SERPINA1 and alpha 1-antitrypsin deficiency: Severe AAT deficiency, defined as an AAT serum level less than 35% of the mean expected value, 50 mg/dL (measured by nephelometry), 11 μM, or 80 mg/dL (measured by radial immunodifusion, although this is now an obsolete technique), is usually associated with Pi*ZZ genotypes, and less frequently with combinations of Z, S, and about 45 “rare” or null alleles.