CHICAGO - Every year, July 6 -- their LDL, or bad-cholesterol level. That figure shapes decisions about whether to start a statin, raise a dose, or add a second medication. A study published Monday in JAMA found that LDL, despite its ubiquity, may not be the most reliable guide for those decisions, and that a less commonly ordered alternative could prevent substantially more heart attacks and strokes.

The alternative is apolipoprotein B, or apoB. Unlike LDL, which measures the concentration of cholesterol in the blood, apoB counts the actual number of cholesterol-carrying particles. That distinction matters because a person can have a normal LDL level while still harboring a large number of small, dense particles, each capable of penetra...