Received on May 2, 2006
Accepted on September 27, 2006

Evidence-Based Laboratory Medicine and Test Utilization

Diagnosis of -1-Antitrypsin Deficiency: An Algorithm of Quantitation, Genotyping, and Phenotyping

¹ Division of Clinical Biochemistry & Immunology, Department of Laboratory Medicine & Pathology, Mayo Clinic College of Medicine, Rochester, MN
² Division of Clinical Biochemistry & Immunology, Department of Laboratory Medicine & Pathology, Division of Hematology, Department of Internal Medicine, Mayo Clinic College of Medicine, Rochester, MN
³ Division of Laboratory Genetics, Department of Laboratory Medicine & Pathology, Mayo Clinic College of Medicine, Rochester, MN
⁴ Department of Health Sciences Research, Mayo Clinic College of Medicine, Rochester, MN

^* To whom correspondence should be addressed. E-mail: Katzmann.Jerry{at}mayo.edu.

Background: Laboratory testing in suspected -1-antitrypsin (A1AT) deficiency involves analysis of A1AT concentrations and identification of specific alleles by genotyping or phenotyping. The purpose of this study was to define and evaluate a strategy that provides reliable laboratory evaluation of A1AT deficiency.

Methods: Samples from 512 individuals referred for A1AT phenotype analysis were analyzed by quantification, phenotype, and genotype. A1AT concentrations were measured by nephelometry. Phenotype analysis was performed by isoelectric focusing electrophoresis. The genotype assay detected the S and Z deficiency alleles by a melting curve analysis.

Results: Of the 512 samples analyzed, 2% of the phenotype and genotype results were discordant. Among these 10 discordant results, 7 were attributed to phenotyping errors. On the basis of these data we formulated an algorithm, according to which we analyzed samples by genotyping and quantification assays, with a reflex to phenotyping when the genotype and quantification results were not concordant. Retrospective analyses demonstrated that 4% of samples submitted for genotype and quantitative analysis were reflexed to phenotyping. Of the reflexed samples, phenotyping confirmed the genotype result in 85% of cases. In the remaining 15%, phenotyping provided further information, including identifying rare deficiency alleles and suggesting the presence of a null allele, and allowed for a more definitive interpretation of the genotype result.

Conclusions: The combination of genotyping and quantification, with a reflex to phenotyping, is the optimal strategy for the laboratory evaluation of A1AT deficiency.