Is it necessary to not eat 8-12 hours before one has their blood drawn to check cholesterol levels or is it okay to draw it after eating? I am asked this question from patients almost on a daily basis. The brief answer is yes. Let me explain why.
The current guidelines from the National Cholesterol Education Panel (NCEP) ATP III recommend initiation of treatment based on one’s LDL-C (the bad cholesterol). This year NCEP ATP IV will present the new guidelines. There is currently debate on which lipid measurement they will recommend for initiating treatment of high cholesterol. Until that is published in November, I will discuss the current guidelines.
So why do we fast when we do lipid profiles? Most laboratories determine LDL–C from a formula dependent on triglycerides. Almost none of one’s LDL-C level is done by direct measurement. If you are doing LDL-C direct measurements, then fasting is not required.
However, as I said, most labs use a triglyceride-dependent formula to calculate LDL-C. This is called the Friedewald formula. LDL-C is equal to the TC minus (HDL-C + VLDL-C). TC is the total cholesterol, HDL-C is the good cholesterol, and the VLDL-C is used as a surrogate measure of the triglyceride level. Labs do not directly measure VLDL-C. They calculate it by dividing serum triglyceride level by five. The rationale is that VLDL particles, under normal circumstances, carry most of the plasma triglycerides.
Since cholesterol is an oil and is not soluble in blood, it must be packaged inside the lipoproteins in order to be trafficked through the blood stream. LDL-C is carried in LDL particles. HDL-C is carried in HDL particles, and triglycerides are primarily carried in VLDL particles. A normally composed VLDL particle should have five times more triglycerides than cholesterol, so TG/5 should be a reasonable equation to calculate VLDL-C. and is accurate as long as the triglyceride level is less than 200mg/dl. So if a lab uses this Friedewald formula to calculate LDL-C, fasting is required.