What’s the best endpoint for prostate cancer studies, anyway? Despite its iffy clinical significance, PSA is often used because it’s just so darn quick and easy to measure. But now there’s a new blood test in town, and last week’s JCO pub aimed to put it to the test. We are, of course, talking about liquid biopsies (aka cell-free or circulating tumor DNA). A secondary analysis of five recent prospective randomized clinical trials for patients with metastatic castration-resistant prostate cancer compared eight different binary ways of interpreting PSA and liquid biopsies at 0 and 13 weeks from trial intervention. The winning correlate of decreased overall survival: any measurable circulating tumor DNA at both 0 and 13 weeks. Despite this test’s own drawbacks, it may eventually enable us to distinguish early which patients merit more intensive therapy.