The concentration of four classes of serum lipoproteins can, when elevated, be regarded as pathological. These are:

• chylomicrons;
• VLDL;
• LDL; and
• beta-VLDL.

The hyperlipoproteinaemias can be classified according to which of the above is increased (see Table 1).

Table 1. WHO, World Health Organization; LDL, low-density lipoprotein; VLDL, very-low-density lipoprotein; IDL, intermediate-density lipoprotein. After [Durrington, 1995a]

Is the classification system useful?

The Fredrickson/World Health Organization (WHO) classification sometimes causes confusion, largely because it may be difficult to remember and is frequently wrongly regarded as a diagnostic classification, when it is simply a way of reporting which of the serum lipoproteins are elevated in a particular blood sample. It is usually sufficient to remember that when cholesterol alone is elevated there is a type IIa hyperlipoproteinaemia. When both cholesterol and triglycerides are elevated the hyperlipoproteinaemia is generally type IIb, but occasionally it is type V hyperlipoproteinaemia (the serum will look milky if this is the case) and rarely, type III. Type I hyperlipoproteinaemia is extraordinarily rare, and an isolated increase in fasting serum triglycerides almost invariably signifies type IV hyperlipoproteinaemia.

Should HDL-C be measured?

All hospital laboratories, in addition to measuring cholesterol and triglyceride levels, should measure HDL-C in those patients whose overall cardiovascular risk is being assessed critically, usually because treatment of their hyperlipoproteinaemia with drugs is under consideration. In women particularly, an elevated cholesterol level may result from a relatively high HDL-C concentration and may not signify any increased risk of CHD. High serum HDL-C does not have a Fredrickson/WHO class, and as evidence suggests it is associated with longevity, it cannot be regarded as an atherogenic hyperlipoproteinaemia. Conversely, low HDL-C is associated with an increased cardiovascular risk, particularly if total serum cholesterol and triglycerides are also elevated or other risk factors are present.