The diagnosis of deficiency of alpha-1 antitrypsin (A1AT) is based on isoelectric focusing of serum proteins and the extent of serum. However, the focusing is technically difficult and a greatly reduced concentration in abnormal A1AT tapeless does not differentiate an unstable variant of a variant called 'null' (that is to say without any phenotypic expression) to 'heterozygous' state. In this study, we compared the results of the assay, the phenotype and genotype of A1AT in 50 patients. Normal A1AT alleles (Pi*M1 to Pi*M4) or loss of the most common (Pi*S and Pi*Z) were clearly identified in phenotyping. However, genotyping was necessary to characterize: (i) certain alleles rarer A1AT (S-Munich, X-Christchurch); (ii) a null allele and; (iii) two new alleles A1AT not yet described in the literature. In conclusion, although the A1AT genotyping is generally not necessary, it is necessary to resolve complex cases and to obtain witnesses validated for isoelectric focusing.