PSA prostate cancer test may be less effective in obese men

In collaboration with the Press Association

The Prostate Specific Antigen (PSA) blood test - which is sometimes used to detect prostate cancer - may be less effective in obese men because they have a higher volume of blood, new research from the US has suggested.

A study by researchers at Duke Prostate Centre and Johns Hopkins Hospital looked at data from more than 13,000 prostate cancer patients to investigate why obese men with the disease often have lower-than-expected PSA concentrations.

PSA is released into the bloodstream by the prostate, but if the gland becomes enlarged due to cancer or other problems, the concentration of PSA in the bloodstream can rise.

PSA concentrations are therefore often used as a first step in the diagnosis of prostate cancer.

However, the US team suspected that the extra blood volume in obese people may dilute levels of the protein to such an extent that the test may not produce reliable results for severely overweight men.

Publishing their findings in the Journal of the American Medical Association, they reveal that PSA concentrations were indeed typically lower in obese patients than normal-weight men, but that the total amount of PSA was approximately the same in both groups of patients.

Dr Stephen Freedland, assistant professor at Duke University, explained: "Obese men have more blood circulating throughout their bodies than normal weight men and, as a result, the concentration of PSA in the blood... can become diluted.

"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 the need for an adjustment in the way we interpret PSA scores that will take body weight into account. If not, we may be missing a large number of cancers each year."

The findings could affect the reliability of other similar blood tests performed on obese people, a number of which are currently being developed.

Dr Freedland added: "For these other tests just starting down the development pipeline, we need to think about the actual total amount of a biological marker rather than concentration."

In the UK, men who are worried about prostate cancer can seek a PSA test from their GP.

Dr Kat Arney, senior science information officer at Cancer Research UK, said: "These are interesting findings but it's important to remember that the reliability of the PSA test is still uncertain.

"Men may have a high level of PSA without having prostate cancer, while a low PSA reading is not a guarantee that they are free from the disease. Furthermore, prostate cancer can sometimes grow very slowly, and may not actually need any treatment for some time.

"Some doctors think that widespread use of the PSA test might result in many men being treated unnecessarily, causing significant side effects such as impotence or incontinence."