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• Describes prevalence of A1AT variants at four testing centres in Canada. • Examines a period from 2011 to 2018. • Reflects the testing strategies used at these centres. • Provides basis of devising the optimum testing strategy. Laboratory confirmation of alpha-1-antitrypsin (A1AT) deficiency may be achieved by multiple methods. Here, we compare the relative comprehensiveness and efficiency of pathogenic variant (PV) detection of four different protocols utilized at different diagnostic centres in Canada. Diagnostic results from 2011 to 2018 at clinical laboratories in British Columbia (BC), Alberta (AB), Ontario (ON), and Québec (QC) were reviewed. The four labs utilize the following protocols: BC-CGID (serum A1AT Concentration/Genotyping/Isoelectric focussing (IEF)/ SERPINA1 DNA sequencing), AB-CID (serum A1AT Concentration/IEF/DNA sequencing), ON-CD (serum A1AT Concentration/DNA sequencing), and QC-G (Genotyping). As the respective catchment areas varied in size and ethnic composition, the comprehensiveness of PV detection was assessed by comparing the frequency of individual genotypes to the ZZ genotype, which is clearly identified by all protocols. Collectively 5399 index patients were tested identifying 396 ZZ genotypes. Serum A1AT concentration as a determinant of further testing efficiently identified PV. ON-CD had the highest detection rate for PV; genotypes with at least one PV, other than S, Z or F, were identified at 0.67/ZZ as compared to <0.2/ZZ (all others). However, ON-CD had the highest rates of undefined molecular variants (UMV) (0.16/ZZ) or likely benign variants (LBV) (0.08/ZZ), compared to all others (<0.12/ZZ and < 0.06/ZZ, respectively). The F variant was identified at 0.10/ZZ, only in the ON-CD and the AB-CID protocols. Collectively, M Malton was the next most common variant, identified as a compound heterozygous genotype at 0.04/ZZ, only in the ON-CD and BC-CGID protocols. Strategies which readily detect variants across the full coding sequence of SERPINA1 detect more PV as well as more UMV and LBV. [ABSTRACT FROM AUTHOR]