How is it possible for me to have low cholesterol when both my hdl and ldl are smack dab right in the middle of the normal ranges?

The math: Hi. In the fasting state, total cholesterol is the sum of LDL-cholesterol + HDL-cholesterol + VLDL-cholesterol. Total cholesterol and HDL-cholesterol are measured directly. VLDL-cholesterol is estimated by 0.2 x triglycerides. LDL-chholesterol is calculated as LDL-C = total cholesterol - HDL-cholesterol - (0.2 x triglycerides). This works reasonably unless triglycerides are high (300's or above).

It's A Good Thing: Total cholesterol measures the cholesterol contained in all lipoproteins including low density lipoprotein (ldl), intermediate density lipoprotein (idl), high density lipoprotein (hdl), very low density lipoprotein (vldl), and chylomicrons. LDL-C and HDL-C can be in the "normal range" and the total cholesterol from all particles can still be relatively low. Overall, this is a favorable finding.