Cardiovascular disease is the leading cause of mortality in the United States and globally. In 2020, 207 of 100,000 people died of heart disease and stroke in the U.S. and an estimated 19.05 million deaths were attributed to cardiovascular disease (CVD) globally, which is an increase from 2010 by 19% [1]. Reducing the risks for cardiovascular diseases is a major focus in public health and patient care.

Blood lipids, such as low-density lipoprotein cholesterol (LDL-C), triglycerides (TG), high-density lipoprotein cholesterol (HDL-C), and total cholesterol (TC), are considered modifiable risk factors for cardiovascular diseases [2]. Lipid-lowering therapies, such as statins and proprotein convertase subtilisin/kexin type 9 (PCSK9) inhibitors, reduce the risk of atherosclerotic cardiovascular diseases (ASCVD) [3]. A meta-analysis of 27 randomized trials found that in individuals with 5-year risk of major vascular events (MACE) lower than 10%, each 1 mmol/L reduction in LDL cholesterol produced an absolute reduction in MACE of about 11 per 1000 over 5 years [4]. Other studies showed that statin-mediated lipid-lowering therapies reduced MACE in high-risk individuals and in individuals with diabetes mellitus [3]. The success of statins in CVD risk reduction stimulated the development of a range of new drugs and changed how clinicians treat elevated LDL-C and reduce ASCVD risk, as summarized recently [5].

As scientific and clinical communities have developed a better understanding of the underlying etiology of CVD, clinical practice guidelines have evolved to incorporate the measurements of new, specific biomarkers central to patient risk assessment and diagnosis. Measurements of blood lipids and lipoproteins have remained central to CVD risk assessment and treatment, but the focus on certain blood lipids and lipoproteins respective concentration targets have changed. The first guideline was developed by National Institutes of Health (NIH) National Cholesterol Education Program (NCEP) in 1985 to create recommendations on the prevention and treatment of CVD [5,6]. In this guideline, known as the Adult Treatment Panel I (ATP I), individuals at higher CVD risk were initially categorized into those with high (≥240 mg/dL, 6.2 mmol/L) and borderline (200 – 239 mg/dL, 5.2 – 6.2 mmol/L) total cholesterol [7]. For patients with high or borderline TC and additional risk factors, such as smoking or high blood pressure, additional blood lipid measurements were recommended, including LDL-C calculation using the Friedewald equation [8]. LDL-C levels were then used as the basis on which drug treatment and/or life-style intervention decisions were made. It was also recommended that patients with LDL-C ≥160 mg/dL (4.1 mmol/L), receive cholesterol-lowering therapies with the goal to lower LDL-C below 130 mg/dL (3.4 mmol/L) [9]. Determination of HDL-C concentrations for all individuals were introduced in the 1993 ATP II guidelines as an addition to routine TC measurements [10]. While TC sub-categorization remained unchanged from ATP I, HDL-C categories of ≥35 mg/dL (0.91 mmol/L) and <35 mg/dL were introduced to guide further investigations and treatment decisions. Patients where then subsequently categorized based on LDL-C values as high risk (LDL-C ≥160 mg/dl, 4.1 mmol/L), borderline high risk (130 – 159 mg/dL, 3.4 – 4.1 mmol/L) and desirable <130 mg/dL (3.4 mmol/L). The treatment target was defined at LDL-C ≤100 mg/dL [10]. The ATP III guidelines were updated in 2004, and LDL-C remained the primary treatment decision biomarker. The ATP III guidelines recommended, for the first time, that physicians obtain a full lipid profile (TC, HDL-C, LDL-C and TG) on all individuals rather than limiting measurements to TC and HDL-C. While primary treatment decisions continued to center around LDL-C, with lower end goal targets of <100 mg/dL (2.59 mmol/L), ATP III also suggested treating individuals with TG ≥200 mg/dL (2.29 mmol/L). HDL-C decision targets were also changed from <35 mg/dL (0.91 mmol/L) to <40mg/dL (1.03 mmol/L) [11]. Additionally, secondary targets, such as non-HDL-cholesterol (non-HDL-C), were added for those with high TG (≥200 mg/dL or 2.29 mmol/L) and metabolic syndrome. The treatment goals for high-risk individuals were set to <70 mg/dL (1.81 mmol/L) for LDL-C or <100 mg/dL (<2.59 mmol/L) for non-HDL-C for individuals with TG >200 mg/dL (2.29 mmol/L) [12]. The 2006 joint American Heart Association and American College of Cardiology (AHA/ACC) guidelines adopted the recommendations from ATP III, in part, and suggested LDL-C targets of <100 mg/dL (2.59 mmol/L) for all patients with coronary artery disease (CAD) and other clinical forms of ASCVD, and suggested LDL-C treatment targets of <70 mg/dL (1.81 mmol/L) in high-risk patients. If attaining LDL-C concentrations of <70 mg/dL (1.81 mmol/L) is not possible, then one should aim for a LDL-C reduction of >50% with lipid lowering drugs [13].

