A colorimetric microassay for opsonins by reduction of NBT in phagocytosing bovine polymorphs.

The adaptation of the reduction of nitro blue tetrazolium (NBT) by phagocytes to the measurement of the opsonising capacity of serum or milk whey is described. A dose-response curve of the absorbance of solubilised reduced NBT to the concentration of opsonins, along with a close correlation (r = 0.94) between the colorimetric NBT assay and ingestion of group B streptococci (as measured by reduction of [3H]thymidine incorporation by extracellular bacteria), was established. This NBT reduction test, employing 96-well flat-bottom microtitre plates, is simple, semiautomated, requires a low number of PMN, and is applicable to a large number of samples, which renders it useful as a screening test for the opsonising capacity of sera or other biologic fluids.