Master fatty acid structure, saturation, and lipid biosynthesis. Understand eicosanoids for ASCP chemistry MCQs.
SECTION 1 — Priority Table & Quick Foundation
| Topic | Priority | Why It’s Tested |
|---|---|---|
| Friedewald equation (LDL-C calculation) | HIGH | Most-asked calculation; trap when TG > 400 mg/dL or chylomicrons present. |
| Lipoprotein density & composition (Chylo, VLDL, LDL, HDL) | HIGH | Classic identification questions: largest/least dense, main lipid carried, apo content. |
| Apolipoproteins (B-100, B-48, A-I, C-II, E) | HIGH | Each has a unique lipoprotein “address” and function — high-yield matching. |
| Familial Hypercholesterolemia (FH) — LDL receptor defect | HIGH | Tendon xanthomas + arcus + family CHD = classic stem. |
| Type I (LPL deficiency) vs Type V hyperlipoproteinemia | HIGH | Massive TG, creamy layer, eruptive xanthomas, pancreatitis risk. |
| Type III dysbetalipoproteinemia (Apo E2/E2) | HIGH | Palmar xanthomas + broad β band = signature. |
| NCEP/ATP III cut-points (TC, LDL, HDL, TG) | HIGH | Direct numeric recall; appears in interpretation questions. |
| Cholesterol enzymatic method (CE-hydrolase → oxidase → peroxidase) | HIGH | Asked: which agent oxidizes 4-aminoantipyrine? Answer: H₂O₂. |
| Triglyceride enzymatic method & free glycerol blank | MEDIUM | Pre-analytical interferent (DM, TPN, glycerol meds). |
| HDL precipitation vs homogeneous (direct) methods | MEDIUM | Tests understanding of method limitations with high TG. |
| Lipoprotein electrophoresis migration patterns | MEDIUM | Origin/β/pre-β/α — must match each band to its lipoprotein. |
| Reverse cholesterol transport (HDL, LCAT, ABCA1, CETP) | MEDIUM | Mechanism question — why HDL is “good cholesterol.” |
| Lp(a) — independent CHD risk | MEDIUM | Apo(a) homology with plasminogen → pro-thrombotic. |
| Tangier disease (ABCA1 defect, near-zero HDL) | MEDIUM | Orange tonsils + low HDL — classic recognition stem. |
| NCEP precision/bias goals (CV%, total error) | LOW | Occasional standardization question. |
| Lipid biosynthesis enzymology beyond HMG-CoA reductase | LOW | Skim — only HMG-CoA reductase and statin link is tested. |
FOCUS vs SKIM
FOCUS hard on: Friedewald math, lipoprotein density order, apo–lipoprotein matching, FH vs Type I vs Type III recognition, NCEP cut-points, cholesterol enzymatic chemistry, and electrophoresis bands. SKIM: detailed cholesterol biosynthesis past HMG-CoA reductase, ABCG5/8 details, exact NCEP CV%, and rare LCAT/Fish-Eye disease genetics.
FOCUS hard on: Friedewald math, lipoprotein density order, apo–lipoprotein matching, FH vs Type I vs Type III recognition, NCEP cut-points, cholesterol enzymatic chemistry, and electrophoresis bands. SKIM: detailed cholesterol biosynthesis past HMG-CoA reductase, ABCG5/8 details, exact NCEP CV%, and rare LCAT/Fish-Eye disease genetics.
Quick Foundation — Read This First
- Lipids are oily — they don’t dissolve in blood (which is water-based). To travel, they ride inside protein-coated “ferries” called lipoproteins.
- 4 main ferries, ordered from biggest/lightest to smallest/densest: Chylomicrons → VLDL → LDL → HDL. As size goes down, density goes up, and protein content goes up.
- Two main cargos: Triglycerides (TG, fuel) and Cholesterol (membranes & hormones).
- Chylomicrons carry dietary (exogenous) TG from gut → cells.
- VLDL carries liver-made (endogenous) TG → cells; once TG is unloaded, VLDL becomes IDL, then LDL.
- LDL = “bad cholesterol” — delivers cholesterol to peripheral cells; if too much, builds up in artery walls → atherosclerosis.
- HDL = “good cholesterol” — picks up excess cholesterol and returns it to the liver (reverse cholesterol transport).
- Apolipoproteins are the “shipping labels” on lipoproteins — they tell receptors what to do (bind, activate, inhibit).
🚢 Petroleum industry analogy: Chylomicrons = giant oil tankers (dietary cargo). VLDLs = tanker trucks (liver cargo to peripheral cells). LDLs = nearly-empty tankers delivering the last cholesterol drops. HDLs = clean-up crew picking up spills back to the refinery (liver).
