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 range of Apo-A1 varies by sex, as follows:
Men: Greater than 120 mg/dL (1.2 g/L)
Women: Greater than 140 mg/dL (1.4 g/L)
levels decrease with age.
Low apolipoprotein A-I level
Other factors that are associated with low Apo A1 level include the following:
Chronic liver disease
High triglyceride level
High apolipoprotein A-I level
High Apo-A1 levels are associated with the following:
Spring and summer seasons 
Collection and Panels
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
CPT Code: 82172
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.
Because it is not clear whether Apo-A1 is an independent predictor of cardiovascular disease, it may be useful to be measured in conjunction with Apo-B to assess the Apo-B/Apo-A1 ratio.
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.
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. 
A defect in the Apo-A1 gene (APOA1) can cause HDL deficiency and systemic nonneuropathic amyloidosis.