Cholesterol Efflux Capacity

Two people can have the same HDL cholesterol level and different cardiovascular risk. Cholesterol efflux capacity examines one aspect of HDL function: its ability to accept cholesterol released by cells. Several human studies have linked higher efflux with fewer coronary events, even after accounting for HDL cholesterol. Not every large study agrees.

This remains an exploratory marker for individual care. The observational evidence is substantial, but there are no standardized clinical cutoffs or validated treatment targets. Studies haven't established that making medical decisions from an efflux result improves outcomes.

How Cholesterol Leaves Artery-Wall Cells

Macrophages are immune cells that take up cholesterol within artery walls. When cholesterol accumulates, they become foam cells and contribute to plaque. These cells can release cholesterol to HDL and its proteins, starting a process that can eventually carry cholesterol to the liver for disposal.

ApoA-I is the main protein on HDL. Cell-surface proteins help transfer cholesterol to apoA-I and HDL particles, and some cholesterol also moves by passive exchange. Particle composition and the route being measured influence efflux, so HDL cholesterol and apoA-I concentrations explain only part of the variation between people.

What the Test Measures

The conventional cell-based assay uses serum from a blood draw after removing particles that contain apoB, including LDL and VLDL. This preparation retains HDL, lipid-poor apoA-I and other proteins that can accept cholesterol. It is called apoB-depleted serum.

The serum preparation is added to laboratory-grown macrophages containing tagged cholesterol. The assay measures how much tagged cholesterol leaves the cells over a set period. Results are commonly normalized to a pooled reference sample; some protocols report a percentage and others report a relative ratio.

This measures cholesterol transfer under laboratory conditions. It doesn't measure cholesterol leaving your own artery walls, the amount of plaque you have, or the liver's subsequent disposal of cholesterol. Movement of a cholesterol label also differs from measuring the net amount of cholesterol removed.

Heart Attack and Coronary Risk

An early study examined 203 healthy volunteers and 793 people undergoing coronary angiography. Higher efflux from apoB-depleted serum was associated with thinner carotid artery walls and lower odds of coronary disease, independent of HDL cholesterol. Later studies examined whether efflux measured before an event was associated with future disease.

These are associations between groups, not estimates of what would happen if you raised your result. The plasma assays provide supporting evidence about the same biological process, but their values aren't interchangeable with serum assay results.

A 2021 analysis pooled 20 studies involving about 25,000 people. Its unadjusted comparison found 37% fewer adverse cardiovascular events in groups with higher efflux. Associations remained after adjustment in several analyses, but the studies used different assays, populations and outcome definitions. The pooled results did not establish a clear association with cardiovascular or all-cause mortality. A 2020 meta-analysis also found large differences between studies and signs of possible publication bias.

A further analysis of the Dallas cohort found that efflux improved statistical risk prediction beyond coronary calcium, family history and hs-CRP. This was an additional analysis of the same cohort, rather than an independent replication, and it didn't test whether ordering CEC changed treatment or prevented events.

If You Already Have Coronary Disease

Some studies in people with established coronary disease found large associations with survival. Among 1,737 people with coronary artery disease, the highest efflux quarter had 76% lower adjusted risk of death from any cause than the lowest quarter. Cardiovascular mortality was 83% lower. This study used a related fluorescent assay with apoB-depleted plasma.

In another study of 1,609 people hospitalized with an ST-elevation heart attack, the highest efflux quarter had about 46% lower adjusted mortality than the lowest quarter over a median of about two years. That finding came from whole serum tested with human macrophage-like cells. Whole serum includes LDL and other cholesterol acceptors removed from the conventional HDL-focused assay, so this is evidence about a broader measurement.

A meta-analysis of five coronary-disease studies involving 5,725 people found about two thirds lower risk in the higher-efflux groups. Its pooled endpoint combined different definitions of cardiovascular events and mortality, including all-cause death in some studies. The size of the association varied substantially between studies.

These findings support further study of efflux as a prognostic marker. They don't establish how much an individual result adds to treatment decisions after a heart attack or stent.

Inherited High Cholesterol

Familial hypercholesterolemia causes high LDL cholesterol from early life. In a study of 227 people with the heterozygous form who were receiving lipid-lowering treatment, higher efflux was associated with less existing cardiovascular disease and thinner carotid artery walls. The association with existing disease was weak: odds were only about 5% lower for each standard step up in efflux. The study used apoB-depleted plasma. Because it compared existing disease at one point in time, it didn't establish that serial efflux measurements predict future events or improve treatment. A separate case-control study found higher, not lower, efflux in people with familial hypercholesterolemia than in controls, so the data conflict.