Beginning in 2013, the AHA/ACC guidelines no longer recommended specific LDL-C treatment target goals and suggested that individuals with LDL-C ≥190 mg/dL (4.91 mmol/L) aim for a 50% reduction in LDL-C as a primary prevention treatment target [14]. For monitoring therapy, LDL-C reductions of ≥50% are anticipated for high intensity treatments in individuals with ≥190 mg/dL (4.91 mmol/L) LDL-C or 70–189 mg/dL (1.81–4.89 mmol/L) for individuals who have diabetes mellitus and a high-risk score. The 2018 AHA/ACC guidelines continued risk-based treatment approaches without providing concentration-specific treatment targets. It was also noted that LDL-C levels calculated with the Friedewald equation exhibit increased unreliability at lower levels of LDL-C, more specifically when LDL-C is <70 mg/dL (<1.81 mmol/L), and suggested calculation adjustments at very low LDL-C concentrations. Additionally, apolipoprotein B (apoB); also referred to as apolipoprotein B100 or apoB₁₀₀) measurement was recommended for individuals with TG ≥200 mg/dL (2.29 mmol/L) [15] as an alternative to LDL-C measurements, and the guidelines noted that an apoB concentration of >130 mg/dL (˃1.3 g/L) would correspond to a LDL-C level ≥160 mg/dL (4.14 mmol/L). Furthermore, the guideline pointed to lipoprotein(a) (Lp(a)) ≥50 mg/dL (≥125 nmol/L) as a CVD risk-enhancing factor [15]. Recommendations for measuring apoB and Lp(a) not only represent a means to overcome the limitations of LDL-C measurements but are also a reflection of the scientific community’s increased understanding of the roles of these proteins in the atherogenic process. The recent 2022 ACC Expert Consensus Decision Pathway on the Role of Non-statin Therapies for LDL-C Lowering in the Management of Atherosclerotic Cardiovascular Disease Risk categorizes individuals with LDL-C ≥100 mg/dL (2.59 mmol/L) and additional conditions as very high risk for future ASCVD events. Furthermore, it suggests that primary hypertriglyceridemia (TG ≥175 mg/dL or 1.98 mmol/L), elevated high sensitivity C-reactive protein (hsCRP) (≥2 mg/L or 190.48 nmol/L), elevated Lp(a) (≥50 mg/dL or ≥125 nmol/L), or elevated apoB (≥130 mg/dL or ˃1.3 g/L) are risk-enhancing factors [16].

