WASHINGTON (AP) — There’s little way to know if you’ll be a fast or slow healer after surgery. Would you be someone who feels back to normal in a week or is out of work for a month with lingering pain and fatigue? That’s one of the big frustrations of surgery.

Now Stanford University researchers have discovered that right after surgery, patients’ blood harbors clues about how fast they’ll bounce back. And it has to do with the activity of certain immune cells that play a key role in healing.