Only unbound cholesterol is active. Serum cholesterol for the most part is bound, to esters, and transported in lipoprotein carriers. While it is not possible to measure unbound and active cholesterol in cells and tissues, it is possible to calculate a rough estimate as follows:
Low Density Lipoprotein...LDL delivers cholesterol to the tissues. High Density Lipoprotein...HDL removes cholesterol from the tissues.
The amount of cholesterol in cells (i.e., Tissue/Cell Cholesterol (TCh)) can thus be calculated:
LDL/HDL x Total Cholesterol