Pharmacogenetic testing is used to uncover genetic causes for variations in drug response. Documentation of the method's usefulness in a clinical setting is scarce. The aim of the study was to systematically categorize the experience from routine CYP2D6 genotyping in a diagnostic laboratory.
All samples submitted to our laboratory for CYP2D6 genotyping in the period 29.06.98-28.12.09 were examined retrospectively. The samples were classified into three indication groups based on clinical information given in the request form. All samples, and a control group consisting of 100 healthy blood donors, were tested for the four most prevalent non-functional CYP2D6 alleles in the European population, and for ultrarapid metabolizer-associated duplications of the gene.
325 samples were included. The proportion of ultrarapid metabolizers was significantly higher in the patient group (4.0 %, p = 0.045) than in the control group (0 %), with the highest proportion among those patients that used a known CYP2D6 substrate. The percentage of poor metabolizers was not significantly higher in the patient group (8.3 %) than in the control group (6.0 %) (p = 0.528).
The CYP2D6 analysis could rarely explain the patients' side effects or lack of drug response, even though the study group was selected because of clinical problems due to drugs they were using. Two explanations may be that the indication(s) for genetic testing is not clearly defined and that the CYP2D6 genotype is only one of many factors that determine individual drug response.