Genetic testing in some form goes back to 1960 when the first chromosomal tests were carried out on the blood of people with Down's syndrome. Natal chromosome analysis was possible, but it was another 10 years before the first DNA tests were carried out to diagnose a range of blood disorders.

Modern genetic testing can be said to have started in the early 1980s with tests for the serious inherited muscle disorder,

muscular dystrophy, followed in the late 1980s by the first tests for cystic fibrosis.

About 4,000 human genetic disorders have now been identified. They are inherited in a relatively simple way, usually through a single abnormal gene. These affect about 2 per cent of the population. In some cases - such as cystic fibrosis, muscular dystrophy, haemophilia, sickle cell disease, thalassaemia and Huntington's chorea - genetic tests are widely available at regional genetics centres (see table 2).

In addition, genetic factors are important in about a third of serious chronic disorders, such as heart disease, asthma, diabetes and arthritis. Specific genes are also being identified in a growing number of common disorders with a familial component. But, with a few notable exceptions, such as breast cancer and familial adenomatous polyposis (pre-malignant bowel polyps), the potential for testing individuals outside a research setting is limited.

Research has shown that people are most likely to ask for genetic tests if someone in the family is affected, particularly if something can be done about the genes that are responsible. So women at high risk of breast cancer are more likely to request tests for BRCA1 and BRCA2 than people with Huntington's chorea, an untreatable form of dementia whose onset is usually in middle age.

The way tests are offered is also important. In a cystic fibrosis study, 70 per cent of people agreed to tests to find out if they were carrying the gene when their GP asked during a consultation, but only 10 per cent responded to a letter from their surgery.1

After the initial approach to the GP, it is usual for a patient to be referred to the nearest hospital-based genetics service. Services vary in what they offer and, although the national picture has grown in a rather piecemeal fashion, local centres have endeavoured to respond to local needs.

What happens after the initial referral will also depend on the particular disease for which the patient appears to be at risk. Extensive investigation of the family tree will include obtaining death certificates and medical notes of affected family members, and discussion with all those involved. Tests, which are analysed at the nearest NHS laboratory, are only carried out when everyone is fully informed about what they involve and what the results

may mean.

For example, in the case of breast cancer, a woman with BRCA1 or BRCA2 has at least an 80 per cent chance of developing breast cancer. What will she do if her test is positive? Will she opt for regular mammography or go for prophylactic mastectomy? How will her test result affect other women in her family? Should they have tests and what treatment will they decide to have if they carry BRCA1 or BRCA2?

Strong economic arguments support the value of breast cancer and familial polyposis testing. Those found to carry the faulty genes may opt for treatment and follow-up, but the large number of negative tests help free up essential services, as patients no longer need regular mammography to look for early breast tumours or endoscopies to check for polyps.

The availability of postal genetic testing - limited at present to cystic fibrosis and breast cancer - is raising ethical and social issues. Hospital research has shown that people contemplating genetic tests need a cooling- off period after initial counselling to decide whether or not to go ahead with tests. They also need advice and support after they receive their results so they understand the implications before deciding what to do next.

Clearly, people who buy tests on the Internet or through other advertising methods risk missing out on essential information and counselling.

REFERENCE

1 Bekker H, Modell M, Denniss G et al. Uptake of cystic fibrosis testing in primary care: supply push or demand pull? BMJ 1993; 306: 1584-86.