Apolipoprotein B (Apo B)

What is apolipoprotein B (Apo B)?

Apolipoprotein B is a structural protein found on the surface of the lipoprotein particles that carry cholesterol and triglycerides in the blood. Each VLDL particle produced in the liver, and each IDL and LDL particle derived from it, carries a single Apo B molecule; the lipoprotein(a) particle also carries Apo B. For this reason, the amount of Apo B in the blood reflects the total number of atherogenic particles, which can settle in the artery wall and contribute to plaque formation.

The LDL cholesterol in the standard cholesterol test shows the amount of cholesterol carried inside these particles; Apo B, on the other hand, shows the number of particles. In most people, these two values move in parallel. In some people, however, LDL cholesterol may appear normal while the particle count is high, because the particles are small and low in cholesterol. In this situation, Apo B provides additional information that complements the LDL measurement and helps the doctor assess cardiovascular risk.

When is the Apo B test ordered?

Apo B is ordered more selectively than the routine cholesterol panel. The doctor may turn to this test when they think that standard lipid values may not fully reflect a person's risk. It can also be used in people on cholesterol-lowering treatment to monitor the effect of the treatment on the number of atherogenic particles.

• Lipid assessment in people with high triglyceride levels
• Diabetes, insulin resistance, obesity or metabolic syndrome
• A family history of heart disease at a young age even though LDL cholesterol is normal or low
• Suspected familial high cholesterol
• Follow-up during cholesterol-lowering treatment
• Ratio assessment together with apolipoprotein A1

The Apo B test does not make a diagnosis on its own; it is one of the pieces of information that help assess a person's overall cardiovascular risk. Your doctor often orders this test together with total cholesterol, LDL cholesterol, HDL cholesterol and triglycerides, and considers the results together with your age, blood pressure, blood sugar status and family history.

How do you prepare for the Apo B test?

Fasting is usually not needed for Apo B measurement; Apo B levels do not change significantly in the hours after a meal. However, if the test is ordered together with a lipid profile that also includes triglycerides and LDL cholesterol, your doctor may ask you to fast for a certain period before the blood draw. In that case, following the instructions you are given makes it easier to interpret the results correctly together.

It is important to tell your doctor about the medications you take, especially cholesterol, thyroid and hormone medications, and whether you are pregnant. A serious illness, recent surgery or sudden weight change can temporarily affect lipid values; doing follow-up measurements under similar conditions makes comparison easier. The blood sample is collected by a nurse at your home or workplace and analyzed at an accredited laboratory.

What does an Apo B result mean?

A high Apo B level suggests that the blood contains a high number of particles that may harm vascular health. This may be associated with dietary habits, physical inactivity, excess weight, diabetes and inherited lipid disorders. An underactive thyroid gland, certain kidney diseases, pregnancy and certain medications can also raise Apo B levels. A high result alone does not mean disease; it is assessed together with other risk factors such as blood pressure, smoking, blood sugar status and family history.

A low Apo B level usually does not indicate a problem. However, an overactive thyroid gland, advanced liver disease, malnutrition or rare inherited conditions can lead to low values. The reference range and target values may vary depending on the method used, the laboratory and the person's overall risk; the laboratory's own range is given in the result report. Your Apo B result should be interpreted by your doctor together with other lipid tests and your health history.