False Positives Common on Cancer Tests, Scientists Find

By Elizabeth Lopatto

May 11 (Bloomberg) -- A third of men and a quarter of women undergoing cancer screening will get false positive results by the time they have undergone four tests, which can lead to inappropriate medical procedures, a study found.

Men’s risk of a false-positive finding was 36.7 percent by the fourth screening, and women’s risk of the wrong result was 26.2 percent by the fourth of the 14 routinely recommended tests possible for each patient, according to the research published in Annals of Family Medicine. Almost 1 in 5 men and 1 in 10 women were likely to undergo an invasive diagnostic procedure such as a biopsy due to a false positive after four tests.

Many cancer screening tests are recommended to the public, as some doctors say that the earlier a malignancy is diagnosed, the easier it is to treat. The American Cancer Society recommends colon cancer screens for men and women ages 50 and older.

“Messages about screening have been oversimplified to ‘Early detection saves lives,’ and that’s the end, and people are made to feel irresponsible if they don’t test,” said Jennifer Croswell, the study’s lead author and acting director of the office of medical applications of research at the Bethesda-based National Institutes of Health, in a telephone interview today. “It’s a more nuanced decision than that. The more times you intervene, the greater the chance is you’re going to have harm.”

Reducing Cancer Deaths

Continued>>>
http://www.bloomberg.com/apps/news?pid=20601124&sid=areU4ct5H4F0&refer=home

heard when a new screening tool becomes available. :shrug: nothing is without risk. a biopsy is not like amputating a limb or anything. people should pull their socks up, and grow up.

Let's say that a screening procedure is 99% accurate in diagnosing the presence or absence of a cancer. Now let's suppose that 1 out of 1000 people screened for that cancer do in fact have it. Now for the math:

99% of the true positives will register as positives (0.99 positives per 1000)

1% of the true negatives will register as positives (9.99 positives per 1000)

So out of 1000 people screened, about 11 will test positive, but only one of those will actually have the cancer (approximately)

This means that about 9% of all positives will be true positives- the rest will be false positives. These aren't the true numbers- just ones I used because the algebra was easy to do in my head- but a little research can turn up more accurate numbers and you can do the calculations to get the ratio of true positives to false positives.

The point is that any time you are using a test to determine the presence or absence of a condition that has a relatively low frequency in the tested population, even with highly accurate tests a high percentage of all positives will be false positives.

and R