Additional guidelines, such as the joint European Society for Cardiology and European Atherosclerosis Society (ESC/EAS) and the Canadian Cardiovascular Society (CCS) guidelines, also recommend LDL-C as primary treatment target with LDL-C treatment goals in line with those in the previous ATP III and AHA/ACC guidelines [17–19]. The 2011 ESC/EAS guidelines pointed out that factors such as lower HDL-C or apoA-I, and higher TG, apoB, Lp(a), or hsCRP are all indicators of higher CVD risk than what is estimated solely using the SCORE algorithm-based risk charts [19]. The 2016 ESC/EAS guidelines recommended measurement of apoB or non-HDL-C in individuals with high TG for risk assessment before treatment [20]. The most recent 2019 ESC/EAS guidelines now recommend Lp(a) testing once in a lifetime to identify those with very high Lp(a) levels (>180 mg/dL or >430 nmol/L) [21,22]. Similarly, the 2021 Canadian Cardiovascular Society guidelines also recommend measuring Lp(a) once in a person’s lifetime for screening purposes and recommends earlier and more intensive health behavior modifications in the primary prevention setting for individuals with Lp(a) ≥50 mg/dL (or ≥100 nmol/L).[23] In patients with familial hyperlipidemia at very high risk, the ESC/EAS guidelines suggested a reduction of ≥50% from baseline LDL-C as a treatment target with an LDL-C goal of <55mg/dL (<1.4 mmol/L). Furthermore, apoB measurements are now recommended for risk assessments in individuals with high TG, diabetes mellitus, obesity or metabolic syndrome, and very low LDL-C. The ESC/EAS guidelines not only provided treatment targets for LDL-C, but also for non-HDL-C (<85, 100, and 130 mg/dL or <2.2, 2.6, and 3.4 mmol/L for very high, high, and moderately high risk, respectively) and apoB (<65, 80, 100 mg/dL or <0.65, 0.80, 1.0 g/L for very high, high, and moderately high risk, respectively). In addition, the CCS guidelines recommend non-HDL-C or apoB measurements in lieu of LDL-C in patients with TG >1.5 nmol/L[23]. For patients with ASCVD who experience a second MACE within 2 years while taking maximally tolerated statin-based therapy, the EAS/ESC guidelines suggest an LDL-C goal of <40 mg/dL (< 1.03 mmoL/L) may be considered [21,22].

Throughout this remarkable evolution of guidelines from 1988 to present, LDL-C has remained the cornerstone of ASCVD management. Treatment goals changed from LDL-C recommendations of <130 mg/dL (3.36 mmol/L) in ATP I to LDL-C levels <40 mg/dL (1.03 mmol/l) in individuals with certain risk-enhancing conditions. Similarly, the LDL-C levels for initiating treatment of patients changed from treating patients with LDL-C ≥160 mg/dL (4.14 mmol/L) to lower LDL-C levels in patients with additional risk factors. Specific treatment targets for LDL-C are no longer recommended in current guidelines. Also, these guidelines no longer use HDL-C as a biomarker for treatment or diagnostic decisions [24,25] and suggest the use of non-HDL-C, apoB, and Lp(a) as new biomarkers in addition or in lieu of traditional blood lipids.

In addition to directly measured CVD risk biomarkers, there are biomarkers that rely on calculated values derived from traditional blood lipid measurements. For example, LDL-C can be measured directly but is often calculated with the Friedewald equation, which uses TC, TG, and HDL-C values to estimate LDL-C concentrations [8]. The equation assumes that very low-density lipoprotein cholesterol (VLDL-C) concentrations can be estimated as being 1/3 of TG values. The equation was derived using lipids data from healthy individuals and provides reliable estimates in individuals with normal TG values; however, the equation is prone to underestimation of LDL-C in patients with high TG and when the actual LDL-C concentration is low [26]. The limitations of the Friedewald equation, especially at high TG levels, are reflected in the most recent clinical practice guidelines, which now recommend the use of alternate calculation algorithms [15], the calculation of non-HDL-C [11], and the measurement apoB in patients with high TG. Over 20 alternate LDL-C equations have been proposed [27] of which 2 equations are currently used in clinical practice, namely the Martin/Hopkins [28–30] and Sampson equations (also referred to as the NIH equation) [31,32]. These new equations showed a higher accuracy than the Friedewald equation, especially at high TG levels and at LDL-C concentrations <70 mg/dL (1.81 mmol/L) [27,33]. A detailed discussion of both equations was recently published [34]. These formulas are still subject to further improvements [35,36] and evaluations [37,38]. Several recent guidelines started recommending non-HDL-C as an alternative to LDL-C for CVD risk assessment [39–41]. Non-HDL-C is determined by calculating the difference between TC and HDL-C measurements and therefore includes other atherogenic particles, such as VLDL [42], that LDL-C measurements do not capture. Common to all of these equations is their dependence on accurate and reliable measurements of TC, HDL-C, TG, and other blood lipids.

