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This article waslast modified on 26 November 2019.
What is pharmacogenetics?

We don’t all respond to the same drugs in the same way. Sometimes a drug will work for one person and not for another, or may cause different side-effects in different people. Our individual responses can be due to the genes we have inherited. With respect to drugs, our unique genetic make-up and our individual characteristics may mean that a drug that is effective for one person is less effective for another, or that a drug that is safe for one person may be dangerous for another person—even at the same dose.

Most drugs are broken down (metabolized) in the body by various enzymes. In some cases, an active drug is made inactive (or less active) through metabolism. In other cases, an inactive (or less active) drug is made more active through metabolism. The challenge in drug therapy is to make sure that the active form of a drug stays present just long enough to do its job. However, some people have enzymes that don’t work in quite the same way as other people, and they may metabolize the drug too quickly or too slowly or not at all — meaning that it may be gone before it has its intended effect, or it may hang around for too long and build up beyond safe levels, leading to side-effects.

A person’s response to a drug may also be related to variation in the way in which a drug reaches or interacts with the part of the body where it has to have an effect – for example, in a protein that the drug binds to in order to produce its particular effect. Also, individuals may experience severe side-effects (known as hypersensitivity reactions) from some drugs due to variations in the proteins involved in the body’s immune response.

Pharmacogenomics is the study of genetic variability that causes differences in individual responses to medications. By analysing the genes that are linked to the the enzymes that metabolize a drug that is to be prescribed or in it interacting the part of the body where it has its effect, a doctor may decide to raise or lower the dose, or even to use a different drug. The decision about which drug to prescribe may also be influenced by other drugs the patient is taking, to avoid interactions between drugs.

The terms “pharmacogenetics” and “pharmacogenomics” are sometimes used interchangeably. There are subtle differences between the two terms, and no consensus on their exact definitions. In general, pharmacogenomics refers to the overall study of the various genes that contribute to drug response, while pharmacogenetics is the study and evaluation of the inherited difference that affect an individual’s response to drugs. The term pharmacogenetics will be used in this article.

On average it takes 7 working days for the blood test results to come back from the hospital, depending on the exact tests requested. Some specialist test results may take longer, if samples have to be sent to a reference (specialist) laboratory. The X-ray & scan results may take longer. If you are registered to use the online services of your local practice, you may be able to access your results online. Your GP practice will be able to provide specific details.

If the doctor wants to see you about the result(s), you will be offered an appointment. If you are concerned about your test results, you will need to arrange an appointment with your doctor so that all relevant information including age, ethnicity, health history, signs and symptoms, laboratory and other procedures (radiology, endoscopy, etc.), can be considered.

Lab Tests Online-UK is an educational website designed to provide patients and carers with information on laboratory tests used in medical care. We are not a laboratory and are unable to comment on an individual's health and treatment.

Reference ranges are dependent on many factors, including patient age, sex, sample population, and test method, and numeric test results can have different meanings in different laboratories.

For these reasons, you will not find reference ranges for the majority of tests described on this web site. The lab report containing your test results should include the relevant reference range for your test(s). Please consult your doctor or the laboratory that performed the test(s) to obtain the reference range if you do not have the lab report.

For more information on reference ranges, please read Reference Ranges and What They Mean.

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About Pharmacogenetic Tests
  • Why is pharmacogenetics important?

    When starting a patient on a drug to treat a particular condition, doctors typically prescribe one of several appropriate drugs. The dose and timing are usually based on the rate of metabolism and clearance from the body in the average person. They begin by prescribing a “standard” dose based on factors such as weight, sex, and age, and then adjust the dose or change the drug over time, depending on whether the patient’s condition is responding to the medication and whether the patient is experiencing unpleasant or dangerous side-effects. Sometimes, a patient may find that a treatment that has been working well for them suddenly does not work so well or causes side-effects when they start taking an additional drug.

    The concentrations of some drugs are monitored with blood tests and the dosages increased or decreased to maintain the drug level in an established “therapeutic” range. Follow-up in this way is called “Therapeutic Drug Monitoring”. If changing the drug dose is not effective in treating or controlling the patient’s condition, or side-effects persist, then the patient may be given a different drug and the process is started again.

    In contrast to this rather practical approach, pharmacogenetics is now beginning to offer doctors the opportunity to individualise drug therapy for patients based on their genetic make-up and get the dosage right from the start. Testing people before treatment is initiated in order to predict the way in which they are likely to respond to certain classes of drugs is an important emerging area of laboratory testing. This genetic information could help both the doctor and the patient when choosing current and future drugs and doses. For certain drugs, pharmacogenetic testing is already helping doctors predetermine drugs and doses that have the best chance of achieving the desired therapeutic effect while reducing the likelihood of side-effects.

  • How does pharmacogenetic testing work?

    Genes are the basic units of genetic material, the segments of DNA that usually code for the production of specific proteins, including the proteins known as enzymes. Each person has two copies of most genes: one copy is inherited from their mother and one copy is inherited from their father. Each gene is made up of a specific genetic code, which is a sequence of basic units called nucleotides. Each nucleotide can be one of four different types (A, T, G, or C). For each nucleotide position in the gene, one of the four nucleotides is the predominant nucleotide in the general population. This nucleotide is usually referred to as "wild type." If an individual has a nucleotide that is different from "wild type" in one copy of their genes, that person is said to have a heterozygous variant. If an individual has the same variant nucleotide in both copies of their genes, that person is said to have a homozygous variant.

