That's correct - the 400mg/dl figure is of crucial importance.
As you know, cholesterol and triglycerides are insoluble and are transported between the intestine, liver and periphery in the form of soluble compounds called lipoproteins. These lipoproteins are classified into chylomicrons, vldl's, idl's, ldl's and hdl's.
The vldl's are synthesised mainly in the liver and represent endogenous triglyceride synthesis using ffa's and glycerol. The ffa�s may be synthesised De novo (N.B biochemistry meaning not literal latin meaning) or by oxidation if sufficient dietary fat is available.
All the lipoprotein particles contain varying amounts of triglyceride, protein and cholesterol. However, each particle has a known, relatively constant, amount of triglycerides, protein and cholesterol unique for that species. Furthermore, vldl's are the largest carriers of triglycerides.
Because of these factors, it is possible to provide a rough estimation of the vldl cholesterol by dividing the measurable triglyceride value, expressed as mg/dl, by five. No direct, automated method of measuring vldl cholesterol has ever been devised, although it can be done in specialised research laboratories.
A further refinement of this method is the so-called Friedwals Equation. This is used to estimate ldl from total triglycerides, hdl and cholesterol:
ldl= (total cholesterol - hdl cholesterol) - (triglycerides/5)
This equation provides a fairly accurate picture of ldl and is indeed linear until we reach the 400mg/dl limit. When the triglycerides present exceed 400mg/dl, very odd figures can result. The calculated figures have been known to be above and below the true figures when analysing the same batch in these circumstances. Consequently, it is not possible to linearly extrapolate the likely error.