While measurements of traditional blood lipids (TC, HDL-C, LDL-C, and TG) remain essential for CVD risk assessments and monitoring treatment, measurements of apolipoproteins, such as Lp(a) and apoB, are now established for use in CVD risk management. Despite inclusion of apolipoproteins in clinical practice guidelines, these biomarkers are not yet widely used in patient care and public health. This can be explained, in part, by their structure, role in CVD risk assessment, and in their standardization status not being widely appreciated, which is discussed herein.

Apolipoproteins are proteins associated with lipoprotein particles, including LDL-C, HDL-C, and others [43]. The surface of the LDL-C particle, for example, is enveloped by a monolayer consisting of phospholipid molecules and apoB. ApoB is a large, hydrophobic protein present as a single copy on LDL particles, VLDL particles, intermediate-density lipoprotein (IDL) particles, as well as Lp(a) particles [44]. Thus, apoB is present in the majority of particles represented by non-HDL. With each particle containing one apoB copy, the measurement of apoB is a convenient approach to evaluating a patient’s total number of circulating atherogenic particles to establish ASCVD risk [45]. Indeed, studies have established that the particle number of apoB-containing, atherogenic lipoproteins is the most important characteristic for determining ASCVD risk [46]. For example, a 2021 study from Johannesen et al. concluded that elevated apoB and non-HDL-C, but not LDL-C, are associated with residual risk for all-cause mortality and myocardial infarction [47].

Lp(a) is a complex protein that consists of an apoB-containing LDL-like particle that is covalently linked to a defining apolipoprotein(a) [apo(a)] protein [48]. Evidence from several high-profile statin and PCSK9 inhibitor clinical trials (4S, AIM-HIGH, FOURIER, among others) previously suggested that when Lp(a) levels are elevated, CVD event rates are higher irrespective of the LDL-C levels achieved through statin and PCSK9 use [49–51]. These findings ultimately led to Lp(a) being recognized as a biomarker of “residual risk” for adverse CVD outcomes in the setting of controlled LDL-C [50,52]. Additionally, Lp(a) is now recognized as an independent, causal risk factor for the development of ASCVD and calcific aortic valvular disease [53,54]. While clinical guidelines have established that elevated Lp(a) levels (≥50 mg/dL or ≥125 nmol/L) constitute enhanced CVD risk [15,22,23].

The primary structural and functional component of HDL-C is apoA-I. HDL-C is hypothesized to mitigate atherosclerosis via the uptake and clearance of excess lipids/cholesterol from tissues, and this reverse cholesterol transport process is facilitated by apoA-I [55,56]. High levels of apoA-I are associated with a reduced risk of cardiovascular events. In contrast, low levels of apoA-I, in example due to apoA-I mutations, can indicate an increased risk for coronary artery disease [57,58]. Despite its inverse relationship with CVD, HDL-raising therapies have not met clinical expectations, suggesting that lipoprotein biology and/or metabolism are more complicated. Indeed, niacin supplements and cholesteryl-ester transfer protein inhibitors, tested in the HPS2-THRIVE and REVEAL trials, respectively, increased HDL-C [59,60]; however, CVD outcomes trials ended due to non-ideal pharmacokinetics, futility, and serious adverse events. Despite these findings, there is an increasing interest in the measurement of apoA-I in lieu of HDL-C, as apoA-I measurements may allow more direct quantification of HDL-C particle numbers.

