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Allergen unit

Pollen unit, also known as the "Noon" unit was defined as the amount of material extracted  from 0,001 mg of pollen (1:1 w/v=1 million pollen units). Noon's definition is extended to include the amount of material extracted from 0,0001 mg of any allergen (rather than pollen only).Various so-called biological units have been proposed in recent years. As long as standards of quality and reliability are quaranteed, there is no need to change the units with which the physician has become familiar, unless a clearly better system can be introduced, reflecting a real standardization, free of methodological or extract specific characteristics. Such an improvement is offered by the system of allergy units, developed by the Laboratory of Allergenic Products of the Food and Drug Administration (F.D.A.) in the U.S.A.

The Allergy Unit (AU) represents a unit value based on skin test reactivity. A consequence of assigning AUs to allergenic extracts is that non-related allergenic  extracts, which have been assigned the same strength in units, will show the same allergenicity in standard patient populations with regard to skin tests.

In short, the method of standardization in AUs implies that reference preparations are characterized by quantitative intracutaneous skin testing. By measuring the dilution which produces a 50mm sum of erythema diameter response (D50) in intradermal skin testing, the allergenic activity of any extract can be defined. The dilution at which the D50 is obtained directly relates to the potency of the extracts in AUs. Once AUs are assigned in this way to the reference extract, any extract obtained from the same source can be assigned AUs by an assay of relative potency in either an in vivo (skin) test or an in vitro (RAST inhibition) potency test.

References

Cordes, B.G. , et al: Preparation of treatment vials for RAST based immunotherapy. In Fadal R.G., Nalebuff, D.J. (eds): RAST in clinical allergy, ed. 2, Corlsbad, C.A., Symposia Foundation, 1989, 348.