LDL Cholesterol Calculations

The level of low-density lipoprotein cholesterol (LDL-C), among other factors, correlates with the likelihood of developing atherosclerotic cardiovascular disease (ASCVD). Thus, LDL-C measurement is useful for assessing ASCVD risk, stratifying individuals into treatment benefit groups, and monitoring risk-reduction therapy.¹

LDL-C is most often measured indirectly, using a calculation based on other blood lipid analytes. Historically, the Friedewald calculation² has been the most common approach. This equation, developed in the 1970s, incorporates total cholesterol, HDL cholesterol (HDL-C), and triglyceride concentrations:

         LDL-C (mg/dL) = total cholesterol – HDL-C – (triglycerides/5)

In this equation, “triglycerides/5” is used to represent very low-density lipoprotein-C (VLDL-C).

LDL-C concentration can also be measured directly or with newer equations. Quest Diagnostics uses the Martin-Hopkins calculation, as described in Question 4, which provides accurate quantitation even in patients with triglyceride (TG) values between 200 mg/dL and 400 mg/dL and LDL-C levels below 70 mg/dL. 

Although the Friedewald equation it is still widely used and generally produces reliable results, it may underestimate LDL-C at the low LDL-C levels that modern treatment guidelines call for and therapies can achieve (e.g., <70-100 mg/dL).¹ PCSK-9 inhibitors can drive LDL-C levels even lower (eg, <40 mg/dL). Such low LDL-C concentrations are below the concentrations considered when the Friedewald equation was developed.²

This limitation relates to the fact that the Friedewald equation uses a fixed ratio of triglyceride to VLDL-C; it does not allow for heterogeneity in the ratio of triglycerides to VLDL-C. This becomes a problem especially at lower LDL-C and higher triglyceride concentrations, when the Friedewald calculation tends to underestimate LDL-C.

In a 2013 study, Martin and colleagues compared the results of direct LDL-C and calculated LDL-C using the Friedewald equation in more than 1.3 million US adults.³ Friedewald-estimated LDL-C tended to be lower than directly measured LDL-C, especially in patients with calculated LDL-C levels below 100 mg/dL. At triglyceride levels ≥150 mg/dL, calculated LDL-C was often <70 mg/dL in patients with directly measured LDL-C in the range of 71 to 80 mg/dL.

These findings suggest that the tendency of the Friedewald equation to underestimate LDL-C at higher triglyceride and lower LDL-C levels could result in high-risk patients being undertreated.

The Martin-Hopkins calculation provides greater customization to a patient’s specific triglyceride level by using a more “personalized” factor to calculate VLDL-C from triglycerides.³ This adjustable factor, which can range from 3.1 to 11.9, was derived from an analysis of triglyceride-to-VLDL-C ratios in more than 1.3 million people.³ The factor is lowest for patients with very low levels of triglyceride and high levels of non-HDL cholesterol (total cholesterol – HDL-C), and highest for those with very high levels of triglyceride and low levels of non-HDL cholesterol.

Compared with the Friedewald equation, the Martin-Hopkins calculation provides better correlation with direct LDL-C measurements.^1,3,4 Concordance with guideline-based risk classification, especially at high triglyceride and low LDL-C levels, is also superior using the Martin-Hopkins calculation. ^1,3,4

In the 2013 validation study mentioned in Question 3,³ the improvement was greatest for people with estimated LDL-C levels below 70 mg/dL, especially those with higher triglyceride levels (Table 1). Thus, the primary advantage of the Martin-Hopkins equation is that it is applicable to low LDL-C levels even in the presence of elevated triglyceride concentrations. 

The improved accuracy at low LDL-C levels allows more accurate assessment of patients in the high-risk categories undergoing aggressive treatment with low LDL goals. In addition, the ability to adjust for high triglyceride levels may improve reliability of LDL-C estimation when fasting is not desired or practical.⁵ This can be convenient for risk assessment, especially for patients who have difficulty fasting (eg, young children and people with diabetes).⁵ 

The need for fasting varies with the indication for testing, and the method used to calculate LDL-C should not affect the decision to require fasting samples. However, as noted in Question 5, the ability of the Martin-Hopkins calculation to adjust for high triglyceride levels may also make LDL-C estimation more reliable in nonfasting patients.⁵

If the LDL concentration could not be calculated because the triglyceride level was too high, direct LDL-C testing may be useful. Direct LDL measurement provides a reliable result even when triglyceride levels are up to 1,000 mg/dL. It can be ordered as a stand-alone test, as a reflex if the patient’s triglyceride level is likely to exceed 400 mg/dL, or as part of one of several panels.

Table 2 lists tests and panels that include direct LDL-C measurement or reflex to direct LDL-C measurement when the triglyceride level is above 400 mg/dL.

Yes. Aside from standard lipid profile tests, Quest offers several advanced lipid testing options, including measurement of LDL particle number and size. In retrospective analyses, lipoprotein subfractions were associated with CVD events.^6,7 Additionally, particle numbers as determined by ion mobility were a significant determinant in defining residual risk.⁸ Quest uses ion mobility separation of subfractions for these measurements.

The ion mobility method directly measures particle size and concentration, unlike other approaches that use algorithms to indirectly calculate lipid subfractions.⁹ And, unlike some methods, ion mobility separation of subfractions does not cause lipoprotein modification that could potentially affect the accuracy of the assay.^7,10 Ion mobility has been used in multiple lipoprotein studies^6,7,11 and is the method used in the Cardio IQ Lipoprotein Fractionation, Ion Mobility test (test code 91604).

Quest also offers measurement of apolipoprotein B (ApoB). ApoB is the main structural protein found on all atherogenic particles, including low-, very low-, and intermediate-density lipoprotein, chylomicron remnants, and lipoprotein(a). ApoB level represents the total burden of atherogenic particles.¹² LDL-C level alone may provide an incomplete assessment of risk and treatment adequacy because CVD risk is more closely related to the number of atherogenic particles in plasma than to their cholesterol content.^13,14

Therefore, relying on LDL-C alone can lead to misclassification of risk and undertreatment.¹⁵  ApoB measurement is included in clinical guidance from several professional societies. It is available for ordering using test code 5224 (Apolipoprotein B) or test code 91726 (Cardio IQ^® Apolipoprotein B).