“It’s sort of a double whammy,” said Dr. Donald Lloyd-Jones, a cardiologist at the Northwestern University Feinberg School of Medicine who helped write the American Heart Association’s cholesterol guidelines. “Biologically, lp(a) both gets into the artery wall and causes damage there more easily.”

Studies suggest that the threshold for high lp(a) begins around 30 milligrams per deciliter of blood. Heart disease risk jumps for those in the 80th percentile, with lp(a) levels above 60, and climbs sharply for the 5 percent of the population with lp(a) levels between 150 and 300, according to Dr. Ginsberg at Columbia. “Those people can be disasters in terms of cardiovascular risk,” he said.

Yet many people at high risk do not fit the typical profile of a person with heart disease. Sandra Revill Tremulis was a health-conscious medical device executive who moonlighted as an aerobics instructor, followed a strict diet, and maintained 16 percent body fat, equivalent to that of an elite athlete. Her LDL and total cholesterol levels were low, and at age 39, her Framingham risk score, which gauges heart disease risk, put her odds of having a heart attack in her 40s at just 1 percent.

But when she started experiencing extreme fatigue and struggled to finish her workouts, she went to an interventional cardiologist and asked for a thorough work-up — which revealed that she had a 95 percent blockage in one of her coronary arteries.

“I was imminent to have a widow-maker heart attack at age 39,” she said.

Further testing showed she had high lp(a), which she believes she inherited from her father, who died of a heart attack at age 50. Determined to raise awareness, Ms. Revill Tremulis started a nonprofit, the Lipoprotein(a) Foundation, and now travels the globe advocating for wider testing.

“Only a small percentage of physicians know about this,” she said. “The biggest challenge for patients is finding knowledgeable physicians who know about this and can help them.”

Dr. Lloyd-Jones at Northwestern said that testing for lp(a) should be considered for people with early-onset cardiovascular disease — which means younger than age 50 for men and age 60 for women — or a strong family history of it. Since high lp(a) is hereditary, those who have it often have a parent, sibling or grandparent who suffered a premature heart attack or stroke. When one person has it, it’s important to test other family members too.