Normal neutrophils--the body's primary defense against bacterial invasion--engulf and destroy bacteria and foreign particles by a process known as phagocytosis. In patients who have repeated bacterial infections, neutrophil function tests may reveal the inability of neutrophils to kill a target bacteria or to migrate to the bacterial site (chemotaxis).
Neutrophil killing ability can be evaluated by the nitroblue tetrazolium (NBT) test, which relies on neutrophil generation of bactericidal enzymes and toxins during killing. This action results in increased oxygen consumption and glucose metabolism, which reduces colorless NBT to blue formazan. The reduced dye is then extracted with pyridine and measured photometrically; the level of reduction indicates phagocytic activity.
Neutrophil killing activity can also be evaluated by noting the neutrophils' chemiluminescence--ability to emit light. After a neutrophil phagocytizes a microorganism, oxygen-containing substances form within the phagocytic vacuoles. As the cell is stimulated, it emits light in proportion to the amount of oxygen-containing substances that are formed, providing an indirect measurement of phagocytosis.
Chemotaxis can be assessed in vitro by placing bacteria in the lower half of a two-part chamber and phagocytic neutrophils in the upper half. After incubation, migrating cells are counted microscopically and compared to standard values."