December 3, 2007

Obesity May Skew Results of Prostate Cancer Test

a photo of a man having a consultation with his doctor

A widely used blood test for detecting the earliest stages of prostate cancer may fail to spot the disease in obese men because of their greater blood volume, according to a recent study. The finding suggests that clinicians may need to rethink the way they interpret the results of prostate cancer blood tests in extremely overweight patients.

Prostate cancer is expected to kill more than 27,000 men nationwide this year, with an estimated 220,000 new cases predicted for 2007. It affects the walnut-sized prostate gland, part of the male reproductive system. Prostate cancer can be detected before symptoms even appear by screening the blood for unusually high concentrations of prostate-specific antigen (PSA), a protein produced by the prostate gland. High PSA levels may prompt additional testing, like a needle biopsy, to confirm or rule out a diagnosis of cancer.

Several studies in recent years have observed that obese men tend to have lower PSA concentrations than normal-weight men, although the underlying cause has been unclear. Some researchers have proposed that hormonal abnormalities may reduce the overall production of PSA in overweight men. But Dr. Stephen J. Freedland of Duke University Medical Center, along with colleagues at other institutions, put forward a different hypothesis. They proposed that the larger blood volume of obese patients dilutes the concentration of PSA, thereby delaying or preventing the detection of cancer.

As reported in the November 21, 2007, issue of the Journal of the American Medical Association, the scientists analyzed the medical records of nearly 14,000 patients who had undergone surgery between 1988 and 2006 to have their cancerous prostate glands removed. The scientists examined the relationship between body mass index — a ratio of weight to height — and blood concentrations of PSA. They also estimated each patient's total blood volume and the total amount of PSA circulating in the blood. Their study was supported in part by NIH's National Cancer Institute (NCI).

As expected, patients with a higher body mass index also had higher blood volumes and lower PSA concentrations. The most obese men had PSA concentrations that were 11-21% lower than those of normal-weight men.

The researchers found that the overall quantity of PSA in the blood of obese men was similar to or greater than the quantity in normal-weight men. This finding challenges the notion that obese men may have reduced production of PSA. Instead, the results suggest that PSA concentration is diluted by their greater blood volume.

The researchers note that additional studies are needed to confirm their findings. Because they relied on medical records rather than directly examining patients, they were unable to use more accurate measures of blood volume and total PSA amounts.

The risks and benefits of PSA-based prostate cancer screening are still uncertain. Nevertheless, this study raises the idea that researchers should take a closer look at how excess weight can affect the reliability of diagnostic blood tests — not only for prostate cancer, but for other disorders as well.

— by Vicki Contie

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