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.