how do they arrive at the cholestoral figure?

Question

fdo they add the ldl and hdl and triglisarides

Answer 1

They measure total cholesterol, HDL, LDL, as well as triglycerides. Measuring LDL is not reliable if the triglycerides are completely out of whack - over 350, I think. HDL plus LDL is most of the total but does not equal total cholesterol. There are other constituents such as VLDL and some other intermediate size particles that contribute as well. Triglycerides are independent of cholesterol - it is not completely inaccurate to think of them as bare products that can be turned into cholesterol.