Why this matters clinically: Lipoprotein levels predict heart attack and stroke risk — every 1% drop in LDL-C ≈ 2% drop in CHD risk.
| Concept | Plain-English Explanation |
|---|---|
| Chylomicron | Biggest, lightest fat ferry from the gut. Makes plasma look milky after a meal. |
| VLDL | Liver-made fat truck. Carries triglycerides made by the liver to muscle/fat tissue. |
| LDL | Cholesterol delivery truck. Too much = plaque in arteries. |
| HDL | Cholesterol garbage truck. Picks up excess and returns it to liver. |
| Apo B-100 | “License plate” of LDL/VLDL — needed for LDL receptor binding. |
| Apo B-48 | Truncated apo B; the unique tag for chylomicrons (intestine origin). |
| Apo A-I | Main protein of HDL; activates LCAT. |
| Apo C-II | Cofactor that turns ON lipoprotein lipase (LPL). |
| Apo E | Receptor ligand for chylomicron remnants and IDL clearance. |
| LPL (lipoprotein lipase) | Enzyme on capillary walls that strips TG from chylomicrons/VLDL. |
| LCAT | Enzyme that esterifies free cholesterol on HDL → matures HDL. |
| CETP | Swaps cholesteryl ester (HDL) for triglyceride (VLDL/LDL). |
| Friedewald equation | Calculates LDL-C: LDL = TC − HDL − (TG/5). Fails when TG > 400 mg/dL. |
SECTION 2 — Core Content Tables
2.1 Lipoprotein Comparison (Master Table)
| Feature | Chylomicron | VLDL | IDL | LDL | HDL |
|---|---|---|---|---|---|
| Density (g/mL) | < 0.93 | 0.93–1.006 | 1.006–1.019 | 1.019–1.063 | 1.063–1.21 |
| Diameter (nm) | 80–1,200 (largest) | 30–80 | 25–35 | 18–30 | 5–12 (smallest) |
| Major lipid carried | Dietary TG | Endogenous (hepatic) TG | TG & cholesterol | Cholesterol | Cholesterol (back to liver) |
| Origin | Intestine | Liver | VLDL → IDL (transient) | IDL/VLDL → LDL | Liver & intestine |
| Major apolipoproteins | B-48, A-I, A-IV, C-II, E | B-100, C-I, C-II, C-III, E | B-100, E | B-100 only | A-I, A-II, C, E |
| Electrophoresis band | Origin (stays at application point) | Pre-β | Broad β (in type III) | β | α (fastest, anode) |
| % Protein | 1–2 | 5–10 | ~18 | ~22 | ~50 (highest) |
| Atherogenic? | Remnants → yes | Yes (esp. small VLDL) | Yes | Most atherogenic | Anti-atherogenic |
Memorize the order from largest/least dense to smallest/most dense: Chylo > VLDL > IDL > LDL > HDL. The smaller the particle, the denser, the more protein, and (after LDL) the less atherogenic.
Pre-analytical — Lipid Panel
Specimen: Serum (preferred) or EDTA plasma. Fasting: 12 hours required for accurate TG and Friedewald LDL-C (current ACC/AHA/pediatric guidelines accept non-fasting for screening). Posture: Sit 5 min before draw — standing falsely raises lipids ~10%. Storage: Refrigerate at 4°C up to several days; freeze for longer storage. Anticoagulant correction: EDTA plasma values are ~3% lower than serum (osmotic dilution) — multiply EDTA cholesterol by 1.03 to compare to NCEP serum cut-points. Average ≥ 2 measurements 1–8 weeks apart for diagnosis.
Specimen: Serum (preferred) or EDTA plasma. Fasting: 12 hours required for accurate TG and Friedewald LDL-C (current ACC/AHA/pediatric guidelines accept non-fasting for screening). Posture: Sit 5 min before draw — standing falsely raises lipids ~10%. Storage: Refrigerate at 4°C up to several days; freeze for longer storage. Anticoagulant correction: EDTA plasma values are ~3% lower than serum (osmotic dilution) — multiply EDTA cholesterol by 1.03 to compare to NCEP serum cut-points. Average ≥ 2 measurements 1–8 weeks apart for diagnosis.
2.2 Cholesterol
| Feature | Details |
|---|---|
| Definition | Amphipathic 27-carbon steroid alcohol with 4 fused rings + side chain. Free or esterified (CE). |
| Tissue source | Diet (animal foods, ~300–450 mg/d) + endogenous synthesis (most tissues, esp. liver). Intestinal absorption ~50%. |
| Reference range (adult) | 140–200 mg/dL (3.6–5.2 mmol/L) — desirable |
| Mechanism of elevation | Genetic (FH, defective apo B), increased dietary saturated/trans fat, secondary to hypothyroidism, nephrotic syndrome, biliary obstruction, diabetes. |
| Clinical significance | Predicts CHD risk; >240 mg/dL = high. ~2% ↓ CHD per 1% ↓ in cholesterol. |
| Rate-limiting enzyme | HMG-CoA reductase (target of statin drugs). |
| Routine assay method | Enzymatic: cholesteryl ester hydrolase → cholesterol oxidase → peroxidase + Trinder dye (read at ~500 nm). |
| Reference method | Originally Abell-Kendall; now isotope-dilution GC-MS (CDC). |
| Key interferences | Vitamin C and bilirubin (reduce H₂O₂ → falsely ↓); hemolysis. |
The Enzymatic Cholesterol Reaction (memorize)
1) Cholesteryl ester + H₂O → cholesterol + fatty acid (cholesteryl ester hydrolase)
2) Cholesterol + O₂ → cholest-4-en-3-one + H₂O₂ (cholesterol oxidase)
3) H₂O₂ + 4-aminoantipyrine + phenol → colored quinoneimine dye (peroxidase)
👉 The agent that oxidizes 4-aminoantipyrine = H₂O₂.
1) Cholesteryl ester + H₂O → cholesterol + fatty acid (cholesteryl ester hydrolase)
2) Cholesterol + O₂ → cholest-4-en-3-one + H₂O₂ (cholesterol oxidase)
3) H₂O₂ + 4-aminoantipyrine + phenol → colored quinoneimine dye (peroxidase)
👉 The agent that oxidizes 4-aminoantipyrine = H₂O₂.
