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.

What does Cannot calculate LDL mean?

Without the HDL level, it is not possible to calculate the LDL level. If your Triglyceride levels are too high (over 4.5), then the formula does not work. If this is the case, it might be worth fasting overnight, and repeating the test in before breakfast as this can affect the triglyceride levels.

What clinical situation can one not use the Friedewald equation?

The Friedewald's formula cannot be used for LDL-C calculation when the subject is not fasting, when serum TG >400 mg/dl or < 100mg/dL [7] or in patients with type III or type I hyperlipoproteinaemia [8].