Hannah Mamuszka and Lena Chaihorsky make the case that diagnostics — not just drugs — are the key to controlling healthcare spending, and that the U.S. systematically underuses them. They lay the groundwork for the series: primary diagnosis (what's actually causing your symptoms), the "diagnosis of exclusion" trap, and two kinds of tests that determine whether a prescribed drug will work for you — pharmacogenomic tests (how your genes affect drug metabolism) and response-predicting diagnostics (whether you'll respond at all). The recurring villain is economics: insurers balk at a $200 test to protect a $10 prescription, ignoring the clinical and downstream costs; physicians skip testing to spare patients surprise bills; and because most drugs work in only a minority of patients, the resulting churn is profitable. They argue this is why response-p ...