As you may already know, the LDL cholesterol that is usually reported when a lipid panel is ordered is not measured but calculated. When the triglycerides are high, LDL cannot be accurately calculated because triglycerides are part of the equation.
How is LDL-C Calculated?
LDL-C is most often calculated using a formula based on other components of the lipid panel. Many lab companies use different equations to calculate LDL-C. A common equation used is the Friedewald equation that has been around since the 1970s. It incorporates the total cholesterol, HDL-cholesterol (HDL-C), and triglyceride concentrations:
LDL-C (mg/dL) = total cholesterol – HDL-C – (triglycerides/5),
where “triglycerides/5” is used to represent very low-density lipoprotein-C (VLDL-C).
Total cholesterol (TC) = LDL + HDL + VLDL. Where VLDL ≅ TG/5.
The formula valid if TG <400 mg/dL. If TG > 400 mg/dL, you have to measure the LDL directly, rather than calculating it.
There are several other newer equations that are slight modifications of the Friedewald equation. TLDL-C concentration can also be measured directly.
If a patient’s LDL cholesterol concentration could not be calculated because the triglycerides were too high (>4.35 mmol/L; >400 mg/dL). What options do you have for getting an LDL cholesterol concentration?
If the LDL concentration could not be calculated because the triglyceride level was too high, direct LDL-C testing may be useful. At Quest Diagnostics, for example, 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.
low-density lipoprotein-cholesterol (LDL-C)
Reference
https://education.questdiagnostics.com/faq/FAQ164