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The protein nitrogen unit measures the potency of the compounds used in allergy skin tests, and is equivalent to 0.01 microgram of phosphotungstic acid-precipitable protein nitrogen. Potency measurements depend on the measurement technique, so that results from different manufacturers cannot be reliably compared: as a result, PNUs are being replaced by bioequivalent allergy units , which are measured by skin testing using reference preparations of standard potency.
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