Protein nitrogen unit

The protein nitrogen unit (PNU) measures the potency of the compounds used in allergy skin tests, and is equivalent to 0.01 microgram (µg) of phosphotungstic acid-precipitable protein nitrogen.

According to Russ Rowlett at the University of North Carolina at Chapel Hill, 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.