The public are being warned by leading health experts about the boom in tests and scans for the "worried well".
A group of doctors in partnership with the Sense About Science charity have hit out at the way DIY testing is being targeted at the public.
The industry is now worth an estimated £99m a year, but the report said tests may be inaccurate and even harmful.
It called on government to improve regulation of the new techniques that are being used.
It comes after government advisors warned in December that private firms should stop offering body scans.
That study, from the Committee on Medical Aspects of Radiation in the Environment (Comare), said people may increase their risk of developing cancer by undergoing unnecessary scans involving radiation.
The Sense about Science report was drawn up in conjunction with the Association of Clinical Biochemistry, the PHG Foundation charity and the Royal College of Pathologists.
HAVE YOUR SAY
If doctors had time to give everyone an annual MOT then there would be no need for these DIY health kits.
It warned that tests taken outside the GP surgery or hospital setting should not be relied upon.
They said many of the tests being offered by private firms looked for the presence of biomarkers, such as a particular genetic sequence or the level of cholesterol in the blood.
But the report said the tests could not determine the implications of these, which after all can be affected by a range of other factors, such as a person's lifestyle.
It also warned self-testing was at risk of producing inaccurate results. The report pointed out that pin-prick blood tests could be invalidated by contamination with other bodily fluids.
And it also pointed out that sending bodily fluids in the post could interfere with them - for example some samples are affected by temperature.
'MY PRIVATE TEST WAS WRONG'
Geoff Hayward started having regular blood testing for high cholesterol five years ago as he has a family history of heart disease. As well as being tested by his local GP, the 61-year-old from Solihull also went to a high street chemists to get a second opinion. Over recent years he has been comparing the readings and at points the NHS test showed his cholesterol levels were twice as high as the private test.
He became so concerned about the differences that he eventually went to see a hospital consultant, who confirmed the NHS was right. Fortunately the confusion has not had any serious consequences - his cholesterol levels are not high enough for him to be placed on statins.
But Mr Hayward is still concerned. "I think it is worrying that test results could differ so much. It can cause people unnecessary worry or even worse indicate they are okay when they are not. Something must be done to ensure there is more consistency."
The report warned: "Unlike medicines and national screening programmes, there is no regulation or requirement for research on the effectiveness of testing.
"With direct-to-consumer tests, anyone can set up a lab and sell testing."
The experts also called for a national system to evaluate diagnostic tests and a publicly accessible database to provide evidence of performance and usefulness.
It said that, while patients could be reassured about the accuracy of NHS tests, there was no evidence about how clinically effective many of them are.
Dr Danielle Freedman, of the Royal College of Pathologists and co-author of the report, said: "The public buy 'testing kits' over the counter and via the internet without knowing the limitations of their results.
"There are 'cowboys in vans' on the high street offering for a price a wide range of tests.
"Do the public know whether tests are performed to the same quality standards as laboratories routinely providing this service to both the NHS and private sector?"
A Department of Health spokesman said the Comare report had raised similar fears and the government was consulting on how best to respond to the issue, which was of "significant concern".