by Troy Goodman, Associated Press Writer

DALLAS (AP) -- Cardiac patients with high levels of a little-known form of ''bad'' cholesterol in their blood are 70 percent more likely to have a heart attack than those with lower concentrations, according to a study released Monday.

The obscure cholesterol particle -- called lipoprotein(a) -- is especially insidious because it's difficult for doctors to measure reliably and because its levels have little to do with the better-known form of ''bad'' cholesterol, called LDL.

The elevated Lp(a) levels also have little to do with more conventional heart disease risk factors such as smoking, high blood pressure and poor diet. It also cannot be directly linked to high cholesterol, or the kind whose levels can be altered through diet or drugs, said lead researcher Dr. John Danesh, of Oxford University in England.

''This study suggests there is a clear association between Lp(a) and an increased risk of heart disease,'' said Danesh, who pointed out that more than a decade worth of research previously failed to link Lp(a) to increased risk of heart disease in the general population.

The Oxford findings were published in Monday's edition of Circulation, a journal of the American Heart Association.

Researchers gathered data used in the study from 27 different studies tracking more than 5,200 people who had heart disease or survived a heart attack. The average age of the people involved in the study was 50, Danesh said.

The number of heart attacks suffered by individuals with the highest Lp(a) concentrations was compared with the number of heart attacks among those with the lowest Lp(a) readings. During a decade of follow-up, the highest group had 70 percent more heart attacks than the low-level Lp(a) study subjects.

''The message here is physicians and cardiologists should be aware and it would be useful to know the Lp(a) levels in patients,'' said Dr. Angelo Scanu, director of the Lipid Clinic at the University of California, who was not involved in the Oxford study.

Lp(a) was first pinpointed in the blood some 40 years ago, but doctors don't normally screen for this lipoprotein because no standardized screening exists and because even when the Lp(a) is known, very little can now be done to modify it. Unlike other kinds of cholesterol, Lp(a) in the blood is 95 percent determined by genes, so drugs and changes in diet have little effect on it, Danesh said.

Lp(a)'s exact role in the blood also is unknown, Scanu said, although researchers do know it's a fat-carrying particle that includes a protein that mimics the body's natural clot-busting properties.

Also, doctors know that blacks have two to three times the rate of elevated Lp(a) compared with whites, yet they do not have double to triple the prevalence of heart disease, Scanu said.

''Some people don't even view it as a risk factor,'' he said of elevated Lp(a) levels. ''But it definitely is.''

AP-NY-09-04-00 1627EDT