The immediate outcome of improved lab standardization and manufacturer certification is an increase in the accuracy of cholesterol testing. The intention of the CDC lipid standardization programs is to improve the accuracy and comparability of research-related testing, primarily through the Lipid Standardization Program (LSP), and to improve the accuracy of general clinical tests directly through the Cholesterol Reference Method Laboratory Network (CRMLN). There is strong evidence that laboratory performance on cholesterol testing has improved through standardization during the past 25 years, although it is difficult to say how much of the improvement has been due to the LSP and CRMLN.

3.1.1 Accuracy of LSP Standardized Laboratories

The goal of the LSP is to ensure that member labs exhibit consistent accuracy in lipid testing over time. Early lipid testing was subject to significant levels of error and bias, so initial efforts of the LSP focused on improving the accuracy of lipid testing through the development and establishment of reference testing methods. As standardization was achieved, the LSP focus turned to maintaining accuracy of lipid testing. Table 3-1 shows the percentage bias and coefficient of variation (CV) of LSP standardized labs (based on part III standardization maintenance surveys among newly enrolled labs that entered the program in each year) since electronic recordkeeping began in 1999. The results show that average bias levels have remained below 2% from 1999 through 2007 and have decreased by 0.7% percentage points over that time period. Labs are considered standardized if neither their percentage bias nor their CV exceeds 3%. Over that same period, less than 10% of surveys exhibited a bias of more than 3% except in 1999 and 2001, and no surveys yielded a CV of more than 3%. Note that the low number of observations may preclude drawing significant conclusions on the trend of "failing" labs over time.

3.1.2 Accuracy of Labs in CRMLN Clinical Laboratory Certification Program

Although the LSP results demonstrate that standardization is being achieved among the limited number of research-oriented LSP standardized labs, the primary mechanism through which the CDC laboratory standardization programs may increase clinical testing accuracy is through the CRMLN. The CRMLN labs are intended to replicate CDC reference methods to extend the reach of standardization, most importantly through the manufacturer certification process. The manufacturer certification process allows manufacturers to calibrate their equipment and supplies against accuracy-based reference values. When used by clinical testing labs, the calibrated supplies will presumably increase the accuracy of clinical testing conducted by these labs. A resource for evaluating how well this program works is to look at data from clinical laboratories that participate in the CRMLN's Clinical Laboratory Certification Program. Table 3-2 shows the average percentage bias and average percentage CV among clinical laboratories participating in this program since 2000. Panel 3.2.a shows results for all labs applying to obtain or maintain certification, and Panel 3.2.b shows results for the subset of labs that obtained certification. As with the LSP standardized labs, the results show consistently low and improving bias and CV values.

Table 3-2. Accuracy of Clinical Laboratories Participating in CRMLN Certification, 2000–2009

3.1.3 CAP Survey Results Show Improvement in Clinical Labs

Although the above tables show that standardization is being achieved among labs participating in the LSP and CRMLN, these results do not directly reflect the accuracy of the many nonprogram labs that conduct patient clinical testing. Table 3-3 shows the results of CAP proficiency testing surveys for total cholesterol for major methods/instruments peer groups between 2000 and 2006. This period was selected because the CAP specimens were relatively free of matrix effects and CDC performed confirmatory testing on the materials used in the program. With the exception of 2001, we used the specimen in the year's A survey whose total cholesterol confirmatory value was closest to 200 mg/dL. Because the 2001 A survey was not available to us, we used the specimen in the B survey with a confirmatory value closest to 200 mg/dL.

Table 3-3 shows that bias and CV have remained consistently low and continue to show improvement at the method/instrument level, with the average absolute value of bias falling from 1.2% in 2000 to 0.7% in 2006, and the CV remaining unchanged. Four out of 34 methods/instruments had biases > 3% in absolute value in 2000 compared with 1 out of 21 in 2006; the corresponding figures for CV were 6 out of 34 in 2000 compared with 2 out of 21 in 2006.

Improvements in standardization have likely been much larger over a longer period, although the available data are not ideal for making long-term comparisons. Table 3-4 shows laboratory performance on CAP surveys in 1985 and 2009. These results show clear improvement in the bias (as measured by the percentage difference between a method/instrument mean and the all method/all instrument mean) and CV for each method/instrument. In 1985, about half of the method/instrument groups had biases greater than 3% in absolute value, and all but two method/instrument groups had CVs greater than 3%. In 2009, only one method/instrument group had a bias greater than 3% in absolute value, and only one group had a CV greater than 3%. However, the specimens used in 1985 and 2009 did not closely resemble patient samples and may have been subject to matrix effects (meaning that an instrument could perform well on patient samples but produce biased results on alterated, nonpatient samples). Nevertheless, the dramatic improvements shown in bias and in particular CV (which may be less susceptible to matrix effects) clearly suggest that standardization of labs occurred over this period.

