How is Alpha 1-antitrypsin deficiency diagnosed?
A diagnosis is established by demonstrating a very low level of alpha1 antitrypsin in the blood and by identifying the abnormal protein by specific testing. In normal individuals the protein is called "M" and in the deficient patient it is called "Z".
Determination of the A1AT serum level by quantitative immunoprecipitation is insufficient evidence for the diagnosis of A1AT deficiency. This is because measurement of serum levels may be falsely elevated because of the particularly robust acute-phase response of this protein. Therefore, determination of the quantitative level of A1AT must be combined with phenotypic analysis. This defines the phenotype of the variant PI proteins in the serum and is performed by isoelectric focusing. Patients with the most severe form of deficiency have an allelic variant that migrates to a higher isoelectric point and can be defined as PI ZZ phenotypes, and therefore by inference as PI ZZ genotypes. Interpretation of the electrophoretic patterns on isoelectric focusing will determine the homozygous or heterozygous states, and will define the specific mutant alleles based upon their relative position between anode and cathode. Finally, the molecular genetic tools for defining the defect in the nucleotide coding sequence for each of the defective alleles have been developed for population studies but are not routinely available in diagnostic laboratories.
In patients with manifestations of liver disease, liver biopsy for light microscopy and histochemistry and possible electron microscopy is valuable for staging liver disease and for identification of the PAS-positive/diastase-resistant globules within the hepatocytes. In neonates, the globules may be very indistinct and ill-developed, but they increase with age. In adult patients in particular, they may be associated with portal and periportal inflammation. Confirmation of the nature of the globules may be provided by immunohistochemical techniques, using immunoperoxidase coupled to A1AT antibody. Finally, the location of these globules within the endoplasmic reticulum may be confirmed by electron microscopy. |
