Apolipoprotein A-I

Apolipoprotein A-I (Apo-A1) is a structural and functional protein that constitutes approximately 70% of the protein in high density lipoprotein (HDL).

The reference ranges of Apo-A1 are as follows ^[1] :

• Adult males: 75-160 mg/dL
• Adult females: 80-175 mg/dL
• Male newborns: 41-93 mg/dL
• Female newborns: 38-106 mg/dL
• Males (aged 6 months-4 years): 67-167 mg/dL
• Females (aged 6 months-4 years): 60-148 mg/dL
• Children aged 5-17 years: 83-151 mg/dL

Low apolipoprotein A-I level

A low Apo-A1 level indicates an increased risk of cardiovascular disease, especially in the presence of an elevated apolipoprotein B (Apo-B) level. ^{[2, 3, 4, 5]}

Other factors that are associated with low Apo A1 level include the following:

• Chronic liver disease

• Chronic kidney disease

• Smoking

• Obesity

• High triglyceride level

High apolipoprotein A-I level

High Apo-A1 levels are associated with the following:

• Pregnancy

• Alcohol use

• Spring and summer seasons ^[6]

Patient instructions: Overnight fasting (12-14 hours)

Collection tube: Lavender top (EDTA)

Unacceptable conditions: Hemolyzed specimens

Specimen preparation: Separate serum from cells as soon as possible or within 2 hours of collection and transfer to 1-mL serum transport tube

Storage/transport temperature: Refrigerated

Stability refrigerated: 8 days unfrozen; 3 months frozen

Panels: None

CPT Code: 82172

Description

Apolipoprotein A-I (Apo-A1) is a structural and functional protein that constitutes approximately 70% of the protein in HDL.

Apo-A1 is produced in the liver and intestines and activates lecithin-cholesterol acyltransferase (LCAT) in the peripheral tissues, which transforms free cholesterol to cholesterol ester and facilitates its transportation to the liver, were it is degraded.

Indications/Applications

Because it is not clear whether Apo-A1 is an independent predictor of cardiovascular disease, it may be useful for Apo-A1 to be measured in conjunction with Apo-B to assess the Apo-B/Apo-A1 ratio.

A higher ratio means an increased likelihood of cholesterol deposition in arteries, leading to atherosclerosis and a higher risk of cardiovascular disease.

A prospective cohort study by Xiao et al indicated that as a risk factor, the Apo-B/Apo-A1 ratio is more strongly associated with abdominal aortic aneurysm (AAA) then with coronary heart disease (CHD), despite the relationship of both conditions to atherosclerosis. In addition, Apo-A1 was found to have a greater protective effect against AAA than CHD. ^[7]

A study by Henson et al indicated that in patients with coronary artery disease, low levels of immune complex consisting of Apo-A1 and immunoglobulin G (IgG) are an independent risk factor for adverse cardiovascular events. Thus, according to the investigators, this immune complex may represent a potential biomarker for predicting cardiovascular disease progression. ^[8]

Apo-A1 is one of many serum markers used in the fibroTest, a noninvasive assessment of the liver that was validated in many liver disease, including hepatitis C, hepatitis B, nonalcoholic fatty liver disease, and alcoholic liver disease.

Considerations

Serum Apo-A1 is not considered a routine test for cardiovascular disease risk assessment.

Overnight fasting might not be necessary to evaluate apo-A1, but most of the laboratories still commend it.

Apo-A1 Milano is a naturally occurring mutant of Apo-A1 associated with a very low HDL level but apparent longevity and much less atherosclerosis than expected for their HDL-C levels. ^[9]

A defect in the Apo-A1 gene (APOA1) can cause HDL deficiency and systemic nonneuropathic amyloidosis.