Pharmacogenomics

Interpreting Pharmacogenomic Tests

Pharmacogenomic test results can be difficult to interpret. There are limitations to interpretation because enzymes involved in drug metabolism arise from multiple genes and that process is often complex. The test results are predictions based on information about the specific genetic variations and on information about the associated diseases, adverse drug reactions, and patient outcomes that have been gathered during studies and clinical trials. In many cases, the predictions will be very accurate, but they cannot say with 100% certainty what will happen with an individual patient and they do not incorporate or make allowances for the other factors in a patient’s life related to the disease condition or to the individual that may also affect their response to treatment. This is one of the reasons why the results are intended to be used in conjunction with other relevant clinical findings.

Pharmacogenetic tests are not yet widely available in the UK, but their availability is increasing. Some currently available pharmacogenomic tests include:

Determination of TPMT activity in blood to predict sensitivity to azathioprine and related drugs

A DNA microarray that tests for about 30 CYP genetic variants. It is meant to be used as an aid in individualizing treatment selection and dosing for drugs metabolized through these genes. It can help predict poor, intermediate, extensive, or ultra-rapid metabolizers.

Tests that detect genetic variants of the CYP2C9 and VKORC1 (vitamin K epoxide reductase) enzymes. These enzymes are involved in the efficacy of warfarin as an anticoagulant. Warfarin is used to prevent dangerous blood clots from forming in the blood vessels of certain patients, but it can significantly increase the risk of bleeding into the head or gastrointestinal tract. These tests identify patients who have genetic variations and so need a reduced dose of warfarin to avoid bleeding episodes.