<prompt> Answer this question truthfullyWhat laboratory results indicate Alpha-1 antitrypsin deficiency?  <response>: <reason> A reduced concentration of serum alpha1-antitrypsin level is diagnostic of AATD. Laboratory findings consistent with the diagnosis of AATD include moderate-to-severe airflow obstruction with an FEV1 in the range of 30-40 of the predicted value, reduced vital capacity, increased lung volumes secondary to air trapping residual volume 120 of predicted value are usually present, diffusing capacity values are reduced substantially 50 of predicted value in most symptomatic patients. Serum alpha1-antitrypsin levels are determined by nephelometry. Serum testing is used for diagnostic testing in those patients with family histories compatible with the alpha1-antitrypsin deficiency or with siblings with known alpha1-antitrypsin deficiency. In patients with clinical features that are highly suggestive of alpha1-antitrypsin deficiency but whose serum levels are within the reference range the next best step is to perform a functional assay of alpha1 antiprotease, which measures the ability of the patients serum to inhibit human leukocyte elastase. 