If you watched the Super Bowl, you probably saw commercials featuring NFL players telling America to “relax your tight end.”

The campaign from drugmaker Novartis — called “Relax, It’s a Blood Test” — featured tight ends Rob Gronkowski and George Kittle and Super Bowl-winning coach Bruce Arians, who’s a prostate cancer survivor.

The real message was targeting men’s fear. A lot of men avoid prostate cancer screening because they assume it automatically means a rectal exam. Relax, all these tough football players said — for many men, screening starts with a simple blood test and a conversation.

The commercial told you that screening starts with a simple blood test. That’s true — but here’s what that blood test actually measures and why one number doesn’t tell the whole story.