Cholesterol, part 3: The Standard Lipid Panel

 

The standard lipid panel is traditionally performed after an 8-12 hour period of fasting, typically an overnight fast.  The measured components of this lipid panel include total cholesterol, high-density lipoprotein (HDL) cholesterol, and triglycerides.

The Lipid Panel Tradition

The standard lipid panel is an annual ritual for adults attending their yearly physicals, and it’s one of the few lab tests that is expected to be covered by one’s health insurance on a yearly basis.  It must be important, then, if the insurance underwriters have decided that it should be done yearly, right?

People assume, as they have been taught for the past 70 years, that the lipid panel is a reflection of their cardiovascular disease risk.  They eagerly await their results and then are often discouraged at being told that they have “high cholesterol” and need to consider starting a cholesterol-lowering medication, then panic sets in.  Unfortunately, it is a poor marker of cardiovascular risk.  If properly interpreted, it can be useful in terms of predicting insulin resistance, but most health professionals look only at the reference ranges and don’t understand what the values indicate.

LDL-C is Calculated, not Measured

Most people are not aware that the LDL-C is a calculated value – it is not measured directly.  That means that treatment decisions based on your LDL cholesterol are nearly always based on a calculated value, not a direct measure.  That alone should give you pause about accepting advice about cholesterol-lowering medication.

The calculation comes from the Friedewald equation, which was developed in 1972.  It looks like this:

LDL cholesterol = total cholesterol (TC) minus high-density lipoprotein-cholesterol (HDL-C) minus triglycerides (TG)/5

The term that divides triglycerides by 5 is the estimate for the very low-density lipoprotein (VLDL) content.  One significant limitation of this equation is that it is known to be inaccurate when the triglyceride level is high (> 400 mg/dL) or the LDL-C is low (< 70).  In that situation, the LDL concentration needs to be measured directly, via a separate laboratory assay.

Worth noting is that, when this equation was developed in 1972, only 35 of the 448 individuals in Friedewald’s sample had an LDL-C that was less than 70 mg/dL.  (ref)  Now, with statins and other medications being used to aggressively lower LDL concentrations, this equation has not fared very well.  Other equations have been developed but have not been validated.

Limitations

The standard lipid panel has quite limited utility in addressing cardiovascular risk, as we will continue to explore.  Far too many people believe that there is a “good” cholesterol and a “bad” cholesterol — that’s an oversimplified perspective on an extremely complex substance.  Suffice it to say, your health is not defined by one number.