Genetic testing
Types of genetic tests
- Diagnostic genetic test: during the clinical investigation of a patient
- For example, HD gene testing in a patient with unexplained chorea
- This can be done by any medical team
- Predictive genetic test: for asymptomatic people at risk of a disease
- For unaffected relatives
- In certain ethnic groups eg. Ashkenazi Jews for particular diseases
- These should be done by a clinical genetics service
- Other types
- Pharmacogenetic tests
- Gene tests to predict response or adverse effects from certain drugs
- HER-2 testing in breast cancer before Herceptin
- HLA-B*5701 testing before abacavir
- TPMT testing for bone marrow suppression with azathioprine or 6-mercaptopurine
- HLA-B*1502 testing for Stevens-Johnson syndrome with carbamazepine
- Prenatal tests
- Preconception genetic test
- Amniocentesis, etc
Genetic counselling
- Performed at a regional genetics centre after referral by a GP or specialist
- There are about 20 regional centres across the UK
- Not offered to children
- Specialised counsellors & doctors will review case and explain testing procedure
- Pre-test counselling
- 2-3 counselling sessions initially, spread out over at least 1 month
- Information given about the disease, the test, the possible results and the implications
- Blood test
- 2 separate blood samples may be taken so results are checked
- DNA from an affected family member (eg. parent) may sometimes be needed
- eg. BRCA genes only tested if a family member has confirmed BRCA1/2 mutation
- Post-test counselling
- Results always given in person by a trained counsellor
- Always offer access to further counselling and patient support organisations, and medical referral (if appropriate)
Structure of a typical scenario
- Ask about any current symptoms
- Is the patient concerned because of a breast lump? Chorea? And so on
- If symptoms are present, then they need to be investigated clinically first
- Ask about family history
- Drawing a family tree can be useful
- Explain the disease
- Clinical features and risk factors
- Treatment options
- Inheritance pattern and risk
- Explain the procedure for the test
- Referral to a clinical genetics service
- Pre-test counselling
- Blood test
- Post-test counselling
- Explain the possible results
- Mention the risk of false-positive and false-negative results
- Mention the relationship of a genetic mutation to the disease
- Disease might not occur despite the mutation (incomplete penetrance)
- Disease may occur by chance despite having no mutation
- Explain other consequences
- May need to disclose to insurance companies or employers in the future
- At present, insurance companies in the UK cannot ask about genetic test results
- May affect other family members
- For example in Huntington’s disease, if the grandchild of a patient with HD is found to have the mutation, that automatically means his parent carries the mutation too
- Ensure the patient can give informed consent without external pressure
Ethical issues
- Should a person be allowed to keep a positive result from his family?
- Non-maleficence: If it is a positive result for a disease then earlier diagnosis or testing of other family members may help them. For example, if you were diagnosed with a mutation in the HD gene, shouldn't your siblings or children know as they are at risk too?
- Autonomy: A patient has the right to keep his medical information confidential, and this can only be breached if there is imminent danger to the patient or others.
- In most countries (including the UK), there is no obligation to disclose results to other family members.
- Should preconception genetic testing be allowed?
- This is when a couple have embryos tested so that a mutation-free embryo can be implanted into the womb
- Beneficence: Parents would prefer a child to have minimal disease risk if possible. Note a parent with a genetic disease may feel guilt if a child is found to have the same disease later.
- Non-maleficence: Embryos may be destroyed during the process.
- Autonomy: A couple have the right to choose what is best. Does an embryo have rights? (This is definitely not the place for me to discuss this further!).
- In the UK, preconception genetic testing is allowed.
Further reading
Patient leaflets and disease information from the Genetic Alliance:
http://www.geneticalliance.org.uk/genetic-resource-centre.htm
