How accurate is cholesterol measurement?
Once high blood levels of cholesterol became an established risk factor for heart disease over ten years ago, government public health officials mounted a nationwide educational campaign. All adults are exhorted to seek testing ("know your cholesterol number"), and those with high readings are told they should be treated first with a low-fat diet and, if that fails, lifelong drug therapy*.
The public didn't learn how inaccurate cholesterol testing can be until The Wall Street Journal did a 1987 investigative series which was picked up by news media across the country. A considerable error rate in cholesterol testing in the U.S. was acknowledged the following year by a panel of experts, appointed by the National Cholesterol Education Program.
The question of whether cholesterol measurement has become more accurate today is the subject of a new government report from the General Accounting Office (GAO). It concluded: "Some progress has been made in improving analytical accuracy in cholesterol measurement, with the development of better methods and materials in recent years. Yet, cholesterol continues to be difficult to measure with accuracy and consistency
across the broad range of devices and settings in which it is analyzed. Studies show that under controlled conditions, particularly research, clinical, and hospital laboratories, measurement is reasonably accurate and precise.
"Considerably less is known, though, about the performance of cholesterol measurement in other settings, such as physicians' office laboratories and public health screenings. Since no overall evaluation of different instruments and laboratories has been conducted, it is impossible to know whether the accuracy goals established for total and HDL [high-density lipoprotein] cholesterol have been or could be met."
Error Rate Reduced
The error rate for laboratory measurement of cholesterol has been reduced considerably, according to a survey cited by the GAO and conducted by the College of American Pathologists. One in four tests was inaccurate; now it's one in 16. Much of the progress can be attributed to quality control methods developed by the Centers for Disease Control and Prevention and the National Institute of Standards and Technology.
In the early days of the cholesterol awareness campaign, total cholesterol was the "number" people were to learn from their test results. Now the total cholesterol is viewed as one step toward learning the more important measurement of blood levels of HDL, high-density lipoprotein ("good cholesterol"), and LDL, low-density lipoprotein ("bad cholesterol"). This is believed to be more predictive of heart disease. A high level of the former (more than 60 mg/dL) and a low level of the latter (no more than 130 mg/dL) are desirable. As for total cholesterol, less than 200 mg/dL is considered the goal. These are the current guidelines of the National Cholesterol Education Program.
Though the GAO did not set out to provide recommendations to the public about how to get the most accurate testing, it contained findings that offer some guidance. Certain factors important to accuracy are beyond consumer control, such as the proper storage of blood to avoid changes in the composition of samples. But others are not; for example, avoid the finger prick type of cholesterol test associated with a desktop analyzer usually offered at health fairs and doctors' offices.
A cholesterol test of blood taken from a vein is more accurate than blood taken from a capillary (in a finger tip). One study cited by the GAO showed that capillary blood total cholesterol is 7% higher than venous source measured with same analyzer.
The length of time a person is sitting or standing prior to having blood drawn can also affect accuracy. A person should remain seated for at least 15 minutes before a venous sample is taken, and if a tourniquet is used, the GAO report found that it should be applied for less than one minute before a specimen is taken.
But even in a perfect world where 100% accuracy in testing could be achieved, the GAO found many other factors such as diet, exercise, illness, and pregnancy that can cause variations in a person's cholesterol level. In fact, some people's cholesterol levels can vary dramatically from week to week while others' remain constant.
As for the Program's initial rallying cry to "know your cholesterol number," the GAO report offered a heavy dose of reality. Instrument error and day-to-day variations from biological and behavioral factors make it highly unlikely that individuals can 'know' their cholesterol levels based on a single measurement.
"Cholesterol levels should be viewed in terms of ranges rather than as absolute fixed numbers. It is important that individuals and physicians be aware of cholesterol measurement variability and that decisions to classify patients and initiate treatment be based on the average of multiple measurements and assessment of other risk factors, as recommended by the National Cholesterol Education Program guidelines.
"This is particularly important when measured cholesterol levels are around the cutpoints that differentiate risk categories and may lead to recommendations for treatment with drugs."
*A controversy exists over the value of lowering cholesterol (see HealthFacts, September 1992 and November 1988).