Apolipoproteins

Apolipoproteins: A, B, and E

Synonym/acronym: Apo A (Apo A1), Apo B (Apo B100), and Apo E.

Common use

To identify levels of circulating lipoprotein to evaluate the risk of coronary artery disease.

Specimen

Serum (1 mL) collected in a gold-, red-, or red/gray-top tube or plasma collected in a green- (heparin) or lavender-top (EDTA) tube for Apo A and Apo B; Plasma (1 mL) collected in a lavender-top (EDTA) tube.

Normal findings

(Method: Immunonephelometry for Apo A and Apo B; PCR with restriction length enzyme digestion and polyacrylamide gel electrophoresis for Apo E) Apolipoprotein A

Age	Conventional Units	SI Units (Conventional Units × 0.01)
Newborn
Male	41–93 mg/dL	0.41–0.93 g/L
Female	38–106 mg/dL	0.38–1.06 g/L
6 mo–4 yr
Male	67–163 mg/dL	0.67–1.63 g/L
Female	60–148 mg/dL	0.6–1.48 g/L
Adult
Male	81–166 mg/dL	0.81–1.66 g/L
Female	80–214 mg/dL	0.80–2.14 g/L

Apolipoprotein B

Age	Conventional Units	SI Units (Conventional Units × 0.01)
Newborn–5 yr	11–31 mg/dL	0.11–0.31 g/L
5–17 yr
Male	47–139 mg/dL	0.47–1.39 g/L
Female	41–96 mg/dL	0.41–0.96 g/L
Adult
Male	46–174 mg/dL	0.46–1.74 g/L
Female	46–142 mg/dL	0.46–1.42 g/L

Normal Apo E: Homozygous phenotype for e3/e3.

Description

Apolipoproteins assist in the regulation of lipid metabolism by activating and inhibiting enzymes required for this process. The apolipoproteins also help keep lipids in solution as they circulate in the blood and direct the lipids toward the correct target organs and tissues in the body. A number of types of apolipoproteins have been identified (A, B, C, D, E, H, J), each of which contain subgroups. Apolipoprotein A (Apo A), the major component of high-density lipoprotein (HDL), is synthesized in the liver and intestines. Apo A-I activates the enzyme lecithin-cholesterol acyltransferase (LCAT), whereas Apo A-II inhibits LCAT. It is believed that Apo A measurements may be more important than HDL cholesterol measurements as a predictor of coronary artery disease (CAD). There is an inverse relationship between Apo A levels and risk for developing CAD. Because of difficulties with method standardization, the above-listed reference ranges should be used as a rough guide in assessing abnormal conditions. Values for African Americans are 5 to 10 mg/dL higher than values for whites. Apolipoprotein B (Apo B), the major component of the low-density lipoproteins (chylomicrons, low-density lipoprotein [LDL], and very-low-density lipoprotein), is synthesized in the liver and intestines. Apolipoprotein E is found in most lipoproteins, except LDL, and is synthesized in a variety of cell types including liver, brain astrocytes, spleen, lungs, adrenals, ovaries, kidneys, muscle cells, and in macrophages. The largest amount is produced by the liver; the next significant amount is produced by the brain. There are three forms of Apo E: apo-E 2, apo-E 3, and apo-E 4, and six possible combinations; of these, Apo-E 3 (e3/3e) is the fully functioning form. The varied roles of Apo E include removal of chylomicrons and very-low-density lipoprotein (VLDL) from the circulation by binding to LDL. The Apo E2 isoform demonstrates significantly less LDL receptor binding, which results in impaired clearance of chylomicrons, VLDL, and triglyceride remnants. The presence of Apo E isoforms E2 and E4 is associated with high cholesterol levels, high triglyceride levels, and the premature development of atherosclerosis. The presence of the E2 isoform is associated with type III hyperlipidemia, a familial dyslipidemia, which is important to distinguish from other causes of hyperlipidemia to determine the correct treatment regimen. Apo E4 is being used in association with studies of predisposing factors in the development of Alzheimer’s disease. Detailed information is found in the study titled “Alzheimer’s Disease Markers.”