For all these biomarkers certain concentrations are stated in guidelines to guide clinical decision making. Therefore, standardization systems need to be created and maintained to ensure the analytical accuracy and reliability of these CVD biomarker measurements and thus to enable correct and consistent clinical decision making using these clinical practice guidelines.

Specific concentrations for blood lipids and lipoproteins described in clinical practice guidelines can only be applied when laboratory tests used in patient care have the same analytical accuracy, precision, and selectivity as those used to create these guidelines. Furthermore, to ensure consistency in clinical decision making over time, the analytical performance of laboratory tests needs to be constant over time. Since its inception in the early 1960s, the Centers for Disease Control and Prevention’s (CDC) CVD Biomarkers Standardization Program (CDC CVD BSP), formerly known as the CDC Lipids Standardization Program, has worked with clinical laboratories, researchers, and other key stakeholders to ensure that the CVD biomarker measurements used for research and performed in patient care settings are comparable and consistent over time. This enabled the effective implementation of evidence-based clinical practice guidelines and facilitated the described advancements in patient care and public health [61,62].

The analytical performance of CVD biomarker tests and, thus, the analytical comparability of test results, can differ across testing systems and can change over time if not monitored in a longitudinal manner and corrected appropriately through standardization activities. Differences and changes in analytical performance can be caused by a multitude of factors [24,63,64]. Addressing these factors requires programs that assess the analytical performance of these tests in a comprehensive, consistent, and scientifically appropriate manner. This is achieved most effectively by comparing test results to a reliable and continuous point of reference. The CDC CVD BSP has provided this point of reference and thorough analytical performance assessments for blood lipids for over 60 years [61,62,65,66].

Analytical performance of tests often varies at different biomarker concentrations, with tests typically aiming for optimal performance at clinical target concentrations. In the past, this was accomplished by the NCEP Expert Laboratory Panel, which developed recommendations for reliable measurements of blood lipids. These recommendations were adopted by the CDC CVD BSP, as well as manufacturers and laboratories. CDC CVD BSP had already standardized measurements conducted in the clinical trials on which the ATP guidelines were based. With advancements in each ATP guideline, the NCEP Expert Laboratory Panel added analytical performance specifications for lipid measurements, including TC [67], LDL-C [68], TG [69], and HDL-C [70]. With the discontinuation of the NCEP, the work performed by the NCEP Expert Laboratory Panel to set analytical performance specifications also stopped. As a result, recommendations on analytical performance requirements are no longer updated to reflect changes in current clinical guidelines and old performance requirements established in the 1990s are still being applied without appropriate modifications (Table 1). A recent study suggests that current NCEP standards may cause up to 10% misclassification of patients. This study also highlights the need for stakeholder collaborations to adjust analytical performance requirements [37].

As clinical practice guidelines evolved, the CDC CVD BSP engaged in research collaborations and generated new data on the analytical performance of CVD biomarker assays to broaden the scientific and clinical community’s understanding of the strengths and weaknesses of traditional lipid biomarker tests used in patient care and research. For example, one key study reported increased measurement inaccuracies in patients with different comorbidities, such as diabetes and hypertriglyceridemia [41,71–74]. Findings from CDC CVD BSP collaborative studies, as well as studies conducted by other groups, are reflected in current guidelines, such as the recently suggested use of apoB or non-HDL-C in patients with high TG [75] to overcome the aforementioned measurement inaccuracies in these patients. Furthermore, based on these research findings, the CDC CVD BSP adjusted its protocols to include samples from individuals with diseases known and/or suspected to affect lipid measurements. In addition, these new protocols [76] now focus on investigating LDL-C measurement accuracy at the low LDL-C concentrations targeted in current clinical practice guidelines [15,76]. CDC CVD BSP is continuing efforts in collaboration with NIH and other key stakeholders to adjust analytical performance criteria for both traditional blood lipids and new CVD biomarkers to facilitate the effective implementation of current guidelines, as proposed recently [15,16,21,22].

