By definition, a genetic test provides information about genetic status - for instance, whether someone has a condition or is a carrier.
Most people use the term genetic testing to mean DNA testing, but it also includes chromosome examination and clinical investigations, such as biochemical measurements or ultrasound, which can give genetic information.
However, for the purposes of this article, we will use the term to mean 'DNA testing'.
When to order a test
Currently, most DNA tests are ordered by specialists to provide information about genetic status in the diagnosis of single gene disorders, such as Huntington's disease, cystic fibrosis and haemophilia.
However, there are situations when a patient may ask about DNA testing in primary care. Family members may approach their GP to request a specific DNA test, because of a genetic condition in the family.
Depending on the condition, a referral to a specialist genetic service may be appropriate.
Other patients may ask for carrier testing when they know there is an increased incidence of a particular condition associated with the population from which they are descended.
Occasionally, patients have seen a news story about a new DNA test - for instance, a 'cancer genetic test' that quantifies their risk of developing cancer.
Clarifying why a patient has asked for a DNA test can help determine if the information generated would affect clinical management.
For example, many of the people who request a 'cancer genetic test' do not have a strong family history of cancer and are surprised to learn that only a small percentage of cancer cases are due to an inherited predisposition.
Jenny is a 29-year-old woman. During a consultation to discuss preconception care, she asks for a DNA test for cystic fibrosis (CF) as her husband is a carrier. She knows that if she is found to be a carrier, there is a one in four chance of their baby having CF. You check the UKGTN website for further information and you see that one of the criteria for CF testing is 'partner a known CF carrier' and that a GP can order a CF carrier test for this clinical indication.
You confirm that your regional genetic laboratory offers this test, what sample it requires, and the estimated reporting time. You explain to Jenny that the DNA test detects 29 of the most common genetic alterations in the CF gene, but as hundreds of mutations have been reported the routine test does not test for all of them.
You advise that results will be available in two weeks, and if she is found to be a carrier this is likely to mean other family members will also be carriers. She gives consent for the test and the sharing of results with other family members if needed. You note her consent in her medical record, take the blood sample and send it to the genetics laboratory.
The results shows that Jenny is not a carrier for any of the 29 mutations tested, which greatly reduces the probability of her and her husband having a child with CF.
DNA testing should be under-taken only when it will assist clinical management.
The UK Genetic Testing Network (UKGTN) has identified five areas of clinical practice where genetic test results may inform management: to confirm a diagnosis; to change a treatment plan; to advise on prognosis and management; to allow presymptomatic testing; and to provide an accurate assessment of genetic risk.
If a GP is considering a DNA test, the UKGTN recommends they clarify their clinical reasoning prior to any discussions with the patient.
Special consideration is also needed before genetic testing in children. The international consensus is that genetic tests should be performed in childhood for diagnosis only.
If the test is for any other reason, such as presymptomatic testing, it should be delayed until the child is of an age when they can decide for themselves if they want this information.
Ordering a genetic test
For some conditions there are national guidelines for ordering DNA tests.
For example, the NICE guidelines on familial breast cancer recommend that DNA testing is co-ordinated through specialist genetic services. There is also an international consensus that predictive testing for conditions such as Huntington's disease should only be conducted through specialist services.
The UKGTN is creating testing criteria for specific genetic conditions outlining the minimal clinical indications and the health professionals eligible to order tests.
Laboratories offering NHS DNA testing are on the UKGTN website (www.ukgtn.nhs.uk).
Genetic testing, like all clinical investigations, needs informed consent. In 2006, the Joint Committee on Medical Genetics (JCMG) published guidance on testing outlining what should be discussed with a patient before they give consent.
As well as discussing the implications of both test results and clinical management, the JCMG report recommends clinicians gain consent for the patient's genetic information to be shared with family members and their healthcare providers.
Other information to discuss includes: the timing of the testing procedure; unexpected results (non-paternity); and that the samples may be used for quality assurance by the laboratory.
Consent must be recorded, either in the medical record or through a written consent form - examples of which are provided in the JCMG report (www.bshg.org.uk).
- Professor Farndon is a director for the NHS National Genetics Education and Development Centre and a professor of clinical genetics and Ms Bishop is an education development officer for the NHS National Genetics Education and Development Centre
- This topic falls under section 6 of the RCGP curriculum 'Genetics in Primary Care'
2. Consider whether it might be more appropriate for a referral to be made to the regional genetics service.
3. Contact your local regional genetics laboratory to find out whether a request for testing for the specific disorder is appropriate from primary care.
4. Discuss with the patient about sharing the test result with other family members and record their decision.