2.3 Triglycerides
| Feature | Details |
|---|---|
| Definition | 3 fatty acids esterified to glycerol; main fat for energy storage. |
| Reference range | < 150 mg/dL (normal); 150–199 borderline; 200–499 high; ≥ 500 very high (pancreatitis risk). |
| Mechanism of elevation | Excess carbs/fat intake, alcohol, diabetes (↑ shunt to fatty acid synthesis), nephrotic syndrome, LPL or apo C-II deficiency, estrogen, β-blockers, thiazides. |
| Clinical significance | > 500 mg/dL → acute pancreatitis; moderate ↑ + low HDL = atherogenic dyslipidemia. |
| Routine method | Lipase → glycerol → glycerokinase → glycerophosphate → glycerophosphate oxidase → H₂O₂ → peroxidase color reaction. |
| Free glycerol interference | Endogenous glycerol ↑ apparent TG by 10–20 mg/dL (more in DM, liver disease, glycerol meds, TPN). Correct via blanking (single- or double-cuvette) or calibration adjustment. |
| Pseudo-hypertriglyceridemia | Glycerol kinase deficiency — TG appears massively ↑ but true TG is normal. |
Pre-analytical — Triglycerides
STRICT 12-hour fasting required (chylomicrons after meals). Avoid alcohol 24 h before. CV biological variation is ~25–30% (much higher than cholesterol). Hemolysis interferes with peroxide-based steps. Glycerol-containing IV fluids (TPN) and DKA falsely elevate TG via free-glycerol pathway.
STRICT 12-hour fasting required (chylomicrons after meals). Avoid alcohol 24 h before. CV biological variation is ~25–30% (much higher than cholesterol). Hemolysis interferes with peroxide-based steps. Glycerol-containing IV fluids (TPN) and DKA falsely elevate TG via free-glycerol pathway.
2.4 Apolipoproteins — Function Map
| Apolipoprotein | Major Lipoprotein | Function | Clinical Tie |
|---|---|---|---|
| Apo A-I | HDL (main) | Structural + LCAT activator + ABCA1 lipid acceptor | ↓ in Tangier disease; antiatherogenic marker. |
| Apo A-II | HDL | Structural | — |
| Apo B-100 | VLDL, IDL, LDL | Structural; LDL receptor ligand | Defective in familial defective apo B-100; one molecule per particle (apo B = particle count). |
| Apo B-48 | Chylomicrons only | Structural; chylomicron remnant receptor ligand | “48% of B-100” — produced by intestinal mRNA editing. |
| Apo C-I | Chylo, VLDL, HDL | Structural | — |
| Apo C-II | Chylo, VLDL, HDL | LPL cofactor (activates) | Deficiency → familial chylomicronemia (mimics LPL deficiency). |
| Apo C-III | Chylo, VLDL, HDL | LPL inhibitor | Excess → ↑ TG. |
| Apo E | VLDL, IDL, HDL, chylo remnants | Ligand for LDL receptor & chylo remnant receptor | E2/E2 → Type III dysbetalipoproteinemia. E4 → ↑ Alzheimer risk. |
| Apo(a) | Lp(a) | Structural; plasminogen-like (pro-thrombotic) | Independent CHD/stroke risk. |
2.5 Lipid & Lipoprotein Disorders
| Disorder | Defect / Mechanism | Lipid Pattern | Clinical Clue |
|---|---|---|---|
| Type I hyperlipoproteinemia (familial chylomicronemia) | LPL deficiency (chr 8) OR apo C-II deficiency (chr 19) | ↑↑↑ Chylomicrons, ↑↑ TG (often > 1,000 mg/dL); ↓ LDL/HDL | Childhood pancreatitis, eruptive xanthomas, lipemia retinalis, creamy supernatant after refrigeration. Not atherogenic. |
| Type IIa (Familial Hypercholesterolemia, FH) | LDL receptor defect (5 classes); autosomal codominant | ↑↑ LDL-C, ↑↑ TC; normal TG | Tendon xanthomas (Achilles), arcus cornealis, premature CHD. Heterozygote 1:500 (TC 300–600); homozygote 1:1,000,000 (TC 800–1,000). |
| Familial defective apo B-100 | Apo B-100 mutation (Arg→Gln 3500); ↓ binding to LDL-R | ↑ LDL-C | Mimics FH but milder; same management. |
| Type IIb (Familial combined hyperlipidemia, FCH) | Hepatic overproduction of apo B-100 | ↑ LDL-C and ↑ TG; ↑↑ apo B | Most common dyslipidemia in CHD patients; phenotype varies within family. |
| Type III (Dysbetalipoproteinemia / broad β disease) | Apo E2/E2 homozygote → ↓ remnant clearance | ↑ IDL/β-VLDL; TC ≈ TG (both 300–600) | Palmar xanthomas (pathognomonic), tuberoeruptive xanthomas, peripheral & coronary disease. Broad β band on electrophoresis. |
| Type IV (familial hypertriglyceridemia) | VLDL overproduction | ↑ VLDL, ↑ TG (200–1,000) | Often with obesity, DM, insulin resistance. |
| Type V | ↑ chylomicrons + ↑ VLDL | ↑↑ TG, ↑ TC | Adult onset; pancreatitis risk; often with DM. |
| Tangier disease | ABCA1 transporter defect (chr 9) | HDL-C 1–5 mg/dL (near absent); low TC | Orange tonsils (pathognomonic), hepatosplenomegaly, neuropathy. |
| LCAT deficiency | LCAT gene defect | ↓ HDL, abnormal HDL (mostly pre-β); free cholesterol ↑ | Corneal opacities, hemolytic anemia, glomerulosclerosis. |
| Hypobetalipoproteinemia | Apo B mutation | ↓ LDL, ↓ apo B | Often asymptomatic; not atherogenic. |
| Abetalipoproteinemia | MTP defect (no VLDL/chylo secretion) | Near-zero LDL/VLDL | Fat malabsorption, fat-soluble vitamin deficiencies, neuropathy. |
| Lp(a) elevation | Genetic; apo(a) attached to LDL via S–S bond | Lp(a) > 30 mg/dL | Independent CHD/stroke risk; only niacin and estrogen lower it. |
| Lipoprotein X (Lp-X) | Cholestasis or LCAT deficiency | Abnormal LDL-like particle (90% PL+free chol; no apo B) | Falsely ↑ LDL-C by Friedewald. |
Pre-analytical — HDL/LDL Methods
HDL precipitation methods fail when TG > 400 mg/dL — supernate becomes turbid/floats → falsely ↑ HDL. Friedewald LDL-C is invalid when TG > 400 mg/dL, in non-fasting samples (chylomicrons), or in Type III (β-VLDL has abnormal cholesterol-to-TG ratio). Use direct LDL-C or β-quantification ultracentrifugation instead. Homogeneous HDL/LDL assays may misread samples from liver/kidney disease patients (atypical lipoproteins).
