Abstract P198: is Directly Measured LDL Clinically Equivalent to Calculated LDL

Background: LDL cholesterol can either be calculated or measured directly. Clinical guidelines recommend using calculated LDL to guide therapy, as the evidence base for lipid management is derived almost exclusively from trials using calculated LDL, with direct measurement of LDL being reserved for those not fasting or significant hypertriglyceridemia. Our aim was to determine the clinical equivalence of directly measured LDL and fasting calculated LDL. Design: Eighty-one subjects had at least 1 calculated and direct LDL performed simultaneously; 64 had a repeat lipid assessment after 4 to 6 weeks of therapy, resulting in 145 pairs of calculated and direct LDL. Correlation between direct and calculated LDL was determined using Pearson's correlation coefficient. The relationship between direct and calculated LDL was also evaluated from a clinical perspective. Direct and calculated LDL were considered “clinically concordant” when the difference between calculated and direct LDL fulfilled 3 criteria: 1) < 6% difference (incremental LDL lowering provided by 1 titration of statin dose, e.g. simvastatin 20 to 40 mg), 2) < 10 mg/dL difference, and 3) placement in the same ATP III LDL cut points (e.g. <100, 100-129). Direct and calculated LDL were considered “clinically discordant” when the difference between calculated and direct LDL fulfilled 3 criteria: 1) ≥ 12% difference (incremental LDL lowering provided by 2 statin titration steps, e.g. from simvastatin 20 to 80 mg), 2) ≥ 10 mg/dL difference, and 3) placement in different ATP III LDL cut points. Results: There was significant correlation between direct and calculated LDL(r=0.93). Clinical concordance between calculated and direct LDL was present in 40% of patients. Clinical discordance was noted in 25% of patients. One-third of patients had > 15 mg/dL difference between direct and calculated LDL, while 25% had > 20 mg/dL difference. In 47% of subjects, the difference between direct and calculated LDL at baseline and follow-up changed by a minimum of 10% or 10 mg/dL. Conclusion: Our findings suggest that directly measured LDL is not clinically equivalent to calculated LDL. This puts into question the current recommendation of using direct LDL in situations where calculated LDL would be inaccurate.