Measuring and Interpreting Serum AAT Concentration.

Deficiency of alpha-1 antitrypsin (AAT) is caused by mutations in the SERPINA1 gene that results in low concentrations of AAT in circulation. The low AAT concentration can result in uninhibited neutrophil elastase activity in the lung, leading to pulmonary tissue damage and lung disease. Clinical evaluation for possible AAT deficiency includes two critical components: measuring AAT concentration in serum and identification of AAT deficiency alleles. In this chapter the methods by which AAT concentration can be measured in the clinical laboratory are described. The two most common methodologies for AAT quantification employ immunometric techniques, specifically nephelometry and turbidimetry, which are both based on light scatter technology. The AAT in the patient sample is combined with an anti-AAT polyclonal antibody solution leading to polymer formation and a proportional amount of subsequent light scatter. Descriptions of each method are presented, and specifics of quality control and assay parameters are discussed. A special discussion focuses on interpretation of results in the context of the different AAT genetic phenotypes and in the context of patients with active inflammatory conditions. Emerging techniques for AAT quantitation by mass spectrometry are also described given that both AAT quantitation and allele identification can be performed on the same assay.