The CDC CVD BSP aims to ensure correct and consistent patient care and to enable effective implementation of research findings in health care and public health. It accomplishes this by creating measurement results in patient care, public health, and clinical research that are comparable across methods, locations, and over time and that are independent of analytical methodologies or technologies. CDC CVD BSP is doing this by assessing, improving, and maintaining the analytical performance of clinical laboratory tests. To conduct these activities appropriately and effectively, it established and maintains unique laboratory capacity and expertise within its clinical reference laboratories (CRL). It uses this capacity and expertise to assist assay manufacturers and clinical laboratories. Furthermore, this capacity is used to monitor the analytical performance of clinical tests as they are used in patient care and research, as detailed below.

CVD biomarkers currently used in patient care and public health range from chemically well-defined molecules, such as cholesterol, to highly complex particles consisting of a range of different small to highly complex molecules, such as LDL-C. The complexity of these particles and the concepts used to define them are not always fully appreciated. However, they determine how certain standardization activities are implemented and, most importantly, the level of agreement among assays that is technically achievable. Herein, we discuss the specific needs of various analytes used in clinical practice. Specifics with respect to the current status of CVD biomarker standardization and improvement efforts underway are discussed in the sections below.

Initial standardization of traditional blood lipids, especially TC, was implemented at the beginning of major clinical trials conducted in the 1970s by the NIH Lipid Research Clinics [62]. Thus, clinical decision points for blood lipids stated in the ATP I guideline were generated using tests already standardized by CDCs. The findings from the NIH study stimulated the development of new clinical tests for blood lipids, which also underwent standardization before they were introduced commercially. This situation profoundly facilitated the implementation of the ATP guidelines in patient care and public health, and majorly contributed to the success and fast evolution of CVD risk assessment and treatment described above.

For apolipoproteins, standardization activities were introduced after clinical trials were conducted, guidelines were published, and commercial tests became available. This situation complicates the effective implementation of standardization activities as existing clinical tests need to be readjusted. Furthermore, it hampers the broad use of clinical decision points mentioned in clinical practice guidelines. This situation may explain in part the current limited use of apolipoprotein measurements in patient care and public health. With apolipoproteins being more recently recognized broadly to be beneficial in CVD risk assessment and treatment, standardization activities are less advanced compared to those available for blood lipids. As mentioned, the situation was further complicated with the loss of the initial reference systems for certain lipoproteins.

In general, Lp(a), apoB, and apoA-I are complex proteins with different isoforms and post-translational modifications [120]; however, they can be more easily quantified compared to HDL-C and LDL-C. Different methodologies are currently available to quantify these proteins by targeting well-defined segments/epitopes of the protein, and when appropriately designed with post-translational modifications and isoforms in mind, can accurately quantify the protein. For example, immunoassays currently utilize antibodies that recognize specific amino acid segments, or epitopes, that are specific to a certain apolipoprotein, while mass spectrometry (MS)-based methods currently measure peptides specific to a certain apolipoprotein protein after proteolytic digest [120–122]. Because the amino acid sequences and primary characteristics of these proteins are generally known, higher order reference materials for these proteins or relevant protein segments/epitopes can be produced. These reference materials can be used for calibration to represent the specific protein, including all known modifications and isoforms, when designed and produced accordingly.

Over the past decades, CVD risk assessment and ASCVD treatments have changed profoundly, as reflected in an evolution of clinical guidelines and new recommendations for CVD biomarker testing. While blood lipids, namely TC, HDL-C, LDL-C, and TG remain central to CVD risk assessment and treatment, apolipoproteins such Lp(a), apoB, and apoA-I are now included as part of the CVD testing panel to provide additional information about residual CVD risk and/or to overcome limitations of traditional blood lipid measurements.