CAP launched its Accuracy Based Lipid (ABL) Survey in 2008 to eliminate or minimize matrix effects and provide better measures of the accuracy and harmonization of cholesterol testing. Results from the 2009 ABL (Table 3-5) suggest that almost all participating laboratories meet current NCEP standards for total cholesterol (total error within 9% of the target level). Most participating laboratories meet National Cholesterol Education Program (NCEP) standards for HDL cholesterol (total error within 13% of the target level), although the performance is not as strong as on total cholesterol. The ABL should be useful for identifying trends in laboratory accuracy as more years of data become available.

Source: College of American Pathologists, 2009b.

Increasing the accuracy of research and clinical testing will result in several medium-term outcomes, including improving clinical diagnosis rates and improving cholesterol-related research. Standardization of research testing has facilitated several important events, from the early research linking elevated total and LDL cholesterol to higher mortality, to more focused, clinical research on the efficacy of treatment and prevention interventions, including drugs and diet and exercise changes. Together, these findings have allowed for the creation of the ATP practice guidelines and provided the impetus for numerous public health campaigns targeted toward increasing physician and public awareness of the risks of high cholesterol and its modifiable risk factors.

3.2.1 Improved Clinical Diagnosis Rates

Better laboratory accuracy facilitates better diagnosis of persons with high cholesterol. If a laboratory produces biased cholesterol readings, some patients who truly need cholesterol reduction may not receive treatment, whereas other patients who do not need treatment may receive it. We used the data on bias from the method/instrument observations underlying Table 3-3 to estimate the percentage of patients whose total cholesterol would be misclassified in 2000 and 2006. We examined the ATP III cutoffs of 200 and 240 mg/dl to distinguish between desirable, borderline, and high total cholesterol. We used cholesterol information for U.S. adults from the 1988–1994 National Health and Nutrition Examination Survey (NHANES III), as reported in the ATP III report (ATP III, 2002), to calculate separate total cholesterol population distributions for men and women. For men, the data are roughly consistent with a normal distribution with mean 202 and variance 40.5. For women, the data are roughly consistent with a normal distribution with mean 206 and variance 43.5. Using the percentage bias for each individual laboratory method in 2000 and 2006 from the data underlying Table 3-3, we calculated the percentage of people misclassified relative to the "true" ATP-III distributions. For example, if the true distribution for males was distributed normally with a mean of 200 and variance of 10, then we would expect to find 50% of men classified as having desirable cholesterol (<200 mg/dl); however, if an individual laboratory method had 5% bias so that its distribution of cholesterol values was distributed normally with a mean of 210 and variance of 10, then too few men would be classified as having desirable cholesterol level and too many would be put into the borderline or high cholesterol categories.

Tables Table 3-6 and Table 3-7 show the percentage of adults whose total cholesterol would be correctly reported (so that a patient's reported value is the same as the true value, as highlighted in green) and the percentage that would be misclassified (so that the patient's reported value is not the same as the true value, as highlighted in red) in 2000 and 2006. The reduced bias in 2006 leads to fewer misclassifications. In 2000, a total of 3.9% of the male population would have been misclassified, including 0.5% of the population with true high cholesterol who would have been reported as having borderline cholesterol. In 2006, misclassifications reduced to 2.3% of the male population, including 0.2% of the population with true high cholesterol who would have been reported as having borderline cholesterol. Values for the female population exhibit the same trend toward fewer misclassifications.

If we conduct this same exercise using the 1985 and 2009 CAP proficiency testing survey data, the share of the population that would be misclassified is much larger (Tables 3-8 and 3-9) in 1985 with misclassifications falling dramatically by 2009. These results should be interpreted cautiously, however, because these specimens may have included matrix effects that distorted the true bias in patient samples.

3.2.2 Practice Patterns

High levels of cholesterol, including LDL cholesterol, were not definitively linked to increased risk of heart disease until the publication of the results of the Lipid Research Clinics Coronary Primary Prevention Trial in 1984 (Lipid Research Clinics Program, 1984). In 1985, NHLBI formed NCEP to organize public health efforts to reduce cholesterol-attributable heart disease. At the heart of these efforts are the clinical practice guidelines, the latest of which is the Detection, Evaluation, and Treatment of High Blood Cholesterol in Adults (ATP III, 2002).

To produce the practice guidelines, NCEP and its partners had to synthesize data from a number of sources to produce a comprehensive cholesterol control strategy. For example, epidemiological and clinical trials have demonstrated the harms of high cholesterol in the overall population and the degree of elevated risk among subpopulations. Other trials have assessed the efficacy of treatment to reduce high cholesterol. Finally, epidemiological surveys such as NHANES are necessary to identify actual cholesterol levels in the population. Combining the outcomes of such disparate studies is only possible when the cholesterol values of each study are directly comparable. To achieve comparability, the major cholesterol studies used to produce the practice guidelines have depended on LSP standardized labs to ensure accuracy and allow comparability.

