After I gave a talk on treating high cholesterol levels, a physician in the audience asked me whether it's worthwhile to send blood for lab tests that measure lipoprotein sizes.

He was concerned because he rarely did so, while several of his practice partners obtained such tests routinely. I told him the test isn't often necessary. Here's why.

High levels of low-density lipoprotein (LDL) cholesterol are associated with a significantly higher risk of heart attack and other manifestations of coronary heart disease (CHD). However, the particles of LDL and the other plasma lipoproteins, like HDL, come in various sizes, and some evidence suggests that the smaller particles are more dangerous than the larger ones.

As a result, a cottage industry has sprung up to measure lipoprotein particle sizes using one of several different methods, and many doctors regularly send blood samples from their patients for these somewhat costly tests.

Two major studies now provide further evidence that tests to measure LDL size are usually not necessary or helpful. Both found that an older test that measures a protein called apolipoprotein B (apoB), which is associated with LDL, may provide ample information at a lower cost.

Investigators in Quebec, Canada, followed 2,072 men aged 35 to 64 years for up to 13 years. Although high LDL cholesterol in these men raised their risk of CHD on average, those with high LDL cholesterol but low apoB levels were found not to be at increased risk for CHD.

By contrast, men with high levels of both LDL cholesterol and apoB doubled their risk of CHD beyond the risk contributed by high LDL cholesterol alone. "We believe that measuring apoB on top of LDL could be of value for assessing future heart disease risk," one of the researchers concluded.

A second study from Stockholm, Sweden, followed 125,000 men and women for more than 10 years. This research team concluded that the size of LDL particles was not an important factor if apoB is measured. In their study, the ratio of apoB to apoA-1, the major protein component of HDL, was the best predictor of CHD risk, with higher ratios corresponding to higher risks.

ApoB adds to the predictive value of LDL cholesterol levels, and measuring apoB may be particularly useful in patients with high triglyceride levels.

Now I am more convinced than ever that these costly tests usually are not needed, but that measuring apoB levels may provide useful additional information at little expense.