Thursday, February 07, 2013

On False Positives in Mammograms

A study about mammograms for older women has some pretty shocking findings:

The experts gathered and analyzed data from 1996 to 2006 on 2,993 older females with breast cancer and 137,949 females without breast cancer.

The data, taken from five Breast Cancer Surveillance Consortium (BCSC) mammography registries in Vermont, Washington, New Hampshire, North Carolina, and California, is "the largest available screening mammography data set in the United States," Braithwaite said.

There were no differences seen in late-stage breast cancer rates between the subjects who received a mammogram annually and those screened biennially.

On the other hand, the scientists discovered that 48% of women between 66 and 74 years old who received annual screening had false positive results, while only 29% of females in the same age group who received screening every other year had false positives.
It  is that false positive rate* which is shocking.  What the study seems to be saying (haven't looked at the original) is that roughly every other woman in that age group who had a mammogram every year had a false positive finding during that ten-year-period.

Now that's not a very good test.  A false positive finding means that the test indicated a possible cancer which then later diagnoses showed was not the case.   To have that happen to half of all women screened annually means not only unnecessary anxiety but also pretty high costs in terms of those extra diagnoses which are needed, and it may be the case that some of those have a potential for negative health consequences, too.

What this suggests to me is an urgent need to develop better tests.
*Because few tests are perfect, the results can fall into four categories.  The ones we want are the true positive findings and the true negative findings.  True positives are the cases where someone has the disease that is screened for and the test tells us so.  True negatives are the cases where someone does not have the disease that is screened for and the test tells us so.

The undesirable outcomes are false negatives and false positives.  False negatives are probably more serious than false positives, because they mean that someone has the disease but the test fails to pick it up.  False positives mean that the screened person does not have the disease but the test indicates that she or he does have it.  False positives result in further medical care which would not be carried out if we knew the true value.