HDL precipitation methods fail when TG > 400 mg/dL — supernate becomes turbid/floats → falsely ↑ HDL. Friedewald LDL-C is invalid when TG > 400 mg/dL, in non-fasting samples (chylomicrons), or in Type III (β-VLDL has abnormal cholesterol-to-TG ratio). Use direct LDL-C or β-quantification ultracentrifugation instead. Homogeneous HDL/LDL assays may misread samples from liver/kidney disease patients (atypical lipoproteins).
2.6 NCEP / ATP III Cut-Points (Memorize)
| Analyte | Optimal / Desirable | Borderline | High | Very High |
|---|---|---|---|---|
| Total Cholesterol | < 200 mg/dL | 200–239 | ≥ 240 | — |
| LDL-C | < 100 (optimal); < 130 (desirable) | 130–159 | 160–189 | ≥ 190 |
| HDL-C | ≥ 60 (protective) | 40–59 | < 40 = risk factor | — |
| Triglycerides | < 150 | 150–199 | 200–499 | ≥ 500 (pancreatitis risk) |
NCEP Analytic Performance Goals
TC: ±3% bias, 3% CV, ≤8.9% total error · LDL-C: ±4%, 4% CV, ≤11.8% · HDL-C: ±5%, 4% CV, ≤12.8% · TG: ±5%, 5% CV, ≤14.8%.
TC: ±3% bias, 3% CV, ≤8.9% total error · LDL-C: ±4%, 4% CV, ≤11.8% · HDL-C: ±5%, 4% CV, ≤12.8% · TG: ±5%, 5% CV, ≤14.8%.
SECTION 3 — Visual Memory Hooks
🧠 Mnemonic — Lipoprotein Density Order (largest → densest)
“Cool Vampires Like Hugs” = Chylomicron → VLDL → LDL → HDL. As you go down the list: size ↓, density ↑, protein ↑.
“Cool Vampires Like Hugs” = Chylomicron → VLDL → LDL → HDL. As you go down the list: size ↓, density ↑, protein ↑.
🧠 Mnemonic — Apolipoprotein Functions
“A-I Activates LCAT” · “C-II Catalyzes LPL” (cofactor) · “C-III Cancels LPL” (inhibitor) · “E for Entry” (E binds remnant + LDL receptor) · “B-100 = Big LDL” · “B-48 = Bowel chylomicron”.
“A-I Activates LCAT” · “C-II Catalyzes LPL” (cofactor) · “C-III Cancels LPL” (inhibitor) · “E for Entry” (E binds remnant + LDL receptor) · “B-100 = Big LDL” · “B-48 = Bowel chylomicron”.
🧠 Mnemonic — Friedewald Equation
“LDL = Total − HDL − TG/5” ⇒ remember: “If TG over 400, kick the door” (don’t use Friedewald). Also fails in non-fasting samples and Type III dysbetalipoproteinemia.
“LDL = Total − HDL − TG/5” ⇒ remember: “If TG over 400, kick the door” (don’t use Friedewald). Also fails in non-fasting samples and Type III dysbetalipoproteinemia.
🧠 Mnemonic — Xanthoma Locations
“Tendon = Type II (FH)” · “Palmar = Type III” · “Eruptive = Hyper-TG (Type I/V)”. Orange tonsils = Tangier.
“Tendon = Type II (FH)” · “Palmar = Type III” · “Eruptive = Hyper-TG (Type I/V)”. Orange tonsils = Tangier.
🧠 Mnemonic — Type I vs Type V Hyperchylomicronemia
Both have ↑↑ TG & pancreatitis. Type I = childhood, only chylo, LPL/C-II defect. Type V = adult, chylo + VLDL, often with DM.
Both have ↑↑ TG & pancreatitis. Type I = childhood, only chylo, LPL/C-II defect. Type V = adult, chylo + VLDL, often with DM.
