The opsonic index is a numerical measure of potency of a given individual's serum to opsonize foreign material - bacteria - ready for phagocytosis.
It is calculated by dividing the number of bacteria in blood per phagocyte when an immune response has been mounted compared to that previously. Each figure is an average for identical unit volumes or smear counts.
Immunisation increases the opsonic index.
Last reviewed 01/2018