Leading genetic testing companies are providing clients with widely divergent and inaccurate predictions of their chances of developing serious diseases. That is the finding from tests conducted by different firms on the same person.

Using my own DNA, I approached three firms who between them provide the majority of genetic tests for common diseases in the UK. They gave contradictory assessments of the risk I faced of developing illnesses, including Alzheimer’s and glaucoma, and a confused verdict on my risk of suffering heart problems.

The findings reveal that those paying up to £825 for the tests may be receiving either misleading assurances that they face low heath risks or are being caused needless anxiety by warnings of high risks.

Lord Taverne, a member of a Lords select committee investigating genetic testing, said: “This [investigation] confirms that some of the commercial genetic tests can be very misleading and harmful. It may cause unnecessary anxiety and people may take action based on an unreliable diagnosis.”

Though in its infancy in the UK, the market for genetic testing is predicted to grow fast.

Several companies are already well established.

The first I approached was GeneticHealth, a British firm which charges £825 for a test and consultation. Its literature states that it can help customers to “live a longer active life, feel and look better, slow the ageing process and avoid disease by effective prediction and detection”.

The other two firms approached were deCODEme, based in Iceland, which charges $985 (£557), and the American group 23andMe, which charges $999.

Viewing their results proved to be more baffling than enlightening. For example deCODEme said my risk of developing exfoliation glaucoma, which causes loss of vision, was 91% below average. Yet according to 23andMe, I was 3.6 times more likely to get it than average.

For age-related macular degeneration, deCODEme put my risk at 20% lower than average, while 23andMe said it was 62% higher.

According to deCODEme, my risk of developing Alzheimer’s was 74% above average, while GeneticHealth said my genes were associated with “a fourfold increased risk of developing Alzheimer’s disease by your late 80s”.

According to deCODEme, my risk of a heart attack, angina or sudden cardiac death is 54.8%, which is 6% above average. By contrast, 23andMe said my risk of a heart attack between the ages of 45 and 84 was 17.5%, below average. Yet another assessment came from Paul Jenkins, the clinical director of GeneticHealth, who said my risk of cardiovascular disease was “low to moderate”.

According to deCODEme, I had a 2% higher than average risk of developing Crohn’s disease, a chronic bowel condition, while 23andMe put it at 47% lower than average.

GeneticHealth said my genes put me at “significantly increased risk” of hypertension, or high blood pressure, yet 23andMe defined me as at “moderately higher risk”.

With so many contradictions and inconsistencies I was left not knowing what to believe. There were, however, five conditions for which both deCODEme and 23andMe put me at similarly reduced risk – celiac disease, colorectal cancer, lactose intolerance, psoriasis and prostate cancer.

Christine Patch, genetic counselling manager at Guy’s hospital in London and a member of the Human Genetics Commission, said the companies had identified my gene types correctly, but that they often based their analysis on different genetic variants.

Even when they were looking at the same variants and research, they could be interpreting them differently. “The science on this is still really work in progress,” she said.

Cecile Janssens, of the Erasmus University Medical Centre in Rotterdam and author of an academic evaluation of the statistical weakness of seven other genetic tests, said: “This is the first time I have seen the results of the same sample being tested.

“It illustrates perfectly how what these companies are doing is premature.

“Although genomic profiling may have the potential to enhance preventative interventions, to date the scientific evidence is insufficient. It could take years if not decades before lifestyle and medical interventions can be responsibly and effectively tailored to individual genomic profiles.”

The firms selling genetic tests are operating in a regulatory vacuum. The Medicines and Healthcare Products Regulatory Agency believes it does not have the resources to oversee the sector, and a senior figure at the Advertising Standards Authority said it felt underqualified.

Asked why the results varied so widely, Kari Stefannson, chairman and co-founder of deCODE Genetics, said: “We are very confident that we understand better than anyone which markers correlate with a given level of disease risk.

“Having worked in this field for well over a decade, I have serious doubts as to whether dot-com companies that out-source both the basic science and the genomic analysis can deliver the sort of quality results that we think the public and medical establishment rightly demand of this new component of healthcare.”

Brian Whitley, founder of GeneticHealth, defended his company’s assessment of my risk of getting Alzheimer’s, saying: “Our risk has been taken directly from the Alzheimer’s Society website and we believe they are the most moderate and core source.”

Brian Naughton, a scientist at 23andMe, said the results depended on “which SNPs [single nucleotide polymorphisms, or genetic variants] you include, so if deCODEme have one more SNP than we do then you expect a difference. Co-founded by Anna Wojcicki, wife of the Google co-founder Sergey Brin, 23andMe asks customers to sign a legal waiver stating they understand their service is for “research and educational use only”.

The firms point to their successes, saying some clients have been given an early diagnosis of their condition.

Lauralee Nygaard, a dentist, in Spokane, Washington state, had a stroke three years ago and doctors could not find the cause. A deCODEme test identified a susceptibility to atrial fibrillation, and she now takes blood-thinning drugs.

Testing for single gene diseases has been available on the NHS for more than a decade. Women with a family history of breast or ovarian cancer receive a test for BRCA gene mutations. Multi-gene disorders are more complex.

So far an estimated 2,000 Britons have taken private gene tests and Stefansson said: “My prediction is within five years every reasonably educated person in western society will have had a genetic profile.”

Helen Wallace, of the campaign group GeneWatch, said: “At present there is a regulatory black hole that allows companies to mislead people about their health. Handing over your genetic information [also] increases the likelihood that insurers will discriminate against you in the future.”

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