The most common blood test for prostate cancer often fails to catch early signs of the disease in obese men because they have more blood than other patients, according to a new study.
The research suggests that levels of prostate-specific antigen, or PSA, might appear lower in obese men, thus delaying diagnosis and treatment of the cancer.
"When we get to the point when PSA is elevated in obese men, we find cancer that's more advanced," said Dr. Alan W. Partin, chairman of urology at Johns Hopkins Hospital and co-author of the study. "When cancer is detected in a later stage, it has much worse outcomes, and it's harder to cure."
Partin said the results of the study, published today in the Journal of the American Medical Association, require confirmation by more rigorous studies. But with further research, he said, PSA tests might be altered to account for a person's weight and avoid the problem of underdiagnosis in obese men.
He said excess weight might also confound blood tests for other cancers, an issue that warrants further study. "Any test that has to do with a concentration [of chemical markers in the blood], people may need to take a close look at them," he said.
Meanwhile, he said, doctors should consider using more aggressive screening techniques on obese men with borderline PSA results.
The researchers studied medical records of about 14,000 men who had cancerous prostates removed at a number of different hospitals. But more than three-quarters of the operations took place at Johns Hopkins.
The records included the patients' PSA levels as well as their body mass index, or BMI - a height-to-weight ratio that indicates how much fat a person is carrying.
The PSA test measures the blood concentration of prostate-specific antigen, a protein produced in excess by cancerous prostate glands. The glands serve as part of the male reproductive system, producing and storing a component of semen.
About 218,000 new cases of prostate cancer will be diagnosed in the United States this year, and more than 27,000 men will die from it, according the Centers for Disease Control and Prevention.
Along with a yearly rectal exam, the PSA blood test is the most common means by which doctors diagnose prostate cancer. When a man's PSA results show elevated or rapidly rising levels of the protein, the next step is usually a biopsy of the gland to look for cancerous cells.
The study of cancer patients, which was led by Duke University researchers, found that obese men - those with a body mass index of 35 or greater - had an 11 percent to 21 percent lower PSA concentration than normal-weight men.
To have a body mass index of 35, a 5-foot 10-inch man would have to weigh 245 pounds.
Partin said the PSA was diluted because the heavy men had 21 percent to 23 percent more blood volume than normal-weight men. As a result, their test results failed to alarm doctors enough to send the men for a prostate biopsy.
"The fat guys had lower PSA levels but had just as much cancer as thin guys," said Dr. Mike Nasland, director of the prostate center at University of Maryland Medical Center.
Experts said the results could help explain why overweight and obese men with prostate cancer fare worse than thinner men.
A study published in February found that overweight and obese men had the same risk of prostate cancer as men of normal weight. But the overweight men were 25 percent more likely to die, and obese men twice as likely.
Late diagnosis was one possibility, causing doctors to question whether heavy men's PSA test levels were somehow skewed. One theory held that hormonal changes caused by excess fat might account for obese men's lower readings. But today's study suggests that increased blood volume is the real reason.
Nasland said the accumulation of fat causes the body to grow more veins and requires more blood volume to carry nutrients to the additional tissue. "You look at an obese guy's neck or arms or belly," he said, "there's a lot more flesh that needs to be nourished."
Dr. Marc Siegelbaum, director of St. Joseph Medical Center's urologic oncology center, speculated that increased blood volume might also obscure rapidly rising PSA production by a diseased prostate.
"If it's low to begin with, and you look at it a year from now," he said, "it might still be diluted out. I think the increased blood volume might make it difficult to pick up the rise."
He noted that age is now factored into PSA screening and that BMI might soon follow suit, based on the results of the study.
"We used to think that 4 [nanograms of PSA per milliliter] is the magic level for anyone," Siegelbaum said, "but that's not the case anymore."
A man's prostate naturally produces more PSA as he ages, so the concentration of protein that triggers alarm is different at age 40 than at 80.
"We have never taken into account body mass index," Siegelbaum said. "But we need to plug that into the equation. There are other tests in the works, but for now PSA is what we've got."
Risks and symptoms
Factors affecting prostate cancer risk
Age: More than 70 percent of men diagnosed with prostate cancer are over 65
Race: African-American men are at substantially greater risk than white and Hispanic men
Diet: Vitamin E and selenium might have a protective effect
Genes: The risk of prostate cancer rises with the number of close relatives who have the disease, particularly if they were diagnosed when they were under 60
Symptoms of prostate cancer
Difficulty urinating or needing to urinate frequently, especially at night
Pain or burning during urination
Blood in urine or semen
Pain in lower back, hips or upper thighs
Impotence [ Source: National Cancer Institute]