To measure or to calculate? Why laboratories should adopt the modified Sampson-NIH equation for LDL-C

As cardiovascular disease prevention guidelines continue to evolve, the pressure on the clinical laboratory to provide accurate Low-Density Lipoprotein Cholesterol (LDL-C) measurements at increasingly lower thresholds has intensified. Now that newly published guidelines reinstated therapeutic targets, which are recommended to be below 70 mg/dL (1.8 mmol/L) and 55 mg/dL (1.4 mmol/L) for very high-risk and secondary prevention patients^{1, 2}, laboratories face a critical challenge: the traditional Friedewald equation systematically underestimates LDL-C at these low levels. Although the  Martin-Hopkins and Sampson-NIH equations are now recommended by the new guidelines over the Friedewald equation,  because of their improved accuracy, neither of these equations were specifically developed for estimating low levels of LDL-C.³ They were both developed on patients who were not treated with the latest lipid-lowering therapies and hence did not contain many patients with very low levels of LDL-C.  Some laboratories rely on direct homogeneous assays as an alternative approach for difficult samples (high triglycerides or low LDL-C).  Direct assays, however, can sometimes lack specificity for cholesterol on LDL on dyslipidemic samples and can suffer from high variability (CV 11%) in low LDL-C samples.⁴

To address this gap, we recently developed the Modified Sampson equation using data from over 34,195 samples from patients on current lipid-lowering therapies, including patients from the FOURIER trial who were on aggressive lipid-lowering therapy with a PCSK9-inhibitor.⁵ The novel equation, shown below, was developed by least-squares regression to match LDL-C (mg/dL) by the β-quantification reference method, by combining terms into non High Density Lipoprotein Cholesterol (nonHDLC (Total cholesterol - HDL-C)), triglycerides (TG) and forcing the coefficient to be one.

This equation provides significantly improved accuracy for estimating LDL-C, particularly at low LDL-C levels, compared to the Friedewald, Martin-Hopkins, and the original Sampson-NIH equation. The Modified Sampson equation achieved a Lin concordance correlation coefficient of 0.992 with the β-quantification reference method, a near-perfect agreement. At clinically relevant LDL-C cutpoints (55 mg/dL and 70 mg/dL), it showed the highest kappa agreement (0.98 and 0.97, respectively), outperforming the other equations, which ranged from 0.92 to 0.96 at these thresholds. The false classification rates at these cutpoints were also the lowest for the Modified Sampson equation, 15% at 55 mg/dL and 18% at 70 mg/dL, compared to 28-37% for the other equations. The net reclassification index showed that the Modified Sampson equation correctly reclassified up to 10-20% more patients with low LDL-C than the alternative equations. 

These findings are clinically important because accurate LDL-C estimation at low levels is increasingly critical for guiding therapy intensification and goal attainment, especially as more patients are treated to lower LDL-C targets with new more potent lipid-lowering agents. The improved accuracy of the Modified Sampson equation reduces the risk of misclassifying high-risk patients who may otherwise be undertreated. Strikingly, this is particularly relevant for patients with LDL-C <125 mg/dL and triglycerides <150mg/dL where all previous equations (Friedewald, Martin and Original Sampson) at the 55 mg/dL cutpoint presented a 30% false classification rate, most being falsely low, whereas with the new Modified Sampson equation it was less than 10%. 

A recent external validation study confirmed that the Modified Sampson equation was superior to other equations and it also outperformed three different direct assays in terms of concordance with the β-quantification reference method at the 55 and 70 mg/dL cutpoints. Like the original Sampson equation, the Modified Sampson equation appears to be suitable for triglycerides up to 800 mg/dL, thus further limiting the need for direct LDL-C assays.⁶

For laboratory implementation, switching to the Modified Sampson equation offers a clear opportunity to enhance patient care by providing more reliable LDL-C results at the low concentrations now recommended by the latest guidelines. A spreadsheet for the Modified Sampson equation (in conventional and SI units) can be freely download at the following website: https://doi.org/10.6084/m9.figshare.28557038 

References

1. Writing Committee Members; Blumenthal RS, Morris PB, Gaudino M, Johnson HM, Anderson TS, Bittner VA, Blankstein R, Brewer LC, Cho L, de Ferranti SD, Gianos E, Gluckman TJ, Gradney KF, Isiadinso I, Lloyd-Jones DM, Marrs JC, Martin SS, McLain KH, Mehta LS, Mora S, Mulugeta WM, Natarajan P, Navar AM, Orringer CE, Polonsky TS, Reynolds HR, Saseen JJ, Shapiro MD, Soffer DE, Tynes SA, Villavaso CD, Virani SS, Wilkins JT. 2026 ACC/AHA/AACVPR/ABC/ACPM/ADA/AGS/APhA/ASPC/NLA/PCNA Guideline on the Management of Dyslipidemia: A Report of the American College of Cardiology/American Heart Association Joint Committee on Clinical Practice Guidelines. Circulation. 2026 Mar 13. doi: 10.1161/CIR.0000000000001423.
2. Visseren FLJ, Mach F, Smulders YM, Carballo D, Koskinas KC, Bäck M, et al. 2021 ESC guidelines on cardiovascular disease prevention in clinical practice. Eur Heart J 2021; 42:3227–337
3. Sampson M, Wolska  A, Cole J, Zubirán R, Otvos JD, Meeusen JW, et al. Accuracy and clinical impact of estimating low-density lipoprotein-cholesterol at high and low levels by different equations. Biomedicines 2022; 10:3156.
4. Miida T, Nishimura K, Okamura T, Hirayama S, Ohmura H, Yoshida H, et al. A multicenter study on the precision and accuracy of homogeneous assays for LDL-cholesterol: comparison with a beta-quantification method using fresh serum obtained from non-diseased and diseased subjects. Atherosclerosis 2012; 225:208–15.
5. Sampson M, Zubiran R, Wolska A, Meeusen JW, Donato LJ, Jaffe AS, et al. A modified Sampson-NIH equation with improved accuracy for estimating low levels of low-density lipoprotein-cholesterol. Clin Chem 2025; 71:1125–1137
6. Meeusen JW, Yi X, Cotten SW, Nielsen JB, Donato LJ, Jones PM, et al. Modern low-density lipoprotein cholesterol formulas outperform direct methods in patients with hypertriglyceridemia and low levels of low-density lipoprotein cholesterol. Clin Chem 2025; 71:1138–1146.