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Why False Positive Results Are So Common In Medicine

Have you ever been surprised and confused by what seem to be conflicting results from scientific research? Have you ever secretly wondered if the medical profession is comprised of neurotic individuals who change their mind more frequently than you change your clothes? Well, I can understand why you’d feel that way because the public is constantly barraged with mixed health messages. But why is this happening?

The answer is complex, and I’d like to take a closer look at a few of the reasons in a series of blog posts. First, the human body is so incredibly complicated that we are constantly learning new things about it – how medicines, foods, and the environment impact it from the chemical to cellular to organ system level. There will always be new information, some of which may contradict previous thinking, and some that furthers it or ads a new facet to what we have already learned. Because human behavior is also so intricate, it’s far more difficult to prove a clear cause and effect relationship with certain treatments and interventions, due to the power of the human mind to perceive benefit when there is none (placebo effect).

Second, the media, by its very nature, seeks to present data with less ambiguity than is warranted. R. Barker Bausell, PhD, explains this tendency:

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Posted in: Science and the Media

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Screening Tests – Cumulative Incidence of False Positives

It’s easy to think of medical tests as black and white. If the test is positive, you have the disease; if it’s negative, you don’t. Even good clinicians sometimes fall into that trap. Based on the pre-test probability of the disease, a positive test result only increases the probability by a variable amount. An example: if the probability that a patient has a pulmonary embolus (based on symptoms and physical findings) is 10% and you do a D-dimer test, a positive result raises the probability of PE to 17% and a negative result lowers it to 0.2%.

Even something as simple as a throat culture for strep throat can be misleading. It’s possible to have a positive culture because you happen to be an asymptomatic strep carrier, while your current symptoms of fever and sore throat are actually due to a virus. Not to mention all the things that might have gone wrong in the lab: a mix-up of specimens, contamination, inaccurate recording…

Mammography is widely used to screen for breast cancer. Most patients and even some doctors think that if you have a positive mammogram you almost certainly have breast cancer. Not true. A positive result actually means the patient has about a 10% chance of cancer. 9 out of 10 positives are false positives.

But women don’t just get one mammogram. They get them every year or two. After 3 mammograms, 18% of women will have had a false positive. After ten exams, the rate rises to 49.1%. In a study of 2400 women who had an average of 4 mammograms over a 10 year period, the false positive tests led to 870 outpatient appointments, 539 diagnostic mammograms, 186 ultrasound examinations, 188 biopsies, and 1 hospitalization. There are also concerns about changes in behavior and psychological wellbeing following false positives.

Until recently, no one had looked at the cumulative incidence of false positives from other cancer screening tests. A new study in the Annals of Family Medicine has done just that. (more…)

Posted in: Clinical Trials

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