Home health kits could be a waste of time, effort and money, according to Which?
The DIY kits for conditions including prostate cancer and stomach ulcers could cause false alarm or provide false reassurance, the consumer magazine found.
"Self-test health kits could be a useful tool, but the lack of clear information about how to use them could do more harm than good," chief executive Peter Vicary-Smith said.
"As your GP may well have to carry out their own tests to confirm a positive diagnosis anyway, you may be better off saving your money and going straight to your GP."
Which? experts examined six kits, available at chemists or online for between £4.99 and £15.99, and interviewed 64 members of the public about thier use.
The results were "hit and miss", with some consumers saying the prostate test results could have led to them not seeking medical help.
The research also found gaps in information which could lead to unnecessary worry.
For example, a Boots blood glucose test kit marketed as helping "in the early detection of diabetes" failed to mention that glucose levels can be raised after a meal, Which? said.
And a Boots bowel test kit did not provide dietary advice such as avoiding red meat for three days before the test.
There were also examples of "baffling language, with consumers in one case asked to draw blood from the "hillside" of the finger.
Other potentially confusing terms included "separation membranes", "desiccant" and "in-vitro diagnostic device".
The Selfcheck Health Test, which tests for an antigen (PSA) linked to prostate problems, did not explain that recent sexual activity, a urine infection or vigorous exercise, could raise PSA levels, the researchers said.
And the Simplicity Stomach Ulcer Screening Test was misleadingly named, Which? said.
It tests for a particular bacteria but only a minority of people with that bacteria are likely to develop a stomach ulcer.
Which? experts and the Plain English Campaign will pass their findings to the Medicines and Healthcare Regulatory Products Agency (MHRA) and self-testing kit manufacturers to try to help improve the quality of information supplied.