APOLIPOPROTEINS A-1 & B

CLINICAL UTILITY:

Apolipoprotein A-1 (Apo A-1) is the main protein component of HDL and accounts for approximately 65% of the total protein content of HDL. Apo A-1 activates lecithin cholesterol acyltransferase which catalyses the esterification of cholesterol. The resulting esterified cholesterol can then be transported to the liver, metabolized and excreted. Persons with atherosclerotic vascular changes frequently exhibit decreased levels of Apo A-1. Even if the concentrations of Apo B are normal, a decreased Apo A-1 level may be a risk factor for atherosclerotic processes. Decreased concentrations of Apo A-1 also occur in dyslipoproteinaemias, acute hepatitis, hepatic cirrhosis and insulin-treated patients.

Apolipoprotein B (Apo B) is the main protein component of LDL and accounts for approximately 95% of the total protein content of LDL. Apo B is necessary for the reaction with LDL receptors in the liver and on cell walls and is thus involved in transporting cholesterol from the liver to the vessel cell. Elevated levels of Apo B are frequently found in atherosclerotic vascular changes and are a risk factor for atherosclerosis. Several studies have shown that the assay of Apolipoporteins A-1 and B is helpful in assessing the risk of atherosclerosis and has greater prognostic power than the sole determination of HDL and LDL cholesterol. A particularly powerful parameter for estimating the risk of atherosclerosis is the quotient Apo B/Apo A-1. The higher the quotient, the greater the risk of atherosclerosis.

Apo A-1 determination are useful in assessing the risk of atherosclerotic diseases. Apo A-1 is generally low in persons with atherosclerotic vascular disease. A low Apo A-1 level can be a risk factor for atherosclerosis even if Apo B is normal. The ratio of Apo B/A-1 may be the best parameter for estimating the risk of atherosclerosis.

Decreased levels of Apo A-1 have also been associated with dyslipoproteinemias such as Tangier disease and LCAT (lecithin cholesterol acyltransferase) deficiency. Apo A-1 is also decreased in acute hepatitis, insulin-dependent diabetes and liver cirrhosis.

Hyperlipoproteinemia is regarded as one of the most important risk factors for the development of arteriosclerotic diseases and studies indicate that in addition to the routine determinations of triglycerides and cholesterol, quantitative determination of the corresponding apolipoproteins is also important. Of special intereist is the Apo B, the carrier protein for low density lipoprotein (LDL b-lipoprotein). Apo B determination is useful in the differential diagnosis of hyperlipoproteinemia as a replacement for the complicated determination of LDL (b) cholesterol by ultracentrifugation. In a normal population, the Apo B determination usually detects only b-lipoprotein or LDL. In hyperlipemic subjects (especially type IV), the results is also affected by the Apo B content of the pre-b-lipoproteins or VLDL.

METHOD DESCRIPTION:

Apolipoproteins A1(or B) in this assay are measured in a nephelometric method.

REFERENCE RANGE:

For Apolipoprotein A-1: Female (18-69 yrs) = 115 - 206 mg/dl; Male (18-69 yrs) = 107 - 187 mg/dl.
For Apolipoprotein B: Female (18-69 yrs) = 61 - 150 mg/dl; Male (18-69 yrs) = 63 - 152 mg/dl.

SPECIMEN REQUIREMENTS:

Serum is the specimen of choice, plasma is acceptable; 0.5 mL minimum, 1.0 mL optimum. The serum or plasma should be separated from the cells as soon as possible.