functional reserve of the oxygen-dependent mechanism of bactericidal action of phagocytes. The test is used to identify the reserve capabilities of the intracellular systems of phagocytes. With preserved intracellular antibacterial activity in phagocytes, there is a sharp increase in the number of formazan-positive neutrophils after their stimulation with latex. A decrease in the values of the activated NBT test of neutrophils below 40% and monocytes below 87% indicates insufficient phagocytosis.
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