The standard formula used for decades to calculate "bad" cholesterol levels is often inaccurate, underestimating potential heart risks, new research shows.
The Johns Hopkins University School of Medicine study found that the standard formula — known as the "Friedewald equation," used since 1972 to calculate low-density lipoprotein (LDL) cholesterol levels — misjudges LDL in the range considered healthy for high-risk patients.
The findings, published online in the Journal of the American College of Cardiology, is likely to come as a a surprise to doctors and patients who routinely depend on cholesterol testing to gauge a patient’s "bad" cholesterol, which at higher levels signal greater risk of clogged arteries and heart attack.
"In our study, we compared samples assessed using the Friedewald equation with a direct calculation of the LDL cholesterol,” said lead researcher Seth Martin, M.D., a clinical fellow at the Johns Hopkins Ciccarone Center for the Prevention of Heart Disease. “We found that in nearly one out of four samples in the 'desirable' range for people with a higher heart disease risk, the Friedewald equation had it wrong."
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As a result, Dr. Martin noted, many patients who think they have healthy LDL cholesterol may, in fact, need more aggressive treatment to reduce their heart disease risk.
"In patients with heart disease, we want to get their LDL level below 70 — that is the typical goal," noted co-researcher Steven Jones, M.D., director of inpatient cardiology at The Johns Hopkins Hospital. He said the findings suggest many people — especially those with high triglyceride levels — have a false sense of assurance that their LDL cholesterol targets have been met.
For the study, the researchers analyzed cholesterol profiles of more than 1.3 million American adults who who provided blood samples from 2009 to 2011. The LDL cholesterol and other blood lipid components in those samples were directly measured with a more accurate technique known as ultracentrifugation. The researchers then compared their findings to the Friedewald formula, used routinely in doctors' offices worldwide, and found significant differences.
The Friedewald equation was introduced into clinical practice in 1972 by William Friedewald, M.D., to cut the significant time and expense of ultracentrifugation.
But Dr. Martin said the study suggests doctors need to consider available alternatives to the Friedewald formula as a more accurate way to assess risk for patients. One alternative involves examining all non-HDL cholesterol — including LDL and other plaque-causing cholesterol particles — and then subtracting HDL from that total to more accurately gauge risks. The non-HDL cholesterol level can be obtained easily using the same test widely available in doctors' offices today at no greater cost than the Friedewald calculation.
"Most specialists in our field agree at this point that all of those non-HDL components are important," he said.
"Non-HDL cholesterol is a much better target for quantifying risk of plaques in coronary arteries," Dr. Jones added. "Looking at non-HDL cholesterol would make it simpler and more consistent, and would enable us to provide our patients with a better assessment."
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