Where the Pattern Differs

The strongest contrary evidence comes from three prospective cohorts with 14,438 people and 1,570 coronary events, all measured with a standard cell-based assay. Once traditional risk factors and HDL cholesterol were accounted for, efflux no longer predicted coronary disease, while HDL cholesterol and apoA-I concentrations kept their associations. A genetic analysis in 20,372 people found no gene variants affecting efflux independently of HDL cholesterol and triglycerides.

Higher efflux hasn't predicted every vascular outcome. The Multi-Ethnic Study of Atherosclerosis used a cholesterol mass-efflux assay that measured net cholesterol movement, a different method from tracking labeled cholesterol. Higher mass efflux was associated with fewer coronary events, but not fewer strokes. It was also associated with more carotid plaque progression, although that association didn't increase steadily across efflux levels.

In a substudy of the JUPITER trial, baseline efflux wasn't significantly associated with subsequent cardiovascular events. Efflux measured during rosuvastatin treatment was associated with events, and HDL particle number was the strongest HDL-related predictor. This difference cautions against assuming that every population or treatment setting will produce the same relationship.

Kidney disease can reverse the pattern. In Dallas Heart Study participants with chronic kidney disease, higher efflux was linked with more cardiovascular events and cardiovascular deaths, while HDL particle concentration stayed protective. Studies in type 2 diabetes and end-stage kidney disease using human macrophage-like cells found no association with events, the association weakens in people with high inflammation, and one angiography cohort found higher efflux linked with more events.

Rheumatoid arthritis studies have also found different associations when measuring separate cholesterol-export routes. ABCA1 and ABCG1 are cell-surface proteins involved in these routes. Their individual efflux measurements had different relationships with plaque and events, influenced by inflammation and treatment. These route-specific assays are different measurements from total CEC.

Conditions Associated With Altered Efflux

• Metabolic syndrome: lower efflux was associated with this combination of abdominal obesity, abnormal blood fats, high blood pressure and impaired glucose regulation in a Dallas study using apoB-depleted plasma.
• Rheumatoid arthritis: a small study testing apoB-depleted serum found lower efflux in 18 people with highly active disease than in 7 with low activity or remission. The average difference between all rheumatoid arthritis participants and controls wasn't significant.
• Obstructive sleep apnea: a small study of 69 patients and 23 controls using apoB-depleted serum found lower total efflux in people with sleep apnea. The association with greater sleep-apnea severity was specific to the ABCA1 route; total efflux didn't differ significantly between severity groups. In a separate study, 12 weeks of CPAP treatment didn't change efflux.
• Kidney disease: HDL isolated from people receiving hemodialysis removed cholesterol less effectively than HDL from healthy controls. Testing isolated HDL is different from testing apoB-depleted serum. In people with chronic kidney disease, a higher efflux result has been linked with more cardiovascular events rather than fewer.
• Type 2 diabetes: findings vary between studies. Some have found preserved efflux despite changes in HDL composition, so diabetes doesn't imply that efflux must be low.

These findings help explain why disease state matters when interpreting efflux. They don't make CEC a diagnostic test for any of these conditions.

When Results Can Be Misleading

• Different assay methods: cell type, cholesterol label, incubation conditions and the cholesterol-export routes being stimulated affect the result. Fluorescent and radiolabeled assays can rank samples differently.
• Different blood fractions: whole serum, whole plasma, apoB-depleted samples and isolated HDL contain different cholesterol acceptors. Their results describe related but distinct measurements.
• Laboratory variation: cell condition, sample preparation, storage and differences between assay runs can change the reading. Small differences between results may reflect measurement variation.
• Acute illness and inflammation: inflammatory activity can change HDL composition and efflux. A sample collected during a heart attack or another acute illness may differ from your usual state.
• Kidney disease: in people with chronic kidney disease, higher efflux has been associated with higher cardiovascular risk, so the usual interpretation may not apply.
• Medication effects depend on the fraction tested: in an 8-week randomized study of healthy men with normal cholesterol, atorvastatin and evolocumab reduced efflux measured with whole plasma while leaving apoB-depleted plasma efflux unchanged. That whole-plasma change doesn't show that HDL function deteriorated.