    Nucleotide or genetic variants (also called polymorphisms or mutations) occur throughout the population. Some genetic variants are benign — they do not produce any known negative effect or may be associated with features like height, hair colour, and eye colour. Other genetic variants may be known to cause specific diseases. Other variants may be associated with variable response to specific medications.

    Pharmacogenetic tests look for genetic variants that are associated with variable response to specific medications. These variants occur in genes that code for drug-metabolizing enzymes, drug targets, or proteins involved in the immune response. Pharmacogenetic tests can determine if a variant is heterozygous or homozygous, which can affect an individual's response or reaction to a drug.

  • When are the tests requested?

    A doctor may test a patient's genes for specific variations that are known to be involved in variable response to a drug at any time during treatment (for example, prior to treatment, during the initial phase of treatment, or later in the treatment). The results of the testing may be combined with the individual's clinical information, including age, weight, health and other drugs that they are taking, to help tailor therapy to the specific individual. Sometimes, the doctor may use this information to adjust the medication dose or sometimes to choose a different drug. Pharmacogenetic testing is intended to give the doctor additional information but may not replace the need for therapeutic drug monitoring.

    Pharmacogenetic testing for a specific gene is only performed once since a person's genetic makeup does not change over time. Depending on the medication, a single gene may be ordered or multiple genes may be ordered. An example of a medication for which multiple genes are usually evaluated is warfarin, which can be affected by genetic variation in genes known as CYP2C9 and VKORC1.

    Testing may be ordered prior to starting specific drug therapies or if someone who has started taking a drug is experiencing side-effects or having trouble establishing and/or maintaining a stable dose. Sometimes patients may not experience such issues until other medications that affect the metabolism or action of the drug in question are added or discontinued.

  • The Pharmacogenetic Tests

    There are currently a variety of pharmacogenetic tests that can be requested on a clinical basis. Some tests may only be applicable to specific ethnic groups. The following are some drugs for which pharmacogenetic tests are available:

    Drug Associated Diseases/Conditions Gene(s) Tested
    Warfarin  Excessive clotting  VKORC1 and CYP2C9
    Thiopurines (azathioprine, mercaptopurine, and thioguanine) (see TPMT) Autoimmune/Childhood leukaemia, inflammatory bowel disease TPMT


  • Is there anything else I should know?

    For most drugs, pharmacogenetic tests are generally not widely requested for a variety of reasons. However, they may be indicated when the medication of interest has a narrow therapeutic range and/or is associated with a high rate of adverse events. Pharmacogenetic testing is in its early stages in the UK, with the exception of TPMT testing which has an established role in treatment of leukaemia and inflammatory bowel disease. The UK 100,000 Genomes Project, which was established to look for previously unknown genetic variation in people with cancer or rare conditions, may provide us with information on other areas where pharmacogenetics could be used to tailor therapy to the patient.

    Pharmacogenetic tests are intended to provide the doctor and patient with additional information when selecting drug treatments and dosages. For a better understanding, patients may want to consult with a genetic counsellor prior to and after having a pharmacogenetic test performed. Genetic counselling and informed consent are recommended for all genetic testing.

    To learn more about the role of pharmacogenetics in personalized medicine, visit the Royal Society’s Personalized Medicine website.

  • Should everyone have pharmacogenetic tests performed?

    Currently they are only indicated in special circumstances if a person is going to take, or is taking, a drug that has an accepted pharmacogenetic test associated with it.

  • Is pharmacogenetic testing required before taking certain drugs?


  • Why am I only tested once?

    Your genetic make-up does not change over time. You may, however, have other pharmacogenetic tests performed if you take a different drug with a different associated pharmacogenetic test.

  • Does this mean my drug levels do not have to be monitored?

    No. Since there are other factors that affect drug levels besides your genetics, therapeutic drug monitoring may still be necessary.

  • What type of sample is used?

    A blood sample is obtained by inserting a needle into a vein in the arm. Saliva samples and buccal swabs (collected by brushing the inner side of the cheek with a swab) can also be used.

  • How do pharmacogenetic tests differ from other genetic tests?

    Pharmacogenetic tests are performed to evaluate a person's potential response to a particular drug therapy. Most other genetic tests have been developed to help diagnose or predict the development of a genetic disease, or to detect bacteria and viruses, for forensic medicine purposes, and in establishing parentage.

  • Will I be monitored differently after the test?

    You may be monitored differently depending on the results of the test, especially when starting the medication, changing the dose, or when adding or discontinuing another medication.

  • Should other family members be tested?

    This is a question to discuss with your doctor and your family members. In some cases it may be useful; in others it may only be relevant if they are going to be taking the same drug or a drug in the same type. Pharmacogenetic test results are useful information for a family member to share with the doctor along with the family's medical history.

  • How do I know whether or not I should have testing done?

    You and your doctor should consider the condition that you have, your history of drug-related side-effects, the drug therapies that are available, and the uses the test is intended for. Pharmacogenetic tests are not meant to stand alone but are meant to be used in conjunction with your other clinical findings.