After watching several hours worth of medical shows like House, M.D., Scrubs, and Doc Martin I have come to observe a diagnostic trope: If a urine sample is left out or misplaced, it will turn purple and the patient will be diagnosed with a porphyria*.
Considering most hospital labs are not likely to have the necessary tests for porphyria, exposing the urine to UV light to see the color change is not a bad idea per se. Most medical shows; however, have the urine change as a result of dumb luck rather than testing for the disease. In fact, the title of the Scrubs episode featuring porphyria is called “My Dumb Luck”. In the US, 1 in 25,000 people have porphyria, so it’s a rare disease and it’s being diagnosed by happenstance on TV.
I was going to link to the clips of various episodes of doctors diagnosing porphyria but I was unable to find the appropriate clips. Instead, here are Penn & Teller performing a trick on Jimmy Fallon.
*Saying you have porphyria is akin to saying you have cancer. Like cancer, there are many variations of porphyria (acute intermittent porphyria, congenital erythropoetic porphyria, variegate porphyria, etc) but saying porphyria is generally easier and gets the point across.