What does my positive test result mean?

To answer, you don't just need to know details on the test.
You need to know yourself well, too.

If you go in for a medical test, you're hoping for more clarity: you might have symptoms and want to know the disease behind them or be checking for something that runs in your family. But when it comes to regular screening tests, like mammograms, you may have no idea how likely you are to have the disease - and often it's fairly unlikely. Is it still worth checking?

To work through the mammogram example, go back to page 1.

You can think of lots of different diagnostic tests in the same way as a mammogram; for instance, this quick HIV test follows a similar pattern of false positives if you're in a very low-risk population, even though it has an incredibly low 0.2% false positive rate.

Here's what it looks like for someone in a low risk population:

You can see that the true positive rate, shown in dark blue, is lower than the false positive rate. That means that you're still unlikely to have HIV even when you get a positive test back.

On the other hand, if you are in a higher-risk population, the test becomes much more accurate:

Now, you can be much more sure of that positive result.

In order to use this visualization for your own screening tests, you need three key pieces of information: your risk, along with the sensitivity and specificity of the test. You can often find those values in the medical literature, or online at CDC.gov.

  • False negatives (have disease, test says no)
  • True positives (have disease, test says yes)
  • False positives (don't have disease, test says yes)
  • True negatives (don't have disease, test says no)
    Risk of having disease: %
    False positive rate: %
    False negative rate: %

    Step 1: Risk Determine your risk for the disease as accurately as possible—as you've seen, it makes a big difference as to how accurate the test results will be. You can talk to your doctor or look up the risk level of the disease for your demographic online. For a mammogram, there's even an online risk calculator. Enter your value, in a percentage, under "risk of having disease."

    Step 2: Specificity The specificity of a test is how often the markers it picks up are truly signs of the disease. So for mammograms, this is the likelihood that any darker areas the mammogram picks up are actually breast cancer. The "false positive rate" is 100% - specificity, so once you find that value online you can enter it into the visualization.

    Step 3: Sensitivity The test's sensitivity is how often it picks up evidence of the disease. 100% - sensitivity is the false negative rate, or how often the test will read negative even though you have the disease. Enter that value into the visualization. The sensitivity and specificity for a mammogram are both available here.

    Now, if you look at the visualization, you can determine what a positive result in your case would mean: because of false positives and false negatives, and based on your risk, a positive result will only reflect a certain chance of having the disease. Good luck!

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    Hand-coded in HTML/CSS and JavaScript by Sarah Lewin. Hosted on GitHub.