Pattern Trigger Table
| If you see… | Think… | Because… |
|---|---|---|
| Creamy layer on top of refrigerated plasma; turbid below | Type V (chylo + VLDL) | Chylos float (creamy top); VLDL stays turbid in plasma. |
| Creamy layer only, clear plasma below | Type I (only chylomicrons) | Chylo is the lightest particle. |
| Tendon (Achilles) xanthomas + premature CHD + family Hx | Familial Hypercholesterolemia (Type IIa) | LDL receptor defect → ↑↑ LDL-C deposits in tendons. |
| Palmar xanthomas (yellow palm creases) + TC ≈ TG | Type III dysbetalipoproteinemia | Apo E2/E2 → β-VLDL accumulation. |
| Orange tonsils + near-zero HDL | Tangier disease | ABCA1 defect → no cholesterol efflux to HDL. |
| Eruptive xanthomas + TG > 1,000 + recurrent pancreatitis | Type I (LPL or apo C-II deficiency) | Cannot clear chylomicrons. |
| “Broad β” band on electrophoresis | Type III | β-VLDL fuses LDL/IDL bands. |
| HDL-C falsely high | Sample TG > 400 — incomplete precipitation | Lipoprotein-precipitate complex floats. |
| Friedewald LDL underestimated | Type III (cholesterol-rich VLDL) | VLDL-C > TG/5 in this disorder. |
| Cholesterol falsely low (enzymatic method) | ↑ Vit C or bilirubin | Reduce H₂O₂ before peroxidase color reaction. |
| TG falsely high in DM patient on TPN | Free glycerol interference | TPN/glycerol meds add to glycerol pool. |
| Lp-X present (cholestasis) | Falsely ↑ LDL-C | Lp-X cholesterol counted as LDL by Friedewald. |
Side-by-Side Comparisons
| Feature | Type I (LPL/C-II def.) | Type V |
|---|---|---|
| Onset | Childhood | Adulthood |
| Lipoproteins ↑ | Chylomicrons only | Chylomicrons + VLDL |
| Plasma after refrigeration | Creamy top, clear infranate | Creamy top + turbid infranate |
| Atherogenic? | No | Variable |
| Trigger | Genetic (rare) | Often DM, alcohol, estrogen |
| Feature | Familial Hypercholesterolemia (FH) | Familial Defective Apo B-100 |
|---|---|---|
| Defect | LDL receptor | Apo B-100 (ligand) |
| LDL elevation | Severe (heterozygote 300–600) | Mild–moderate |
| CHD aggressiveness | Aggressive | Less aggressive |
| Treatment | Statin ± LDL apheresis (homozygote) | Statin (similar to FH) |
Diagnostic Window — There Is No Time Marker, but…
Lipids are not time-based markers, but two key time windows matter for the lab:
| Marker / State | Onset | Peak | Duration | Best Use |
|---|---|---|---|---|
| Postprandial chylomicrons | 1–2 h after meal | 3–6 h | Cleared 8–12 h (need fasting) | Why 12-h fast is needed for TG/Friedewald LDL. |
| Acute illness ↓ HDL/TC | Within hours | 1–3 days | Returns to baseline weeks after recovery | Defer lipid panel ≥ 2–3 months after MI/surgery/infection. |
| Statin LDL-C drop | Within 1–2 wks | 4–6 wks | Sustained while on therapy | Recheck LDL 4–8 wks after starting/changing statin. |
SECTION 4 — Diagnostic Flowcharts
Flowchart 1 — Patient with Chest Pain & Suspected Dyslipidemia (Differential)
| Patient with chest pain, family history of premature CHD → Order fasting lipid panel (TC, HDL, TG, calculated LDL) |
| → TC 300+ mg/dL, LDL 200+, normal TG, tendon xanthomas → Familial Hypercholesterolemia (FH) |
| → TC ≈ TG (both 300–600), palmar xanthomas → Type III dysbetalipoproteinemia (confirm with apo E2/E2 isoform analysis or β-quantification ratio > 0.30 VLDL-C/TG) |
| → ↑ TC + ↑ TG, ↑ apo B, varied family pattern → Familial Combined Hyperlipidemia (Type IIb) |
| → TG > 1,000, eruptive xanthomas, pancreatitis → Type I (LPL or apo C-II deficiency) if child; Type V if adult + DM |
|
⚠️ Common mistake: Calculating LDL by Friedewald in Type III — VLDL-C is much higher than TG/5, so LDL-C is falsely ↑. Always use β-quantification ultracentrifugation for definitive Type III diagnosis.
|
Flowchart 2 — Lipoprotein Identification by Plasma Appearance (After Overnight Refrigeration)
| Refrigerate plasma overnight at 4°C → Inspect |
| → Clear plasma → Normal or only LDL ↑ (Type IIa / FH) |
| → Turbid (uniform) plasma, no creamy top → ↑ VLDL (Type IV) |
| → Creamy top + clear infranate → ↑ Chylomicrons only (Type I) |
| → Creamy top + turbid infranate → ↑ Chylo + ↑ VLDL (Type V) |
|
⚠️ Common mistake: Confusing Type IV (turbid only — VLDL stays suspended) with Type V (creamy top from floating chylomicrons + turbid infranate).