This procedure is contraindicated for

N/A

Indications

Evaluation for risk of CAD

Potential diagnosis

Apolipoproteins are the protein portion of lipoproteins. Their function is to transport and to assist in cell surface receptor recognition and cellular absorption of lipoproteins to be used as energy. While studies of the exact role of apolipoproteins in health and disease continue, there is a very strong association between Apo A and HDL “good” cholesterol and Apo B and LDL “bad” cholesterol.

Apolipoprotein A

Increased in

Familial hyper-α-lipoproteinemia
Pregnancy
Weight reduction

Decreased in

Abetalipoproteinemia Cholestasis Chronic renal failure Coronary artery disease Diabetes (uncontrolled) Diet high in carbohydrates or polyunsaturated fats Familial deficiencies of related enzymes and lipoproteins (e.g., Tangier’s disease) Hemodialysis Hepatocellular disorders Hypertriglyceridemia Nephrotic syndrome Premature coronary heart disease Smoking

Apolipoprotein B

Increased in

Anorexia nervosa
Biliary obstruction
Coronary artery disease
Cushing’s syndrome
Diabetes
Dysglobulinemia
Emotional stress
Hemodialysis
Hepatic disease
Hepatic obstruction
Hyperlipoproteinemias
Hypothyroidism
Infantile hypercalcemia
Nephrotic syndrome
Porphyria
Pregnancy
Premature CAD
Renal failure
Werner’s syndrome

Decreased in

Acute stress (burns, illness) Chronic anemias Chronic pulmonary disease Familial deficiencies of related enzymes and lipoproteins (e.g., Tangier’s disease) Hyperthyroidism Inflammatory joint disease Intestinal malabsorption α-Lipoprotein deficiency (Tangier’s disease) Malnutrition Myeloma Reye’s syndrome Weight reduction

Critical findings

N/A

Interfering factors

Drugs and substances that may increase Apo A levels include anticonvulsants, beclobrate, bezafibrate, ciprofibrate, estrogens, furosemide, lovastatin, pravastatin, prednisolone, simvastatin, and ethanol (abuse).
Drugs that may decrease Apo A levels include androgens, β-blockers, diuretics, and probucol.
Drugs that may increase Apo B levels include amiodarone, androgens, β-blockers, catecholamines, cyclosporine, diuretics, ethanol (abuse), etretinate, glucogenic corticosteroids, oral contraceptives, and phenobarbital.
Drugs that may decrease Apo B levels include beclobrate, captopril, cholestyramine, fibrates, ketanserin, lovastatin, niacin, nifedipine, pravastatin, prazosin, probucol, and simvastatin.
Drugs that may decrease Apo E levels include bezafibrate, fluvastatin, gemfibrozil, ketanserin, lovastatin, niacin, nifedipine, oral contraceptives, pravastatin, probucol, and simvastatin.
Failure to follow dietary restrictions before the procedure may cause the procedure to be canceled or repeated.

Nursing Implications and Procedure

Pretest

Positively identify the patient using at least two unique identifiers before providing care, treatment, or services.
Patient Teaching: Inform the patient this test can assist in assessing and monitoring risk for coronary artery (heart) disease.
Obtain a history of the patient’s complaints, including a list of known allergens, especially allergies or sensitivities to latex.
Obtain a history of the patient’s cardiovascular system, symptoms, and results of previously performed laboratory tests and diagnostic and surgical procedures.
Obtain a list of the patient’s current medication, including herbs, nutritional supplements, and nutraceuticals (see Effects of Natural Products on Laboratory Values online at DavisPlus).
Review the procedure with the patient. Inform the patient that specimen collection takes approximately 5 to 10 min. Address concerns about pain and explain that there may be some discomfort during the venipuncture.
Sensitivity to social and cultural issues, as well as concern for modesty, is important in providing psychological support before, during, and after the procedure.
Instruct the patient to abstain from food for 6 to 12 hr before specimen collection.
Note that there are no fluid or medication restrictions unless by medical direction.

