Synonyms:
Related tests: Triglycerides, Total cholesterol, APO A, HDL, LDL, Lipoprotein (a), LDL subclasses.

Clinical significanceClinical indicationsSample typePreparationNotes

Clinical significance
Apolipoprotein B100 (Apo B) is the main structural protein of LDL (low-density lipoprotein); the role of Apo B is not only to allow esterified cholesterol (and, to a much lesser extent, triglycerides) to be transported in the blood, but also to recognise the specific LDL receptors present on the membranes of some cells.
Raised serum Apo B can be seen in the following conditions: familial hyperlipoproteinaemia, diabetes, hypothyroidism, nephrotic syndrome, kidney failure, liver disease, cholestasis, Cushing’s syndrome, porphyria and pregnancy.
Reduced serum Apo B can be seen in the following conditions: hyperthyroidism, malnutrition, intestinal malabsorption, chronic anaemia, severe hepatocellular dysfunction, myeloma and chronic lung disease.
The ratio of Apo A1 (the main apolipoprotein of HDL) to Apo B can be an indirect index of the ratio of HDL to LDL, and therefore of HDL cholesterol to LDL cholesterol.

Clinical indications
Assessment of the risk of ischaemic cardiovascular disease.

Sample type
The patient must have a blood sample taken

Preparation
You must fast for 12 hours; a small amount of water is allowed. You must have been standing or sitting upright for at least 30 minutes.
In the days before the blood test, you are advised to keep to your usual diet as far as possible.

Notes
A true increase in serum Apo B can be caused, in vivo, by treatment with beta-blockers, androgens, catecholamines, ciclosporin or diuretics, as well as by diets high in saturated fat and cholesterol and by alcohol abuse.
A true decrease in serum Apo B can be caused, in vivo, by treatment with colestyramine, oestrogens, fibrates or statins, as well as by diets high in polyunsaturated fatty acids and low in cholesterol.
As with Apo A1, the limitation of Apo B (and even more so of the ratio between the two) is that it is an indirect index: it gives no direct information on the size of LDL particles (see LDL subclasses).