Obesity can skew key prostate cancer test results

By Will Dunham

WASHINGTON (Reuters) - Doctors reading the results of a blood test widely used to screen for prostate cancer can be fooled into thinking obese men are disease-free, researchers said on Tuesday.

The test may yield falsely reassuring results because obese people have more blood in their bodies due to their girth, thus diluting the concentration of the protein doctors use to detect the presence of prostate tumors, the researchers said.

The prostate gland produces a protein called prostate-specific antigen, or PSA. Only prostate cells produce it and if levels are higher it suggests the cells are growing -- which can be a sign of cancer although an enlarged prostate can also send PSA levels up.

The researchers examined medical records for nearly 14,000 men who had undergone surgery to treat prostate cancer between 1988 and 2006 at Johns Hopkins Hospital in Maryland, Duke University in North Carolina or five U.S. Veterans Affairs hospitals in California, Georgia and North Carolina.

Men with a body mass index, or BMI, indicating obesity had a higher blood volume and lower PSA concentrations. The most obese men had PSA concentrations 11 to 21 percent lower than those recorded in men of normal weight, the researchers reported in the Journal of the American Medical Association.

These men could have a total amount of PSA in the blood that might signal prostate cancer, but because they had so much more blood, the PSA concentration was so diluted that the test results seemed to show no cause for alarm, they added.

Thus, PSA concentrations that might be no worry for a thin man might suggest cancer for an obese one. "It's not that PSA is a bad test in obese men. Rather, we just need to learn how to use it better," Duke urologist Dr. Stephen Freedland, one of the researchers, said in a telephone interview.

"So whatever (PSA level) you consider abnormal, you just have got to adjust it by about 15 to 20 percent downwards for obese people," Freedland added, or risk missing many cancers.