|
Flowchart 3 — Method Selection for LDL-C
| Lipid panel ordered → Check TG result first |
| → TG < 400 mg/dL + fasting + no Type III → Use Friedewald: LDL = TC − HDL − TG/5 |
| → TG ≥ 400 mg/dL → Use direct (homogeneous) LDL-C method or β-quantification (gold standard) |
| → Suspected Type III (broad β band, palmar xanthoma) → β-quantification + apo E genotyping |
| → Non-fasting sample → Friedewald invalid; use non-HDL-C = TC − HDL instead. |
|
⚠️ Common mistake: Reporting Friedewald LDL on a non-fasting specimen — chylomicron TG inflates the TG/5 estimate of VLDL-C.
|
Flowchart 4 — Hypertriglyceridemia: Primary vs Secondary?
| Patient with TG > 500 mg/dL → Rule out secondary causes first |
| → Diabetes (↑ glucose), hypothyroid (↑ TSH), nephrotic syndrome (proteinuria), alcohol, estrogen, β-blocker, thiazide → Treat underlying cause |
| → No secondary cause + family history + childhood onset + low LPL activity post-heparin → Type I (LPL deficiency) |
| → Same picture but LPL activity restored after adding apo C-II to assay → Apo C-II deficiency |
| → Adult onset, often with DM/obesity → Type IV or V |
|
⚠️ Common mistake: Missing pseudo-hypertriglyceridemia (glycerol kinase deficiency) — TG appears extreme but cholesterol panel and clinical picture are normal. Use a glycerol-blanked TG method to confirm.
|
Flowchart 5 — Selecting the Right HDL Method
| Lipid panel includes HDL-C → Assess sample TG level |
| → TG < 400 mg/dL → Direct (homogeneous) HDL or dextran sulfate-Mg²⁺ precipitation are reliable |
| → TG > 400 mg/dL with precipitation method → Pre-treat with ultracentrifugation to remove VLDL/chylo, then precipitate; or use direct assay validated for high TG |
| → Liver/kidney disease specimen → Direct homogeneous methods may misclassify; verify with reference method (CDC RM = ultracentrifugation + heparin-Mn²⁺ + Abell-Kendall) |
|
⚠️ Common mistake: Reporting HDL from a turbid supernate after precipitation — incomplete sedimentation of apo B particles falsely ↑ HDL-C.
|
SECTION 5 — 25 High-Yield Facts
| # | Fact |
|---|---|
| 1 | Friedewald: LDL-C = TC − HDL-C − (TG/5); invalid if TG > 400, non-fasting, or Type III. |
| 2 | Lipoprotein density order (low→high): Chylo < VLDL < IDL < LDL < HDL. |
| 3 | Chylomicrons carry dietary (exogenous) TG; contain apo B-48. |
| 4 | VLDL carries endogenous TG from liver; contains apo B-100. |
| 5 | LDL is the major cholesterol carrier to peripheral cells; only apolipoprotein is apo B-100. |
| 6 | HDL has the highest protein content and lowest TG; main protein apo A-I. |
| 7 | Apo C-II activates lipoprotein lipase (LPL); deficiency mimics Type I. |
| 8 | Apo E2/E2 homozygosity → Type III dysbetalipoproteinemia. |
| 9 | Familial Hypercholesterolemia = LDL receptor defect; tendon xanthomas, premature CHD. |
| 10 | HMG-CoA reductase = rate-limiting enzyme of cholesterol synthesis; statin target. |
| 11 | Cholesterol enzymatic assay: cholesteryl ester hydrolase → cholesterol oxidase → H₂O₂ + 4-AAP/phenol → quinoneimine dye (~500 nm). |
| 12 | Triglyceride enzymatic: lipase → glycerol → glycerokinase → glycerol-3-P → glycerophosphate oxidase → H₂O₂ → color. |
| 13 | Free glycerol falsely ↑ TG (esp. DM, TPN, glycerol meds); fix with blanking. |
| 14 | Type I (LPL deficiency): childhood, TG > 1,000, eruptive xanthomas, pancreatitis. Not atherogenic. |
| 15 | Tangier disease: ABCA1 defect → near-zero HDL, orange tonsils, ↑ CHD risk. |
| 16 | Lp(a) is LDL + apo(a); apo(a) homologous with plasminogen → pro-thrombotic; independent CHD risk. |
| 17 | Electrophoresis bands (anode → origin): α (HDL) → pre-β (VLDL) → β (LDL) → origin (chylomicrons). |
| 18 | NCEP HDL-C: < 40 mg/dL = positive risk factor; ≥ 60 mg/dL = negative (protective). |
| 19 | NCEP TG: < 150 normal · 150–199 borderline · 200–499 high · ≥ 500 very high (pancreatitis). |
| 20 | NCEP LDL goals: < 100 (CHD/equivalent), < 130 (≥ 2 risk factors), < 160 (0–1 risk factor). |
| 21 | Reference method for cholesterol: Abell-Kendall (historical) → now isotope-dilution GC-MS at CDC. |
| 22 | HDL reference method: ultracentrifugation + heparin-Mn²⁺ precipitation + Abell-Kendall. |
| 23 | LDL reference: β-quantification = ultracentrifugation (remove VLDL) + heparin-Mn²⁺ HDL precipitation. |
| 24 | EDTA plasma values are ~3% lower than serum (osmotic effect); multiply by 1.03 to compare. |
| 25 | Non–HDL-C = TC − HDL-C; better marker than LDL when TG is elevated; secondary ATP III target. |
🎯 Most Testable Pattern
A patient stem with tendon (Achilles) xanthomas + family history of early MI + TC > 300 + LDL > 200 + normal TG is virtually always Familial Hypercholesterolemia (Type IIa) due to LDL receptor defect. If the stem swaps “tendon” for “palmar,” switch your answer to Type III dysbetalipoproteinemia (apo E2/E2).
