Apolipoprotein B (ApoB) test: what it shows

Apolipoprotein B is a protein produced in two forms. The shorter form, apolipoprotein B-48, is a component of chylomicrons — the particles that carry fats from the intestine into the blood; it helps the body absorb dietary fats and fat-soluble vitamins. The longer form, apolipoprotein B-100, is produced in the liver and is a component of several lipoproteins: very low density (VLDL), intermediate density (IDL) and low density (LDL, also called the "bad" cholesterol particles). It is ApoB-100 that allows LDL particles to bind to receptors on the surface of cells, especially those of the liver, and this is how the body removes cholesterol from the circulation. Because every atherogenic particle (LDL, VLDL, IDL) carries exactly one ApoB molecule, the concentration of this protein in the blood directly shows the total number of such particles — even if the amount of cholesterol they contain (measured by the usual lipid panel) differs. For that reason ApoB is sometimes assessed as an additional marker alongside the usual lipid panel when the state of the cardiovascular system is being assessed. The 2019 ESC/EAS dyslipidaemia guidelines recommend ApoB for risk assessment particularly when triglycerides are raised, with diabetes, obesity or metabolic syndrome, or when LDL cholesterol is very low (according to a 2024 review in Circulation).

• When the number of atherogenic (LDL, VLDL) particles in the blood is to be assessed additionally, and not only the amount of cholesterol they contain
• When there is a family history of early heart disease, or LDL cholesterol stays high despite treatment — in similar cases MedlinePlus also mentions testing a related marker, lipoprotein (a)
• Together with the whole lipid panel (total cholesterol, LDL, HDL, triglycerides), when the risk of cardiovascular disease is being assessed
• When inherited disorders of lipid metabolism are suspected, such as familial hypercholesterolaemia

How to prepare

Preparation requirements depend on the laboratory and may be the same as those for a usual lipid panel, so it is best to check with your own doctor or laboratory before the test. For example, Cleveland Clinic states that no fasting is needed for an ApoB test on its own, but if a lipid panel is done at the same time you may be asked to fast for 12 hours.

The specific reference interval depends on the laboratory's method, so you will find the exact figure on your own result sheet. There is no single limit that applies to everyone: the 2019 ESC/EAS dyslipidaemia guidelines give ApoB as a secondary treatment goal — below 65 mg/dl for people at very high cardiovascular risk, below 80 mg/dl at high risk and below 100 mg/dl at moderate risk (0.65, 0.80 and 1.00 g/l when converted; the risk category is set by a doctor), while in the US AHA/ACC guidelines an ApoB of 130 mg/dl or more corresponds to LDL cholesterol of 160 mg/dl or more (according to a 2024 review in Circulation). In general, this marker is assessed together with LDL, HDL, triglycerides and total cholesterol.

A raised ApoB indicates more atherogenic (LDL, VLDL) particles in the blood. MedlinePlus states that the causes which increase total and LDL cholesterol include fatty food, excess weight, too little physical activity, as well as conditions such as diabetes, kidney disease or thyroid disorders, and inherited disorders such as familial hypercholesterolaemia.

• Fatty food, excess weight, insufficient physical activity
• Diabetes mellitus, kidney disease, thyroid function disorders
• Inherited disorders of lipid metabolism, for example familial hypercholesterolaemia
• Some medicines that affect lipid metabolism (discuss the medicines you take with your doctor)

What to discuss with a doctor:

• The whole lipid panel (total cholesterol, LDL, HDL, triglycerides) for a more precise picture
• An assessment of cardiovascular risk factors together with a doctor
• Family history of early heart disease
• Lifestyle factors — diet, physical activity, weight
• When to repeat the test to assess the change

A lowered ApoB

ApoB reflects the number of atherogenic (LDL, VLDL) particles. According to MedlinePlus, one of the known reasons why ApoB can be lowered is inherited familial hypobetalipoproteinaemia — a condition caused by mutations in the APOB gene and associated with impaired fat absorption. Discuss the specific significance with a doctor.

• An inherited APOB gene mutation causing familial hypobetalipoproteinaemia
• The impaired absorption of fats and fat-soluble vitamins (for example E, A) associated with this condition

What to discuss with a doctor:

• The whole lipid panel (total cholesterol, LDL, HDL, triglycerides) for an overall picture
• Liver markers and an assessment of nutrition
• Whether any lipid-lowering medicines are being taken that could affect the result
• When to repeat the test to assess the change