With treatment targets now focusing on LDL-C concentrations that are profoundly lower than those in earlier guidelines, the assessment of analytical performance of LDL-C tests is now focused on those lower concentrations. While these assessments revealed that the analytical performance of LDL-C measurements at the old treatment targets are appropriate, the analytical performance at the new treatment targets needs to be improved. Furthermore, LDL-C measurements require further improvements to provide more accurate measurement results in patients with certain conditions, such as hypertriglyceridemia.

The data collected in CDC’s CVD BSP indicate that the achievable level of improvement depends, in part, on how well a biomarker can be defined and on the consistency of the measurement principles used by clinical assays and RMPs. The highest level of agreement among different assays was achieved for total cholesterol, a well-defined analyte, while the same level of agreement may not be technically feasible with less well-defined biomarkers, such as LDL-C, especially since clinical tests for LDL-C use different, proprietary techniques to emulate ultra-centrifugation. Traditionally, cholesterol in LDL particles is assumed to reflect the number of LDL particles. With a better understanding of the composition of lipoprotein particles and their role in the atherosclerotic process, measurements of apoB concentrations are now considered a more accurate parameter for the number of atherogenic particles present in blood. One of technical advantage of apoB assays is that they appear less affected by triglycerides and other comorbidities than LDL-C. This may explain why the described IQR for apoB are smaller than for LDL-C. Despite these limitations, LDL-C remains a primary biomarker used for CVD risk assessment; thus, further standardization activities for LDL-C are focused on improving analytical performance at LDL-C levels targeted in current clinical treatment guidelines, while maintaining the level of accuracy achieved at the higher levels.

Lp(a) is a biomarker that captures residual CVD risk not captured by traditional blood lipids measurements. While guideline recommendations differ with respect to Lp(a) clinical decision cut-off concentrations, units and frequency of measurement, and patient populations for which measurements are recommended, the overarching consensus is that Lp(a) should be measured. This is important given that recent studies estimate that over 1.4 billion people worldwide have elevated levels of Lp(a), which represents ~20% of the world’s population [50]. Indeed, a recent multicenter, cross-sectional, epidemiological study demonstrated that >25% of established ASCVD patients from 48 countries had Lp(a) levels exceeding the aforementioned clinical decision thresholds for increased CVD risk [132,133]. With clinical decision threshold being defined, the assays need to be accurate and comparable to consistently use these decision thresholds. Despite previous standardization efforts, Lp(a) assays are still not sufficiently accurate and comparable. The previous reference system, which is no longer available, is being replaced by a new, more sustainable SI-traceable reference system. Standardization activities for Lp(a) will continue as a collaborative effort among stakeholders.

Fundamental to the efficient and reliable use of traditional and new CVD biomarker measurements in patient care, clinical research, and public health is the consistent accuracy and comparability of biomarker test results across measurement systems, time, and location. The CDC CVD BSP has successfully performed these activities for traditional CVD biomarkers for decades. As a result, total cholesterol measurements, standardized since the early 1960s, are highly accurate globally. The current lack of reference systems for apolipoproteins that initially were maintained by a single laboratory highlight the importance of maintaining networks of reference laboratories to ensure sustainability of reference systems once established. Thus, the success of standardization of traditional blood lipids over decades, which is mainly attributed to the work conducted by CRMLN, will provide a template for maintaining a sustainable system for apolipoproteins. As a first step towards this goal, CDC CVD BSP is collaborating with stakeholders to establish a new, sustainable mass spectrometry reference system for apoB, apoA-I, and Lp(a). With formal, more sustainable standardization programs for apolipoproteins being available soon, more data will be generated to further solidify their role in CVD risk assessment.

The success of previous clinical practice guidelines can, in part, be attributed to successful coordination of guideline implementation and related laboratory standardization activities. Building on these experience, CDC CVD BSP will collaborate with stakeholders to continue facilitating successful implementation of new guidelines.