A patient stem with tendon (Achilles) xanthomas + family history of early MI + TC > 300 + LDL > 200 + normal TG is virtually always Familial Hypercholesterolemia (Type IIa) due to LDL receptor defect. If the stem swaps “tendon” for “palmar,” switch your answer to Type III dysbetalipoproteinemia (apo E2/E2).
Quick Interpretation Table
| Lab Finding | Clinical Interpretation |
|---|---|
| Fasting plasma with creamy top + clear infranate | Type I (chylomicrons only) — LPL or apo C-II deficiency. |
| Creamy top + turbid infranate | Type V (chylomicrons + VLDL). |
| TG > 1,000 mg/dL in a child + recurrent pancreatitis | LPL deficiency (Type I) — confirm by LPL activity post-heparin. |
| Apo E2/E2 + TC ≈ TG (both 300–600) + palmar xanthomas | Type III dysbetalipoproteinemia. |
| HDL-C < 5 mg/dL + orange tonsils | Tangier disease (ABCA1 defect). |
| Falsely high HDL on direct method | Sample TG > 400 mg/dL → incomplete precipitation/turbid supernate. |
| Friedewald LDL invalid | TG > 400, non-fasting, or Type III — switch to direct LDL or β-quantification. |
| Falsely low cholesterol on enzymatic method | Vitamin C, bilirubin, or hemolysis interfering with peroxidase color step. |
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SECTION 8 — Rapid Revision Sheet
📖 READ THIS 2 HOURS BEFORE YOUR EXAM
Friedewald
- LDL = TC − HDL − TG/5
- INVALID if TG > 400
- INVALID non-fasting
- INVALID Type III
- Use: direct LDL or β-quant
NCEP Cut-Points
- TC: <200 / 200–239 / ≥240
- LDL: <100 opt · <130 · 130–159 · 160–189 · ≥190
- HDL: <40 risk · ≥60 protective
- TG: <150 · 150–199 · 200–499 · ≥500
Lipoprotein Density Order
- Chylo (<0.93)
- VLDL (0.93–1.006)
- IDL (1.006–1.019)
- LDL (1.019–1.063)
- HDL (1.063–1.21)
- Smaller = denser = more protein
Apo → Lipoprotein
- B-48 → Chylomicron
- B-100 → VLDL/IDL/LDL
- A-I → HDL (LCAT activator)
- C-II → LPL activator
- C-III → LPL inhibitor
- E → Remnant + LDL receptor
- (a) → Lp(a), pro-thrombotic
Electrophoresis Bands
- Origin → Chylomicrons
- β → LDL
- Pre-β → VLDL
- α (fastest) → HDL
- Broad β = Type III
Type I vs Type V
- I: child, only chylo, LPL/C-II def
- V: adult, chylo + VLDL, often DM
- Both: pancreatitis, eruptive xanthoma
- Plasma I: creamy top, clear below
- Plasma V: creamy top + turbid
FH (Type IIa)
- LDL receptor defect
- Hetero 1:500, TC 300–600
- Homo 1:10⁶, TC 800–1000
- Tendon (Achilles) xanthoma
- Arcus cornealis
- Premature CHD
- Tx: statin ± LDL apheresis
Type III (Dysbetalipoproteinemia)
- Apo E2/E2
- TC ≈ TG (both ~300–600)
- Palmar xanthoma (path-gnomonic)
- Broad β band
- VLDL-C/TG ratio >0.30
- Friedewald overestimates LDL
Tangier Disease
- ABCA1 defect (chr 9)
- HDL near 0 (<5 mg/dL)
- Orange tonsils (path-gnomonic)
- Hepatosplenomegaly
- Peripheral neuropathy
Cholesterol Assay
- CE hydrolase → free chol
- Cholesterol oxidase → H₂O₂
- H₂O₂ + 4-AAP + phenol → quinoneimine (peroxidase)
- Read ~500 nm
- Interferents: Vit C, bilirubin, Hb
- Ref method: GC-MS (was Abell-Kendall)
Triglyceride Assay
- Lipase → glycerol
- GK → glycerol-3-P
- GPO → DHAP + H₂O₂
- Peroxidase color step
- ↑ Glycerol false ↑ TG (DM, TPN, gly meds)
- Fix: blanking method
HDL Methods
- Reference: UC + heparin-Mn²⁺ + Abell-Kendall
- Routine: dextran sulfate-Mg²⁺ ppt OR direct homogeneous
- Fail when TG > 400 (turbid supernate)
- Direct may misread liver/kidney pts
LDL Methods
- Reference: β-quantification
- Routine: Friedewald (cheap)
- Direct homogeneous (TG > 400)
- Calculate non-HDL-C if non-fasting
Confusing Pairs
- Apo A-I (HDL) vs Apo B-100 (LDL)
- Apo C-II activates vs C-III inhibits LPL
- Tendon (FH) vs Palmar (Type III) xanthoma
- Type I (child) vs Type V (adult)
- FH vs FDB (apo B defect)
- LCAT vs ACAT
Opposites — Risk
- ↑ LDL = atherogenic
- ↓ HDL (<40) = risk factor
- ↑ HDL (≥60) = protective
- ↑ TG (severe) = pancreatitis
- ↑ apo B = ↑ particle # = risk
- ↑ apo A-I = protective
Specimen Notes
- Fast 12 h for TG and LDL calc
- EDTA × 1.03 to compare to NCEP
