ObjectiveSmall dense low-density lipoprotein cholesterol (sdLDL-C), as an emerging atherogenic factor of cardiovascular diseases, requires additional tests. We aimed to establish a sdLDL-C equation using standard lipid profile and evaluate its capacity of identifying the residual cardiovascular risk beyond LDL-C and apolipoprotein B (ApoB).MethodsThis cross-sectional study included 25 435 participants from Health Management Cohort and 11 628 participants from China Health and Retirement Longitudinal Study (CHARLS) to construct and evaluate the sdLDL-C equation by least-squares regression model. The equation for sdLDL-C depended on low-density lipoprotein cholesterol (LDL-C) and an interaction term between LDL-C and the natural log of triglycerides (TG).ResultsThe modified equation (sdLDL-C = 0.14*ln(TG)*LDL-C - 0.45*LDL-C + 10.88) was more accurate than the original equation in validation set (slope = 0.783 vs. 0.776, MAD = 5.228 vs. 5.396). Using the 80th percentile (50 mg/dL) as a risk-enhancer rule for sdLDL-C, accuracy of the modified equation was higher than the original equation in validation set (90.47% vs. 89.73%). The estimated sdLDL-C identified an additional proportion of high-risk individuals in BHMC (4.93%) and CHARLS (1.84%).ConclusionThe newly developed equation in our study provided an accurate tool for estimating sdLDL-C level among the Chinese population as a potential cardiovascular risk-enhancer.