Obesity-linked high blood volumes render PSA test less effective

The extra blood volume produced in obese patients may so dilute levels of PSA that the PSA test may be significantly less effective for diagnosing prostate cancer in men carrying extra pounds, suggests a recent study in JAMA (2007; 298:2275-80).

The new research, combining data from more than 13,000 prostate cancer patients at Johns Hopkins Hospital, Baltimore, and elsewhere, could eventually affect the reliability of scores of other blood tests for cancer and other diseases in obese people or at least alter the way those tests are analyzed, investigators say.

The current research was designed to determine which of two dueling hypotheses explained this, said co-author Alan Partin, MD, of Johns Hopkins. One idea was based on the possibility that obese men make less PSA because they tend to have less testosterone. The other attributed the phenomenon to the increased amount of blood that obese men produce to support their size, which has the effect of thinning out the concentration of PSA.

Using records of patients treated for prostate cancer between 1988 and 2006 at Johns Hopkins, Duke University, Durham, NC, and various Veterans Affairs hospitals, researchers compiled information on PSA concentration and body mass index. Using a standard calculation, the researchers used BMI to estimate the amount of blood circulating in each patient's body. A different calculation used this blood volume, along with PSA concentrations, to estimate the total amount of PSA each patient had.

As expected, PSA concentrations were typically lower in the obese patients than in the normal-weight patients, although the total amount of PSA was about the same in both groups.

"It's clear to us that excess blood had diluted PSA concentrations in [the obese] group," Dr. Partin said.

In a related study, researchers from Massachusetts General Hospital in Boston found that men who are overweight or obese when diagnosed with prostate cancer are at greater risk of death after treatment.

Results of the study, led by Jason Efstathiou, MD, PhD, showed that overweight and obese men (those with BMI >25 kg/m²) at the time of diagnosis were nearly twice as likely to die from locally advanced prostate cancer as were patients who had a normal BMI at diagnosis.

Compared to men with normal BMI (<25), men with BMI between 25 and 30 were more than 1.5 times more likely to die from their cancer. Similarly, men with BMI >30 were 1.6 times more likely to die from their disease compared to men with a normal BMI. After 5 years, the prostate cancer mortality rate for men with a normal BMI was <7% compared to about 13% for men with BMI >25.

The study was reported in the online edition of Cancer.