PNU

   abbreviation for protein nitrogen unit, a measure of the potency of the compounds used by doctors in allergy skin tests. One PNU is defined as 0.01 microgram (µg) of phosphotungstic acid-precipitable protein nitrogen. Unfortunately, the potency measurements depend on the technique of measurement used, so results of one manufacturer are not comparable to those of another manufacturer. As a result, although PNU's are still used, they are being replaced by bioequivalent allergy units (BAU), which are measured by actual skin testing using reference preparations of standard potency.