Lipoprotein Phenotyping Profile

Evaluate hyperlipidemia to determine abnormal lipoprotein distribution and concentration in the serum.

Type I: Hyperchylomicronemia. A large, lipid-stained fraction remains at the application point. Other lipoproteins stain less intensely. It is a very rare pattern. The patient serum is very lipemic and shows a floating chylomicron band, after remaining overnight in the refrigerator. The infranatant is clear.

There are two conditions that give a false type I pattern. The first is dysglobulinemia. In this condition, a large portion of the lipid is bound to immunoglobulins (IgM). Agarose gel protein electrophoresis of serum should be performed to confirm or exclude a diagnosis of dysglobulinemia. In the case of a positive result, an immunoelectrophoresis can determine the type of lipoprotein or lipoproteins involved. The second condition that can give a false type I pattern is delipidation of lipoproteins. Some drugs act as delipidizing agents and can result in patterns that are very misleading. It is imperative to confirm that the sample had been drawn from a fasting (>12 hours) patient before one classifies the electrophoretic pattern because hyperchylomicronemia is induced by food intake. The definitive test for confirmation of type I hyperlipoproteinemia is the postheparin lipase activity (PHLA) measurement. In contrast to type V hyperlipoproteinemia, PHLA is reduced or absent in true type I hyperlipoproteinemia.

Type IIa: Hypercholesterolemia. In this condition, there is a high concentration of β-lipoprotein with normal pre−β-lipoprotein. The α-lipoprotein is normal or sometimes slightly decreased. The calculated LDL-cholesterol value should be >160 mg/dL. This is a relatively common disorder.

Type IIb: Hypercholesterolemia with elevated triglycerides. This is a common type of hyperlipoproteinemia. Besides a high β-lipoprotein concentration, there is an elevated pre−β-lipoprotein concentration. Chylomicrons are not present, and the α-lipoprotein band is normal or slightly decreased. On nonfasting samples, a distinction cannot be made between the type IIa and type IIb patterns.

Type III: Hypercholesterolemia with hypertriglyceridemia. Serum total cholesterol and triglyceride levels are present at about a 1:1 ratio. Chylomicrons are not usually present. The β-lipoprotein, present at high concentration, forms a broad band and has a faster mobility than the usual β-lipoprotein. The separation between the β-lipoprotein and pre−β-lipoprotein bands is very poor because of the increase in the concentration of abnormal intermediate-density lipoprotein (which lacks apolipoprotein E). This abnormal lipoprotein migrates between the β-lipoprotein and pre−β-lipoprotein bands. The α-lipoprotein usually is decreased. Final diagnosis of type III may require Apo E genotype testing.

Type IV: Hypertriglyceridemia. Elevated pre−β-lipoproteins are present with a normal or low β-lipoprotein fraction. Sometimes the mobility of the β-lipoprotein fraction is faster than normal. The α-lipoprotein is usually decreased. There are no chylomicrons present. If the serum triglyceride concentration is 200−300 mg/dL, a “mild type IV” designation can be used. The patient should be completely fasting (> 12 hours); otherwise the type IV and type V patterns will be very difficult to distinguish.

Type V: Hyperlipidemia with chylomicronemia. Chylomicrons, β-lipoprotein, and pre−β-lipoprotein are usually increased. Sometimes the serum triglyceride can be >10,000 mg/dL. The serum total cholesterol concentration can also be elevated. The presence of a floating-chylomicron band in serum stored at 4°C constitutes a positive identification for the presence of chylomicrons. The infranatant serum varies from very opalescent to lipemic in appearance. This appearance is caused by pre−β-lipoproteins being present. Because a chylomicron band is present, protein electrophoresis on agarose gel can allow detection of the abnormal globulin-lipoprotein complex.