LDL cholesterol on most lab reports is not directly measured. It’s calculated from three other numbers on your standard lipid panel: total cholesterol, HDL cholesterol, and triglycerides. The formula your lab most likely used has been around since the 1970s, and while it works well for most people, it has blind spots that can lead to inaccurate results in certain situations.
The Friedewald Equation
The classic formula for calculating LDL is straightforward:
LDL = Total Cholesterol − HDL − (Triglycerides ÷ 5)
All values are in mg/dL. The “triglycerides divided by 5” part is an estimate of VLDL cholesterol, another type of cholesterol-carrying particle in your blood. The logic is simple: if you know your total cholesterol and can account for HDL and VLDL, whatever is left over is your LDL.
This equation, known as the Friedewald formula, has been the default method in clinical labs for over 50 years. When your doctor orders a standard lipid panel, the lab measures total cholesterol, HDL, and triglycerides directly, then plugs those numbers into this formula to produce your LDL result. That’s why your lab report may say “LDL cholesterol (calculated)” rather than just “LDL cholesterol.”
When the Calculation Becomes Unreliable
The Friedewald equation assumes that dividing triglycerides by 5 gives a reliable estimate of VLDL. That assumption breaks down when triglycerides are high. The CDC considers the formula invalid for triglyceride levels above 400 mg/dL, and many labs will simply leave the LDL field blank rather than report a misleading number.
But accuracy problems can start well before that 400 mg/dL cutoff. When triglycerides run between 200 and 399 mg/dL, the Friedewald equation tends to underestimate LDL, sometimes significantly. This matters most for people whose LDL is near a treatment threshold, like 70 mg/dL. In that range, the formula can make your LDL look lower than it actually is, potentially affecting treatment decisions.
Several factors increase the gap between calculated and actual LDL: triglycerides at or above 150 mg/dL, low HDL (below 40 mg/dL), diabetes, obesity, and taking cholesterol-lowering medications. If you have one or more of these, your calculated LDL may be off by 10% or more compared to a directly measured result.
Newer Calculation Methods
Two updated formulas have emerged to address the Friedewald equation’s weaknesses. The Martin-Hopkins equation, developed at Johns Hopkins, replaces the fixed divisor of 5 with an adjustable factor based on your individual triglyceride and non-HDL cholesterol levels. This makes it more accurate across a wider range of patients, especially those with low LDL or moderately elevated triglycerides.
The Sampson equation, developed at the NIH, uses a more complex formula that accounts for the nonlinear relationship between triglycerides and VLDL. It remains accurate with triglyceride levels up to 800 mg/dL, double the ceiling of the Friedewald formula. Among patients with triglycerides between 200 and 399 mg/dL and LDL below 70, up to 44% were reclassified into a higher LDL category when switching from the Friedewald to the Sampson equation, and 65% when switching from Friedewald to Martin-Hopkins. That’s a substantial number of people whose LDL was being underreported.
Many major labs have begun adopting one of these newer formulas, though the transition is not universal. If your lab report doesn’t specify which equation was used, it’s reasonable to assume the Friedewald formula unless your triglycerides are very high.
Calculated LDL vs. Direct Measurement
A directly measured LDL test uses a chemical assay to quantify LDL particles without relying on any formula. It costs more and isn’t part of the standard lipid panel, but it sidesteps the accuracy issues that plague calculated results. Direct measurement is most useful when triglycerides are elevated, when calculated LDL seems inconsistent with other risk markers, or when precise LDL tracking is needed during treatment.
That said, direct LDL assays have their own limitations and can vary between manufacturers. For most people with normal triglyceride levels, the calculated result is perfectly adequate.
Non-HDL Cholesterol as a Simpler Alternative
There’s an even simpler number already sitting on your lipid panel that avoids the Friedewald equation’s problems entirely: non-HDL cholesterol. The formula is just total cholesterol minus HDL. No triglyceride estimate needed, no room for that particular source of error.
Non-HDL cholesterol captures LDL plus all the other cholesterol-carrying particles that contribute to cardiovascular risk, including VLDL and other remnant particles. It becomes especially valuable when triglycerides are above 200 mg/dL, precisely the situation where calculated LDL is least trustworthy. Some research suggests non-HDL cholesterol is actually a better predictor of cardiovascular risk than LDL alone, because it reflects the full spectrum of harmful particles rather than just one type. It requires no additional testing and no extra cost, since it’s derived from two values already on every standard lipid panel.