Intratest

Potential complications: N/A
Ensure that the patient has complied with dietary or activity restrictions, and pretesting preparations; assure that food has been restricted for at least 6 to 12 hr prior to the procedure.
Avoid the use of equipment containing latex if the patient has a history of allergic reaction to latex.
Instruct the patient to cooperate fully and to follow directions. Direct the patient to breathe normally and to avoid unnecessary movement.
Observe standard precautions, and follow the general guidelines in Patient Preparation and Specimen Collection. Positively identify the patient, and label the appropriate specimen container with the corresponding patient demographics, initials of the person collecting the specimen, date, and time of collection. Perform a venipuncture.
Remove the needle and apply direct pressure with dry gauze to stop bleeding. Observe/assess venipuncture site for bleeding or hematoma formation and secure gauze with adhesive bandage.
Promptly transport the specimen to the laboratory for processing and analysis.

Post-Test

Inform the patient that a report of the results will be made available to the requesting health-care provider (HCP), who will discuss the results with the patient.
Instruct the patient to resume usual diet as directed by the HCP.
Nutritional Considerations: Decreased Apo A and/or increased Apo B levels may be associated with CAD. Nutritional therapy is recommended for the patient identified to be at risk for developing CAD or for individuals who have specific risk factors and/or existing medical conditions (e.g., elevated LDL cholesterol levels, other lipid disorders, insulin-dependent diabetes, insulin resistance, or metabolic syndrome). Other changeable risk factors warranting patient education include strategies to encourage patients, especially those who are overweight and with high blood pressure, to safely decrease sodium intake, achieve a normal weight, ensure regular participation of moderate aerobic physical activity three to four times per week, eliminate tobacco use, and adhere to a heart-healthy diet. If triglycerides also are elevated, the patient should be advised to eliminate or reduce alcohol. The 2013 Guideline on Lifestyle Management to Reduce Cardiovascular Risk published by the ACC and AHA in conjunction with the NHLBI recommends a “Mediterranean”-style diet rather than a low-fat diet. The new guideline emphasizes inclusion of vegetables, whole grains, fruits, low-fat dairy, nuts, legumes, and nontropical vegetable oils (e.g., olive, canola, peanut, sunflower, flaxseed) along with fish and lean poultry. A similar dietary pattern known as the DASH diet makes additional recommendations for the reduction of dietary sodium. Both dietary styles emphasize a reduction in consumption of red meats, which are high in saturated fats and cholesterol, and other foods containing sugar, saturated fats, trans fats, and sodium.
Social and Cultural Considerations: Numerous studies point to the prevalence of excess body weight in American children and adolescents. Experts estimate that obesity is present in 25% of the population ages 6 to 11. The medical, social, and emotional consequences of excess body weight are significant. Special attention should be given to instructing the child and caregiver regarding health risks and weight-control education.
Recognize anxiety related to test results, and be supportive of fear of shortened life expectancy. Discuss the implications of abnormal test results on the patient’s lifestyle. Provide teaching and information regarding the clinical implications of the test results, as appropriate. Educate the patient regarding access to counseling services. Provide contact information, if desired, for the American Heart Association (www.americanheart.org) or the NHLBI (www.nhlbi.nih.gov).
Reinforce information given by the patient’s HCP regarding further testing, treatment, or referral to another HCP. Answer any questions or address any concerns voiced by the patient or family.
Depending on the results of this procedure, additional testing may be performed to evaluate or monitor progression of the disease process and determine the need for a change in therapy. Evaluate test results in relation to the patient’s symptoms and other tests performed.

Related Monographs

Related tests include Alzheimer’s disease markers, antiarrhythmic drugs, AST, ANP, BNP, blood gases, CRP, calcium and ionized calcium, cholesterol (total, HDL, and LDL), CK and isoenzymes, CT scoring, echocardiography, glucose, glycated hemoglobin, Holter monitor, homocysteine, ketones, LDH and isoenzymes, lipoprotein electrophoresis, magnesium, MRI chest, myocardial infarct scan, myocardial perfusion heart scan, myoglobin, PET heart, potassium, triglycerides, and troponin.
See the Cardiovascular System table at the end of the book for related tests by body system.