Protein nitrogen unit precipitation procedure for allergenic extracts: collaborative study

Protein nitrogen unit (NU) determination is one of the methods used to test and label the concentration of allergenic extracts. This recently standardized method is applicable to all allergenic extracts. One PNU/mL is equivalent to 1 X 10(-5) mg nitrogen determined to be in the material precipitated from 1 mL allergenic extract by phosphotungstic acid (PTA), a protein precipitant. The nitrogen is quantitated by the Kjeldahl method or another analytical method of equivalent accuracy and precision. A collaborative study of the optimized PNU precipitation method in which 6 samples were analyzed in duplicate by 6 laboratories using the Kjeldahl method for the determination of nitrogen yielded a mean of 0.1358 mg N/mL, a repeatability standard deviation and coefficient of variation of 0.0071 mg N/mL and 5.23%, respectively, and a reproducibility standard deviation and coefficient of variation of 0.0188 mg N/mL and 13.84%, respectively. The method has been adopted official first action.