Alpha-1-antitrypsin testing algorithm. The first step is to measure the level in serum by radial immunodiffusion or nephelometry. If the serum level is below 11 μ M or 50 mg/ml (by nephelometry) further analysis by either isoelectric focussing or genetyping is warranted. Very low serum levels or levels below the detection limit of the serum assay suggest the presence of a Null variant (no production by the hepatocytes, and therefore distinct from the Z deficiency). A Null variant may be further analysed by isoelectric focussing and/or gene sequencing (as described in Fregonese et al., Respir Med; in press).