2.7 Lipids, Lipoproteins, and the Friedewald LDL Calculation

Lipids, Lipoproteins, and Coronary Risk Assessment

Lipids are a dedicated AMT Chemistry work-area topic. Medical Laboratory Scientists measure cholesterol fractions and triglycerides to assess atherosclerotic cardiovascular disease (ASCVD) risk and to monitor therapy.

Lipid Metabolism Essentials

• Triglycerides (TG): Glycerol + three fatty acids; primary energy storage lipid. Elevated after meals; fasting specimens are preferred for classic risk panels and Friedewald calculation.
• Cholesterol: Structural membrane lipid and steroid/bile-acid precursor. Travels in plasma only inside lipoproteins.
• Lipoprotein classes (largest/lightest → smallest/densest):
  1. Chylomicrons — dietary TG from intestine (apoB-48).
  2. VLDL — endogenous TG from liver (apoB-100).
  3. LDL — major cholesterol carrier to tissues (apoB-100); high LDL-C drives atherogenesis.
  4. HDL — reverse cholesterol transport to liver (apoA-I); higher HDL-C is generally protective.

Desirable Levels and Risk Context

Exact cutoffs evolve with guidelines, but MLS exams commonly expect these adult reference concepts (mg/dL):

• LDL-C: Optimal <100; near optimal 100–129; borderline 130–159; high 160–189; very high ≥190.
• HDL-C: Low (risk) <40 (men) or <50 (women); higher is better.
• Triglycerides: Desirable <150; very high ≥500 raises pancreatitis risk.
• Total cholesterol: Desirable <200.

Non-HDL-C (total − HDL) and apoB are increasingly used clinically, but classic board items still emphasize LDL-C and the Friedewald equation.

Analytical Methods

• Enzymatic assays dominate routine total cholesterol, HDL-C, and triglyceride measurement.
• HDL is measured after precipitating apoB-containing lipoproteins or by direct homogeneous methods.
• Direct LDL assays measure LDL-C without calculation and are preferred when Friedewald assumptions fail.
• Severe icterus, hemolysis, or lipemia can interfere optically—lipemic specimens may need ultracentrifugation or clearing protocols per lab policy.

Friedewald LDL Calculation (High-Yield)

When a direct LDL is not reported, LDL-C is often estimated:

LDL-C (mg/dL) = Total cholesterol − HDL-C − (Triglycerides / 5)

The TG/5 term estimates VLDL-C in mg/dL (use TG/2.2 if units are mmol/L—know which unit system the question uses).

Worked Example

Total cholesterol = 240 mg/dL, HDL-C = 40 mg/dL, TG = 150 mg/dL. VLDL-C ≈ 150/5 = 30. LDL-C ≈ 240 − 40 − 30 = 170 mg/dL (high).

When Friedewald Is Invalid

• Triglycerides ≥ 400 mg/dL
• Nonfasting specimens (for this formula)
• Suspected type III dyslipoproteinemia (remnant accumulation)
• Very low TG where VLDL estimation is unreliable in some protocols

In these cases, order or report a direct LDL-C.

Clinical Correlation Scenarios

• Isolated high TG + low HDL: metabolic syndrome / diabetes pattern.
• Extremely high TG (>1000 mg/dL): consider acute pancreatitis risk and chylomicronemia.
• High LDL with tendon xanthomas / premature CAD: consider familial hypercholesterolemia workup.

Mastering lipid panels means calculating correctly, knowing method limitations, and linking patterns to coronary risk—not merely memorizing a single cutoff.

Specimen Collection and Preanalytical Issues for Lipid Panels

For classic Friedewald reporting, collect a fasting specimen (typically 9–12 hours, water allowed). Nonfasting triglycerides rise and can falsely lower calculated LDL-C. Note the collection time and fasting status on the requisition when policy requires it.

Preanalytical traps:

• Prolonged tourniquet use concentrates proteins and lipids.
• IV contamination from lipid emulsions (TPN) can create extreme lipemia unrelated to endogenous metabolism.
• Standing vs supine posture slightly alters concentrations; use consistent outpatient technique.
• If the specimen is grossly lipemic, document interference checks; some assays need dilution, ultracentrifugation, or lipemia-clearing reagents before releasing results.

Metabolic Pathways Worth Memorizing

1. Exogenous pathway: Dietary TG → chylomicrons → lipoprotein lipase (LPL) releases free fatty acids → chylomicron remnants cleared by the liver.
2. Endogenous pathway: Hepatic VLDL → LPL → IDL → LDL.
3. Reverse transport: Peripheral cholesterol → HDL → liver (SR-B1) for excretion as bile acids.

ApoB-100 marks atherogenic particles (VLDL, IDL, LDL). ApoA-I marks HDL. Questions may ask which apolipoprotein is on LDL vs HDL—answer apoB-100 vs apoA-I.

Secondary Causes and Drug Effects

Secondary dyslipidemia patterns commonly tested:

• Uncontrolled diabetes mellitus → high TG, low HDL.
• Nephrotic syndrome → high cholesterol/LDL from hepatic overproduction.
• Hypothyroidism → elevated total cholesterol/LDL.
• Thiazides, beta-blockers, corticosteroids, and retinoids can worsen lipid profiles—correlate with medication history when results suddenly change.

Quality Control and Reporting Tips

• Verify the arithmetic: Total cholesterol should approximately equal HDL + LDL + (TG/5) when Friedewald is used; large imbalances suggest transcription or assay error.
• If TG ≥400 mg/dL, cancel/suppress calculated LDL and reflex or recommend direct LDL.
• Panic or critical callbacks are lab-specific, but extremely high TG (e.g., >1000 mg/dL) warrant rapid clinician notification because of pancreatitis risk.
• Report units clearly (mg/dL vs mmol/L). Never mix unit systems inside the Friedewald equation.

With these preanalytical, pathway, secondary-cause, and QC details, lipid testing becomes a complete Chemistry domain skill rather than a single formula to memorize.