Apolipoprotein A & B

Test Description
Lipoproteins are important transporters of cholesterol. High-density lipoprotein
(HDL or “good cholesterol”) picks up cholesterol in the tissues and transports it back
to the liver for either recycling or excretion in the bile. Since it works to get rid of
excess cholesterol, high levels of HDL are preferred. Low-density lipoprotein (LDL,
or “bad cholesterol”) also transports excess cholesterol, however it has a tendency
to transport it to the arteries, where it can result in the development of atherosclerosis.
Thus, it is best to have low levels of LDL to decrease the risk of atherosclerotic
disease.
Apolipoproteins (apo) are the protein components of lipoproteins. Two of the
apolipoproteins, apo A and apo B, in particular serve important functions in control
of cholesterol in the body. Apo A activates the enzymes that cause movement of
cholesterol from the tissues into HDL and also causes HDL to be recognized by
receptor sites in the liver where the cholesterol is deposited. There are two subtypes
of apo A: apo A-I and apo A-II. Apo A-I is the more plentiful of the two and can be
measured directly. Apo A-I tends to correlate with HDL levels and is thought to perhaps
be better than HDL as an indicator of coronary artery disease (CAD) risk.
There are also two forms of apo B: apo B-100 and apo B-48. Apo B-48 is part
of the structure of chylomicrons, the large lipoproteins that initially transport lipids to
the liver. Once there, the lipids are combined with apo B-100 to form very low-density
lipoproteins, which eventually become LDL. Apo B-100 levels correlate with LDL levels
and can be measured directly. Apo B-100 is considered an indicator of CAD risk.
In addition to individual measurement of apo A-I and apo B-100, an apolipoprotein
evaluation also includes calculation of the apo A/apo B ratio. The lower the ratio, the
greater the risk of developing CAD. In addition to assessment of CAD risk, these apo
tests can also be used to monitor patient response to treatment for hyperlipidemia.


Normal Values
A polipoprotein A-1

Male:                    Low risk:                                  > 123 mg/dL
                              Borderline/High Risk:                109–123 mg/dL
                             High Risk:                                  <109 mg/dL

Female:                 Low risk:                                   > 140 mg/dL
                              Borderline/High Risk:                  123–140 mg/dL
                               High Risk:                                 <123 mg/dL


A polipoprotein B

Male :                    Low risk:                                    52–110 mg/dL
                              Borderline/High Risk:                  111–127 mg/dL
                             High Risk:                                     > 127 mg/dL

Female:                  Low risk:                                    49–103 mg/dL
                               Borderline/High Risk:                104–127 mg/dL
                               High Risk:                                   > 127 mg/dL


A polipoprotein A-1/B Ratio
Male:                        Low risk:                                   > 1.11
                                 Borderline/High Risk:              0.86–1.11
                                 High Risk:                                 < 0.86
  Female:                  Low risk:                                   > 1.35
                                 Borderline/High Risk:              0.97–1.35
                                 High Risk:                                  < 0.97

Possible Meanings of Abnormal Values

Apo A-I

Increased                                                                      Decreased

Familial hyperalphalipoproteinemia                      Coronary artery disease
Pregnancy                                                               Diabetes mellitus
Weight loss                                                             Hepatocellular disease
                                                                                Hypertriglyceridemia
                                                                                Nephrotic syndrome
                                                                                 Renal failure
                                                                                Tangier disease


Apo B
Increased                                                                        Decreased

Biliary obstruction                                                  Chronic anemia
Cigarette smoking                                                   Chronic pulmonary disease
Cushing’s syndrome                                                Hyperlipidemia (Type I)
Diabetes mellitus                                                     Hyperthyroidism
Hemodialysis                                                           Inflammation
Hepatic disease                                                        Malnutrition
Hyperlipoproteinemia                                              Reye’s syndrome
(Types II, III, V)                                                      Tangier disease
Hypothyroidism                                                       Weight loss
Increased risk of CAD
Nephrotic syndrome
Porphyria
Pregnancy
Renal failure

Contributing Factors to Abnormal Values
• Apo levels may be affected by unstable weight and dietary habits.
• Apo levels should not be drawn for at least 3 months following surgery or myocardial
  infarction.
• Drugs that may increase apo A-I levels: carbamazepine, cholesterol lowering
   agents, estrogens, ethanol, oral contraceptives, and phenobarbital.
• Drugs that may decrease apo A-I levels: androgens, beta-blockers, diuretics, and
   progestins.
• Drugs that may increase apo B-100 levels: beta-blockers, corticosteroids,
cyclosporine, diuretics.
• Drugs that may decrease apo B-100 levels: cholesterol lowering agents, estrogens,
  indapamide.

Interventions/Implications

Pretest

• Explain to the patient the purpose of the test and the need for a blood sample to be
  drawn.
• Fasting for 12 hours is required before the test. Water is permitted.
• No smoking is allowed for at least 12 hours before the test.

  Procedure

• A 7-mL blood sample is drawn in a gold-top (serum separator) collection tube.
• Gloves are worn throughout the procedure.

Posttest

• Apply pressure at venipuncture site. Apply dressing, periodically assessing for continued
  bleeding.
• Label the specimen and transport it to the laboratory.
• Report abnormal findings to the primary care provider

Clinical alert                   

• Increased risk of CAD occurs with decreased apo A-I levels and increased apo B

levels.