It's not the cholesterol, but the HDL that's 'good'

Q: After I finally understood what was meant by "good" and "bad" cholesterol in the blood, my doctor came up with a new one when she told me my lipoprotein(a) is too high. I would like to know more about this lipoprotein.

A: Let me begin with a brief refresher course. There is, in fact, no "good" or "bad" cholesterol; all the cholesterol in the blood can be considered bad. Instead, there are good and bad protein carriers (lipoproteins) of the cholesterol and other fats in the blood. The main carrier of cholesterol is low density lipoprotein (LDL), and LDL is a "bad" lipoprotein because it deposits cholesterol in arteries and initiates the process of atherosclerosis. High density lipoprotein (HDL) is a "good" lipoprotein because it can remove cholesterol from the arterial wall and thus protect against atherosclerosis. Still controversial is whether to consider very low density lipoprotein (VLDL), which carries most of the triglycerides in the blood, consistently "bad." High levels of VLDL are clearly a risk for heart disease in women and diabetics, but not necessarily in men.

Lipoprotein(a) (Lp(a)) is very similar to LDL, but it contains an additional protein, apo a, which has a structure much like parts of yet another blood protein, plasminogen. Plasminogen is converted to plasmin, which helps prevent blockage of arteries by breaking down fibrin, a major component of clost (thrombi) within arteries. Because of its similarity to plasminogen, Lp(a) can interfere with the conversion of plasminogen to plasmin and the actions of plasmin in the removal of blood clots. As a result, levels of Lp(a) above 25 mg/dl are a risk factor for coronary artery disease, and this risk is separate from those imposed by high LDL or low HDL levels.

Dr. Margolis is professor of medicine and biological chemistry at the Johns Hopkins School of Medicine and associate dean for faculty affairs at the school.