WASHINGTON — Too often, people pass a cardiac checkup only to collapse with a heart attack days later. Now scientists have found a clue that one day may help doctors determine if a heart attack is imminent, in hopes of preventing it.

Most heart attacks happen when fatty deposits in an artery burst open, and a blood clot then forms to seal the break. If the clot is too big, it blocks off blood flow.

The problem: Today’s best tests can’t predict when that’s about to happen.

“We don’t have a way to get at whether an artery’s going to crack, the precursor to a heart attack,” said Dr. Eric Topol, director of California’s Scripps Translational Science Institute.

On Wednesday, Scripps researchers reported a new lead — by searching people’s blood for cells that appear to flake off the lining of a severely diseased artery.

Topol’s team measured high levels of those cells, deformed ones, floating in the blood of 50 people who’d just had a heart attack. The research is reported in the journal Science Translational Medicine.

Topol said his team soon will begin needed studies to learn how early those cells might appear before a heart attack, and if spotting them could allow use of clot-preventing drugs to ward off damage.