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DEAR DR. ROACH: I am 85 and healthy apart from polycythemia vera, for which I take aspirin. I was interested in the new blood test for cancer. Is it worthwhile? -- M.N.

ANSWER: When screening patients for cancer, the goal is to find a test that leads to a beneficial outcome for the person being screened -- ideally an outcome where they are less likely to die from cancer. These kinds of studies are large, difficult and expensive to do, and we only have this kind of information for a few screening tests, such as mammograms, colon cancer screening, and Pap smears (although there is controversy even among these). We don't have these kinds of studies for the new blood testing, which is called cell-free DNA testing.

For the new cell-free DNA testing, a small amount of a person's blood is tested for a large number of cancers. A positive test could indicate that the person has a certain type of cancer in theory before other screening tests are positive.

Unfortunately, the technology is not perfect. The test is reported to be 99% specific, meaning that only 1% of people will get a false positive test. But it's not as good at finding cancer when it's there. It will help diagnose somewhere between 10% and 30% of all cases of people with cancer. However, because cancers in asymptomatic people are uncommon, many of the positive tests will be false positives.

For example, stomach cancer is expected in about 100 people among 100,000 Americans between the ages of 50-70. The test is expected to find roughly 30% of them, so 30 people out of 100,000 will be true positives. One percent of the 100,000 will have a false positive result, which equals 1,000 people. So, of all the people with a positive result, only 3% will actually have stomach cancer. (This is called the positive predictive value.)

This means that 97% of people with a positive test will be very anxious and may need additional tests to prove that they didn't have stomach cancer. Also, 70 people who actually have stomach cancer will be told that they don't, and this might mean that they are less likely to see their doctors once they get symptoms.

Cancers that are more common have better results, but even for the most common cancer in this age group -- colon cancer -- the positive predictive value is about 30%. This is not as good as the standard tests for colon cancer, such as a colonoscopy or multitarget stool testing (Colorguard).

Successful screening tests lead to early diagnoses. The cancers still need to be removed, usually surgically, and in an 85 year old, this always includes some risk, even in the best hands. Screening for cancers should only be done in healthy people whose life expectancy is long enough that the benefit of getting rid of the cancer outweighs the initial risk of harm from surgery.

Mathematical models suggest that 85 is an age where most people would not benefit from screening. However, screening can be appropriate for some very healthy people who may have an increased risk for cancer. For all these reasons, I don't recommend the new cell-free DNA cancer screening tests for you.

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Dr. Roach regrets that he is unable to answer individual letters, but will incorporate them in the column whenever possible. Readers may email questions to ToYourGoodHealth@med.cornell.edu or send mail to 628 Virginia Dr., Orlando, FL 32803.

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