- Sit 5 min before draw
- Avoid alcohol 24 h
- Avoid lipid panel within 2–3 mo of acute illness/MI
Drugs Quick
- Statins → ↓ LDL 20–40%
- Niacin → ↑ HDL 15–35%, ↓ Lp(a)
- Fibrates → ↓ TG 20–50%
- Bile-acid resins → ↓ LDL 15–30%
- Ezetimibe (NPC1L1) → ↓ LDL ~20%
- PCSK9 inhibitors → ↓ LDL ~50%
- Fish oil → ↓ TG up to 45%
NCEP Performance
- TC: 3% CV, ±3% bias, ≤8.9% TE
- HDL: 4% CV, ±5%, ≤12.8%
- LDL: 4% CV, ±4%, ≤11.8%
- TG: 5% CV, ±5%, ≤14.8%
SECTION 9 — Final Memory Map
📘 LIPIDS & LIPOPROTEINS
► Lipid Classes
• Fatty acids — saturated / mono/poly-unsaturated; trans = atherogenic; essential = linoleic + linolenic
• Triglycerides — 3 FA + glycerol; main fuel storage; neutral lipid (core)
• Phospholipids — amphipathic; surface of lipoproteins/membranes
• Cholesterol — 27C steroid; precursor of bile acids, steroids, vit D; HMG-CoA reductase rate-limiting
► Lipoproteins (low → high density)
• Chylomicron — apo B-48 · dietary TG · origin band · creamy top after fasting
• VLDL — apo B-100 · endogenous TG · pre-β band · turbid plasma
• IDL — transient · accumulates in Type III
• LDL — apo B-100 · cholesterol delivery · β band · most atherogenic
• HDL — apo A-I · reverse cholesterol transport · α band · antiatherogenic
• Lp(a) — LDL + apo(a) · plasminogen-like · pro-thrombotic · >30 mg/dL = risk
• Lp-X — abnormal · cholestasis or LCAT def · falsely ↑ Friedewald LDL
► Apolipoproteins
• A-I → HDL, LCAT activator
• B-48 → chylomicron only (intestinal)
• B-100 → VLDL/IDL/LDL, LDL-receptor ligand, 1 per particle
• C-II → LPL activator; C-III → LPL inhibitor
• E (E2/E2) → remnant clearance defect → Type III
• apo(a) → Lp(a), independent CHD risk
► Metabolic Pathways
• Exogenous: gut → chylomicron → LPL strips TG → remnant → liver via apo E + LDL-R
• Endogenous: liver → VLDL → LPL → IDL → LDL → LDL-R uptake
• Reverse cholesterol transport: HDL ← ABCA1 efflux ← apo A-I ← LCAT esterifies → CETP swaps CE/TG with LDL → liver
► Disorders (Fredrickson Classification)
• Type I: ↑ chylomicrons (LPL or apo C-II def), childhood, TG >1000, pancreatitis, NOT atherogenic
• Type IIa (FH): ↑ LDL only, LDL-R defect, tendon xanthoma, arcus, premature CHD
• Type IIb (FCH): ↑ LDL + ↑ VLDL/TG, ↑ apo B, common in CHD
• Type III (dysbetalipoproteinemia): apo E2/E2, TC ≈ TG, palmar xanthoma, broad β band
• Type IV: ↑ VLDL, adult onset, often with DM/obesity
• Type V: ↑ chylo + VLDL, adult, often with DM, pancreatitis
• Tangier: ABCA1 defect → near-zero HDL, orange tonsils, neuropathy
• LCAT def: corneal opacity, hemolytic anemia, renal disease
• Hypo-/Abetalipoproteinemia: low LDL/apo B; abetalip = MTP defect, fat malabsorption
► Lab Methods
• Cholesterol enzymatic: CE hydrolase → cholesterol oxidase → H₂O₂ + 4-AAP/phenol → quinoneimine (~500 nm). Reference: GC-MS (CDC).
• Triglyceride enzymatic: lipase → glycerol → GK → glycerol-3-P → GPO → H₂O₂. Free glycerol blank required in DM/TPN.
• HDL: precipitation (heparin-Mn²⁺ ref; dextran-Mg²⁺ routine) or direct homogeneous. Fails if TG >400.
• LDL: β-quantification (reference) · Friedewald (routine) · direct (TG >400 or non-fasting).
• Apo: turbidimetric / nephelometric immunoassays.
• Electrophoresis: agarose or polyacrylamide; identify abnormal bands (broad β = Type III).
► Standardization
• NCEP/ATP III sets cut-points and analytic goals
• CDC Cholesterol Reference Method Laboratory Network (CRMLN) certifies labs/manufacturers
• Reference materials must be commutable with native samples
► Pre-Analytical & Reporting
• 12-h fast for TG & Friedewald; serum or EDTA plasma (×1.03)
• Sit 5 min, avoid alcohol 24 h, defer panel 2–3 mo after acute illness/MI
• Average ≥ 2 measurements 1–8 wks apart for diagnosis
► Treatment Targets
• Statins (HMG-CoA reductase) — ↓ LDL 20–40%, primary therapy
• Niacin — ↑ HDL most effectively, ↓ Lp(a)
• Fibrates — ↓ TG 20–50%
• Ezetimibe (NPC1L1) — ↓ LDL ~20%
• Bile-acid resins — ↓ LDL 15–30%; contraindicated if TG >400
• PCSK9 monoclonal antibodies — ↓ LDL ~50%, esp. for FH