Doctors must take body weight into account when reading test results for prostate cancer as obesity may distort the findings, a US study argues.
Obese men have more blood so the concentration of antigen, a marker for the disease, is lower, a team found.
The North Carolina study, published in the Journal of the American Medical Association, involved 14,000 patients.
It may explain why obese men seem to have more aggressive cancers, as tumours may initially be missed.
The test for prostate-specific antigen, or PSA, is known to be notoriously unreliable.
It's as if you dissolve a tablet in a cup of water versus a tub of water
Dr Stephen Freedland
About a third of men with raised PSA levels will not have cancer, and will undergo unnecessary invasive tests.
Meanwhile, the test sometimes misses prostate cancer, as highlighted in this study.
"We've known for a while that obese men tend to have lower PSA scores than normal weight men, but our study really proposes a reason why this happens, and points to a need for an adjustment in the way we interpret PSA scores to take body weight into account," said Dr Stephen Freedland, a urologist at the Duke Prostate Center.
"If not, we may be missing a large number of cancers each year."
Working it out
At the extreme, the men in the most obese category had PSA concentrations as much as 21% lower than those of normal weight men.
Dr Chris Hiley, of the Prostate Cancer Charity, said: "This study shows us yet another downside to obesity. An obese man's true PSA level is diluted by his increased blood volume caused by excess weight.
"Doctors now need to work out how to take this into account so they can make an accurate estimate of the PSA level - important in the diagnosis and management of prostate cancer."
She added: "This finding could also be of wider significance in interpreting blood tests in other conditions that affect both men and women."
In the UK, the disease now accounts for 13% of male cancer deaths, and is the second most common cause of cancer death in men, after lung cancer.