Apolipoprotein B

Apo B is central to lipoprotein transport, being essential for the secretion of triglyceride-rich lipoproteins from the liver and gut. One molecule of apo B is present in each chylomicron, VLDL or LDL particle. It exists in two forms; apoB100 and apoB48 encoded by the same gene on human chromosome 2. ApoB48 is secreted by enterocytes and is the major protein constituent of chylomicrons (CM). ApoB100 is the major component of all lipoproteins except CM and HDL and 90% of circulating apoB100 is found in LDL. It is essential to the formation of VLDL particles and their release into the circulation. ApoB100 is the ligand for the LDL receptor on hepatocytes and in peripheral tissues. Mutations of the apo B gene terminus at or close to codon 3500 is associated with abnormal ligand binding and a familial form of hypercholesterolaemia (familial defective apo B)

Elevated plasma apo B100 is a marker of increased numbers of LDL particles and is a risk factor for coronary artery disease even in the presence of a relatively normal LDL cholesterol concentration.

Apo B100 is raised in hyperlipoproteinaemia types IIa, IIb, IV and V, hyperapobetalipoproteinaemia (normal LDL, elevated apo B) hepatic obstruction, renal disease, diabetes, hypothyroidism, Cushings syndrome, anorexia and pregnancy. Drugs which cause an increase include cyclosporin, diuretics, corticosteroids, beta blockers, alcohol, androgens, progestins and catecholamines. Diets rich in saturated fats and cholesterol also increase plasma apo B concentrations.

Decreased apo B levels are found in abetalipoproteinaemia (Tangier disease), heterozygous hypobetalipoproteinaemia, LCAT deficiency, hyperlipoproteinaemia type I, lipoprotein lipase cofactor (apo CII) deficiency, hyperthyroidism, malnutrition, malabsorption, severe hepatocellular dysfunction, Reye’s syndrome, inflammatory joint disease, pulmonary disease, myeloma and weight reduction.

Clinical Indications:
Apo B determination is useful in estimating the adequacy of endogenous pathway inhibition by drug therapy as assessed by LDL particle numbers; the diagnosis of certain primary disorders of lipoprotein metabolism (eg. abetalipoproteinaemia, homozygous hypobetalipoproteinaemia – see above) and as a research tool in the investigation of lipoprotein metabolism.

Approximate reference range:
0.5 – 1.0g/L;
(See individual laboratory report)

Ranges related to CHD risk:
< 0.9g/l Target for secondary prevention
<1.05 g/l desirable
1.05-1.245g/l borderline risk
1.25-1.39 g/l high risk
>1.40 g/l very high risk

Patient preparation:
Patients should follow their normal diet for 3 weeks prior to sampling. A fasting sample is preferred, but non-fasting is acceptable. Standardise posture to reduce effect of change in plasma volume – seat the patient for 5 minutes before sampling. Avoid venous stasis – apply tourniquet briefly before inserting needle and release before drawing sample.

Sample details:
EDTA plasma or serum (min. vol. 0.5ml).
Stable 4 days at 4°C, 2months at -20°C
Transport – First Class Post (avoid weekends)

Information required:
Age, sex, NHS/Hospital No.
Medication
Lipid profile results including LDL-cholesterol (if available)

References:
Bhatnagar D, Durrington PN. Does measurement of apolipoproteins add to the clinical diagnosis and management of dyslipidemias?. Curr Opin Lipidol 1993; 4: 299-304.

Wald NJ, Law M, Watt HC et al. Apolipoproteins and ischaemic heart disease; implications for screening. Lancet 1994; 343: 75-79

Sniderman AD, Cianflone K. Measurement of apoproteins; time to improve the diagnosis of atherogenic dyslipidaemias Clin Chem 1996; 42: 489-91

Miremadi S, Sniderman A, Frohlich J. Can measurement of serum apolipoprotein B replace the lipid profile monitoring of patients with lipoprotein disorders? Clin Chem 2002;48: 484-488

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