Mechanism of the heparin effect on the nitroblue-tetrazolium slide test

The nitroblue-tetrazolium (NBT) slide test has been applied widely to assist in the diagnosis of bacterial infection. The test involves mixing a small amount of heparinized blood with a solution of NBT and noting the percentage of neutrophils that contain reduced NBT after a fixed period of incubation. Although it has been recognized that anticoagulants can influence the scores obtained, the mechanism of this effect has not been widely appreciated. We have observed that particles are formed when NBT and heparin are mixed, and that these particles are ingested by human neutrophils. Major changes in human neutrophil metabolism result from minor changes in the relative concentrations of heparin and NBT. These observations may explain many discrepancies that have been reported on the NBT slide test by various investigators.