Measuring Bad Cholesterol: The News Isn’t Good
Date: June 7, 2013
The standard formula used for decades to calculate low-density lipoprotein (LDL) cholesterol levels is often inaccurate, according to a recent study by Hopkins scientists. Of most concern, the researchers say, is their finding that the widely used formula underestimates LDL where accuracy matters most: in the range considered desirable for high-risk patients.
LDL is known as “bad” cholesterol, with higher numbers signaling greater risk of plaque accumulating in heart arteries, which can lead to heart attacks. Since 1972, a formula called the Friedewald equation has been used to gauge LDL cholesterol. It is an estimate rather than an exact measurement. However, physicians use the number to assess their patients’ risk and best course of treatment.
"In our study, we compared samples assessed using the Friedewald equation with a direct calculation of the LDL cholesterol. We found that in nearly 1 out of 4 samples in the ‘desirable’ range for people with a higher heart disease risk, the Friedewald equation had it wrong," says Seth Martin, a clinical fellow at Hopkins’ Ciccarone Center for the Prevention of Heart Disease and lead author of the study, which appeared in the Journal of the American College of Cardiology.
“As a result,” Martin says, “many patients may think they achieved their LDL cholesterol target when, in fact, they may need more aggressive treatment to reduce their heart disease risk.”
For the study, the researchers obtained detailed lipid profiles of more than 1.3 million American adults—almost one out of every 180 adults in the United States—analyzed from 2009 to 2011. The LDL cholesterol and other blood lipid components in those samples had been directly measured with a technique known as ultracentrifugation. The researchers then evaluated those samples using the Friedewald equation that is used routinely in doctors’ offices worldwide. When they compared the results, the differences came to light.
As a more accurate alternative to Friedewald, Martin and his colleagues suggest looking at non-HDL, which is acquired by subtracting HDL from total cholesterol.
That non-HDL number, which includes LDL and other plaque-causing cholesterol particles called VLDL, would typically be about 30 points higher than when LDL cholesterol is calculated under the Friedewald method, and it could vary. But, Martin says, it would provide a better way to assess whether patients need to modify their medications or make more substantial lifestyle changes. Stephanie Desmon