(Agencies)
Updated: 2007-11-21 11:21

WASHINGTON - 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 US 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.

The prostate is a walnut-sized gland that produces seminal fluid. It is found below the bladder.

Dr. Carmen Rodriguez, an American Cancer Society epidemiologist who participated in the study, said the findings were particularly important considering the rising rates of obesity in the United States and worldwide.

Rodriguez said doctors had known obese men were at higher risk of developing more aggressive prostate cancer. She said this study indicates one of the reasons may be that some obese men could have had false negative results in PSA tests, with their cancer then detected much later after it had grown more advanced and more dangerous.

Freedland said the findings could affect the way doctors look at other tests for cancer and other diseases that depend on concentrations of disease markers like PSA in the blood.

He said it might be helpful to consider the total amount of a disease marker in the body rather than its concentration in a certain volume of blood, in order to account for the dilution that can occur in the obese.

Worldwide, prostate cancer is estimated to kill about 221,000 people annually, with 679,000 new cases diagnosed.

The American Cancer Society estimates that about 27,000 men will die from prostate cancer in the United States this year and about 219,000 men will be diagnosed with it.