Playing the genome game


23andMe - it sounds more like a twitter handle or a dating website than a source of controversy. But that's the name of the saliva testing kit just launched in the UK, which aims to inform people of how their DNA affects their risk of getting a variety of medical conditions. With a name based on the 23 pairs of chromosomes we carry in every cell in our bodies, 23andMe gives reports on more than 100 health traits. These include genetic variations that could increase your risk of Parkinson's diseasecystic fibrosis, sickle cell anaemia and blood clotting disorders.

The Food and Drug Administration (FDA) in the USA last year banned the test amid concerns about accuracy and what the results might mean to patients. The UK version does not give information on as many conditions, and the makers claim they have addressed the FDA's concerns.

But I have serious concerns. Human DNA is extremely complicated, and one gene rarely acts in isolation from others. What's more, in the vast majority of cases, both 'nature and nurture' play a role. While having certain genes makes it almost inevitable that you'll inherit some medical conditions, like Huntington's chorea, in most cases the risk is not that clear. Even carrying the high- risk BRCA gene doesn't mean you are certain to get breast or ovarian cancer.

In most medical conditions, carrying a certain gene may increase the chance of getting a condition but by no means makes it inevitable. For instance, if you have a parent or sibling with multiple sclerosis, you're 10 times more likely than the rest of the population to get the condition yourself, but you still have a 99% chance of not being affected. Many autoimmune diseases are more common in people with certain genes, but the condition is thought to be triggered by something that happens to you during your lifetime, such as being exposed to a particular virus. Having a test with no guidance about what the results really mean to you could cause far more worry than it solves.

All screening tests carry a risk of 'false positive' tests - where your result suggests you could have a condition when you don't have it, or where the risk identified by the test isn't as high as it seems. Two years ago there was a major controversy in the UK breast screening service, when results suggested that for every woman whose life was saved by breast screening, three would be diagnosed with breast cancer and treated unnecessarily . At best, false positives can cause huge concern while you're waiting for further tests; at worst, they can result in unnecessary major surgery. If I identify a positive result like this, I bring my patient in so I can explain exactly what it means. Getting a result without being forewarned could be hugely distressing.

Then there's the issue of 'false negatives' - tests that give you the all clear when in fact there is something to worry about. Telling someone who drinks alcohol to excess that their liver function tests are normal - without explaining that this doesn't mean it will stay that way if they keep drinking - has been shown to reinforce unhealthy lifestyle patterns. Discovering you don't have one risk for heart disease certainly doesn't mean you'll never be affected, regardless of whether you sit on the couch with a cigarette in one hand and a pie in the other.

Finally, what do you do with the results? Suppose you do the test in your 20s, and discover you're more likely to develop Parkinson's disease 40 years down the line? Embracing science and technology is one thing - jumping in with both feet without thinking through the consequences is quite another.

Disclaimer: This article is for information only and should not be used for the diagnosis or treatment of medical conditions. Patient Platform Limited has used all reasonable care in compiling the information but make no warranty as to its accuracy. Consult a doctor or other health care professional for diagnosis and treatment of medical conditions. For details see our conditions.