Reduced circulating alpha-1-antitrypsin concentration (Human Phenotype Ontology id HP_0032025) is a reduced concentration or activity of circulating alpha-1 antitrypsin, which is a 52-kDa glycoprotein mainly synthesized and secreted by hepatocytes into the bloodstream. Alpha-1 antitrypsin is a serine-proteinase inhibitor that it is crucial in maintaining protease-antiprotease homeostasis in the lungs. Also known as: Reduced serum alpha-1-antitrypsin.