Diagnostic Test Cutoffs

Below you can see a hypothetical distribution of white blood counts (WBC) in an enhanced urinalysis for patients who do and do not have urinary tract infections (UTI).

Imagine that we chose 12 WBC as the cutoff score for this test, and diagnose anyone with 12 WBC or higher as having a UTI. You can see that 50% of the people with UTIs will have 12 WBC or higher, because 50% of the area under the red curve is to the right of 12 WBC. With this cutoff, the test will be 50% sensitive.

You can also see that about 2% of the people without UTIs will be diagnosed incorrectly -- false positives -- because 2% of the area under the blue curve is to the right of 12 WBC. Because 98% of the people who are healthy will test negative, the test is 98% specific.

Click in the image to choose a cutoff score for the enhanced urinalysis and learn the implications of your choice.

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