Post-heparin LPL activity testing with Apo C-II stimulation is a classic functional assay used to differentiate the two most common genetic causes of severe chylomicronemia: deficiency of lipoprotein lipase itself versus deficiency of its essential activator, apolipoprotein C-II. The test measures LPL catalytic activity in plasma obtained shortly after an intravenous heparin bolus, first in the patient’s native sample and then again after adding purified exogenous Apo C-II. If activity is low at baseline but rises to normal when Apo C-II is supplied, the defect lies in the activator; if activity remains low, the LPL enzyme itself is defective.
The core diagnostic value lies in the reversal of LPL inactivity by Apo C-II supplementation. This simple add‑back experiment cleanly separates LPL gene defects from Apo C‑II gene defects using a single blood draw, providing a functional answer that genetic panels cannot always resolve on their own.
Why Post‑Heparin Plasma Is Required
Lipoprotein lipase is not freely circulating in the blood. It is anchored to the capillary endothelium by heparan sulfate proteoglycans.
The Heparin Challenge
An intravenous injection of heparin releases LPL (and hepatic lipase) into the plasma. Blood collected 10–15 minutes after the injection becomes a “post‑heparin” sample that contains measurable lipolytic activity.
Without Heparin, the Baseline Is Irrelevant
Measuring LPL in pre‑heparin plasma yields near‑zero activity in healthy individuals because little active enzyme is present. The post‑heparin sample is therefore mandatory to detect any catalytic function.
How the LPL Activity Assay Works
The principle is straightforward: the lab incubates the patient’s post‑heparin plasma with a triglyceride substrate and quantifies the released free fatty acids.
Substrate and Detection
Assays typically use a radiolabeled triolein emulsion or a fluorogenic triglyceride analog. The reaction is run under conditions that favour LPL over hepatic lipase, often by including a specific LPL inhibitor or by adjusting salt concentrations.
The Baseline Measurement
The lab first measures activity without adding any exogenous Apo C‑II. In a healthy person, enough endogenous Apo C‑II is present in the plasma to fully activate LPL, giving a normal value. In a patient with severe hyperchylomicronemia, the baseline activity is often markedly low or undetectable.
The Apo C‑II Stimulation Test: A Stepwise Differential Diagnosis
When baseline post‑heparin LPL activity is low, the key question is whether restoring the activator can rescue the enzyme.
Adding Exogenous Apo C‑II
A second aliquot of the same post‑heparin plasma is mixed with purified recombinant or plasma‑derived Apo C‑II at saturating concentration. The LPL activity assay is then repeated.
Interpreting the Pattern
- LPL deficiency: Activity stays low even after Apo C‑II addition. The enzyme molecule itself is either absent or catalytically dead.
- Apo C‑II deficiency: Baseline activity is low, but it normalizes completely upon adding Apo C‑II. The LPL enzyme is structurally normal but has been inactive for lack of its cofactor.
Follow‑Up Genetic Testing
While the functional pattern strongly points to one gene or the other, clinicians often confirm with sequencing of the LPL and APOC2 genes. The biochemical result can also help classify a variant of uncertain significance found in a genetic panel.
Pitfalls and Limiting Factors in the Assay
Despite its elegant design, the test is not foolproof. Several real‑world variables can blur the interpretation.
Hepatic Lipase Interference
Post‑heparin plasma contains both LPL and hepatic lipase. If the assay conditions do not fully suppress hepatic lipase activity, the measured “LPL” activity may appear falsely high, potentially masking a partial deficiency.
Presence of Circulating Inhibitors
Some patients develop antibodies against LPL or have other plasma inhibitors. These can cause low baseline activity that may not be fully reversed by Apo C‑II, simulating an LPL defect. Specialist labs must rule this out.
The Test Is Increasingly Rare
Today, many centres skip the functional assay in favour of simple gene panels. However, the post‑heparin stimulation test remains invaluable when genetic results are ambiguous, when novel mutations are found, or when a rapid functional proof is needed before genetic results are available.
Logistical Demands
The test requires intravenous heparin, timed phlebotomy, and immediate plasma separation at 4 °C to preserve enzyme activity. It is therefore confined to a handful of reference laboratories.
Making the Right Choice for Your Diagnostic Goal
The post‑heparin LPL / Apo C‑II stimulation assay is a targeted functional tool, not a screening test. Here is when it delivers the most value:
- If your primary focus is a fast, functional classification: Use the stimulation assay on a post‑heparin sample. A clear “rescued” or “not rescued” result gives a near‑instant differential diagnosis without needing a genetic report.
- If your primary focus is resolving a genetic variant of uncertain significance: Pair the functional result with the DNA finding. For example, if a novel LPL missense variant is identified and the stimulation test shows no rescue, the variant is highly likely to be pathogenic.
- If your primary focus is screening and genetic results are already conclusive: The assay adds little value, because pathogenic nonsense or deletion mutations in LPL or APOC2 are self‑explanatory. The functional test is reserved for the exceptional, ambiguous cases.
A single tube of post‑heparin plasma, tested without and with Apo C‑II, can functionally settle a diagnostic dilemma that genetics alone sometimes cannot.
Summary Table:
| Diagnostic Pattern | Baseline LPL Activity | Activity After Apo C-II Addition | Clinical Diagnosis |
|---|---|---|---|
| LPL Gene Defect | Low / Undetectable | Remains Low (No Rescue) | Primary Lipoprotein Lipase Deficiency |
| Apo C-II Gene Defect | Low / Undetectable | Normalizes (Full Rescue) | Apolipoprotein C-II Deficiency |
| Healthy Control | Normal | Minimal to No Change | Normal Lipolytic Function |
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