3.2.3 Cholesterol Awareness

An important goal of public health programs focused on cholesterol is to increase awareness of the risks of high cholesterol among physicians and the general public. NCEP has focused efforts in two key areas: (1) improving clinical practice to increase the detection of high cholesterol and improve cholesterol treatment, and (2) increasing public awareness of the risks of high cholesterol to promote cholesterol-reducing lifestyle choices. By working with physician associations, continuing to refine the ATP guidelines, and conducting national conferences on cholesterol, NCEP continues to focus on physician education and training. NCEP has also focused on public health information campaigns, including the Know Your Number campaign to highlight the risks of high cholesterol among the general public and the Healthy People 2000 and Healthy People 2010 campaigns, which set defined targets for population cholesterol control.

NCEP efforts to improve physicians' understanding of the risks of cholesterol on CHD appear to have been largely successful. The NHLBI Cholesterol Awareness Surveys found that the number of patients who had ever had their cholesterol levels checked increased from 35% to 75% between 1993 and 1995 (NHLBI Cholesterol Awareness Surveys press release, December 1995). This survey also found that physicians had lowered the threshold for initiating cholesterol reduction treatment and were generally in compliance with the ATP guidelines. A CDC study using Behavioral Risk Factor Surveillance System data found that the proportion of people who reported having their blood cholesterol screened in the preceding 5 years increased from 67.6% in 1991 to 73.1% in 2003 (Saddlemire et al., 2005). However, recent evidence finds that lower rates of dietary and pharmacologic therapy initiation remain among certain physician groups, indicating that education efforts need to continue (Yarzebski, Bujor, & Goldberg, 2002).

As with the development of the clinical guidelines, public health information campaigns are ultimately the product of multiple and disparate sources of data on the risks of high cholesterol and the effectiveness of different treatment and prevention strategies; as with the formation of the practice guidelines, this is possible only when the data used are directly comparable due to the underlying accuracy of the cholesterol measurements. Thus, the CDC lipid standardization programs have played an important role in facilitating the research necessary to inform, guide, and bolster public health information efforts.

3.2.4 Cholesterol-Lowering Drugs

There have been clear improvements in drug therapies to reduce LDL cholesterol levels and/or increase HDL cholesterol levels in recent years (the LSP does not standardize LDL testing, although the CRMLN does; to estimate LDL levels, most U.S. laboratories use the Friedewald equation, which depends on total cholesterol, HDL cholesterol, and triglyceride measures that are standardized by both programs). In particular, the introduction and widespread adoption of statins has revolutionized cholesterol management. Currently, six statins (atorvastatin, fluvastatin, lovastatin, pravastatin, rosuvastatin, and simvastatin) are approved for use in the United States. These statins have been shown to reduce LDL cholesterol by 34% to 55%, with the most recently approved statins producing the largest reductions (Senior Journal, 2005). Other cholesterol-lowering drugs include ezetimide, nictonic acid, fenofibrate, and gemfibrozil.

The standardization of cholesterol measurement has played an important but difficult-to-quantify role in the development of cholesterol-lowering drugs. Cholesterol-lowering drugs are approved based primarily on their safety and their efficacy in lowering LDL cholesterol levels. To assess efficacy, it is necessary to accurately and reliably measure cholesterol levels. Standardization of cholesterol testing allows a large number of patients to be tested in large, multicenter clinical trials. Standardization also facilitates comparisons across trials and allows improvements in cholesterol to be assessed in the context of previous epidemiological studies showing the relationship between standardized cholesterol levels and clinical outcomes.

3.2.5 Diet and Exercise

In addition to pharmacological cholesterol reduction treatment, diet and exercise are important for ensuring reductions in cholesterol levels. On the basis of observational study findings, ATP III lists physical inactivity and an atherogenic diet (which generally includes high cholesterol) as major modifiable risk factors for high levels of LDL cholesterol and low levels of HDL cholesterol. The consumption of saturated fats and cholesterol has been falling since the early 1970s. In 1972, the average American consumed 355mg of cholesterol and 13.2g of saturated fat with a total energy intake of 1,983 kilocalories per day. By 1990, the cholesterol and fat intake measures had improved to 291mg of cholesterol and 12.6g of saturated fat with 2,199 kilocalories consumed per day (Ernst, Sempos, & Briefel, 1997). So while total caloric intake has markedly increased, cholesterol and saturated fat have decreased both in proportional and absolute levels. In the following decade, the proportion of calories from saturated fat continued to fall, although total cholesterol intake decreased only in men and actually increased by 11g per day in women (Carroll, Lacher, & Sorlie, 2005). However, the consumption of LDL cholesterol has decreased (Carroll, Lacher, & Sorlie, 2005). As with cholesterol medications, the evidence for the efficacy of lifestyle interventions to mitigate these risk factors came from clinical and epidemiological research, which, in most cases, benefited from increased accuracy due to the LSP.

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