Cholesterol tests: how accurate are they?

Nutrition Action Healthletter, May, 1988 by Anne Montgomery

Cholesterol Tests: How Accurate Are They?

Several months ago, 58-year-old Sandra Miller (*1) had her blood cholesterol measured for the first time. The result--198 (*2)--sounded like good news. Her doctor told her this meant that she fell into the "desirable blood cholesterol" category (under 200).

If Sandra's cholesterol level had topped 200, her doctor would have urged her to go on a cholesterol-lowering diet. As it is, she'll continue to eat foods such as ice cream, bacon, and cheese as often as she wants.

What Sandra, her doctor, and millions of others don't know is that cholesterol tests are not as accurate as many believe. Assuming her doctor's lab meets the government's guidelines, Sandra's blood cholesterol might actually be anywhere from 168 to 228.

Interest in cholesterol and its role in heart disease has never been higher. Over the next five to seven years, industry analysts estimate that there will be a 20-fold increase in cholesterol testing. [1] But because of laboratory error, many people may get inaccurate results--so inaccurate that, like Sandra, they are given the wrong advice or treatment.

Recently, NIH set guidelines for how accurately labs should measure cholesterol. The current goals mean the results from your blood test could be as much as 15 percent higher or 15 percent lower than your actual cholesterol level. This wide margin of error raises the specter that millions of Americans now having their blood cholesterol measured could be misclassified--and mistreated--as low-risk when they should be high-risk...or vice versa.

For example, the average blood cholesterol is 211 for men and 215 for women. Some "average" men will get cholesterol test results as high as 243--which misclassifies them as "high-risk." Many others will get a test result as low as 179--which misclassifies them as "low-risk." In fact, the average man should be classified as "borderline high" risk (see graph at right).

Assessing Accuracy. Since 1963, the College of American Pathologists has conducted proficiency surveys to determine how much cholesterol test results vary from lab to lab. The labs' performance is judged in two ways. "Precision" is defined as the ability of a lab to get similar results on a single test sample each time it is run. "Accuracy" is defined as a lab's ability to match the target, or true, value of a test sample. Proficiency testing surveys measure accuracy only.

In a recent survey, CAP sent a blood sample with a cholesterol level of 265 to 5,434 labs. Even after excluding about five percent of the results which were far off the target, the remaining results ranged from a low of 229 to a high of 312--as much as 18 percent off the target value, and a total difference of more than 80. [2]

According to Herbert Naito, chairman of the National Institutes of Health Laboratory Standardization Panel on Blood Cholesterol Measurement, most manufacturers of the machines that analyze cholesterol have already achieved good precision. "What labs need to work on is accuracy," he says.

But the current standard for accuracy allows for a five percent testing error. And the current standard for precision allows for roughly a 10 percent error. Together, they mean that in 95 percent of cases, cholesterol test results may be as much as 15 percent higher or 15 percent lower than the actual value. [3] (In the remaining five percent of cases, the results may be even more off target.)

That isn't good enough, especially when other factors cause test results to vary. Among other things, cholesterol levels in a person's blood naturally fluctuate over time.

Congressional Criticism. Fortunately, Congress has recently become a severe critic of poor lab performance. Representative Ron Wyden (D-OR), who chairs the House Subcommittee on Regulation and Business Opportunities, held hearings on medical laboratories last March. Under Wyden's questioning, Guy C. Glenn of the College of American Pathologists acknowledged, "lack of accuracy in lab testing is a serious problem."

"Clearly, the lab industry's current self-policing system is not working," charges Wyden. "Right now, there are thousands of labs all over the country that are essentially unregulated."

Currently, an estimated 15,000 to 20,000 hospital and independent labs, and 100,000 to 150,000 physician office labs, perform dozens of diagnostic tests ranging from Pap smears, to white blood cell counts, to cholesterol tests.

But federal regulations cover only those hospital and independent labs that participate in the Medicare program. Under the Clinical Laboratory Improvement Act (CLIA), the government also regulates labs that do interstate business. But it has failed to set standards for all other labs--including those in doctors' offices.

Outside the System. Physician office labs, the fastest-growing segment of the lab industry, are not subject to any federal regulation at all. Over the last several years, the number of physician office labs in the U.S. has skyrocketed, in part because doctors prefer to do their own lab work. That way, they can cut out the middleman and get the full profit themselves. And even though the government doesn't regulate their labs, doctors with Medicare patients can still bill the government for full reimbursement.