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Apolipoprotein B (ApoB)
(Primary Atherogenic Apolipoprotein - LDL Particle Number Marker)
Synonyms
- ApoB
- Apolipoprotein B-100
- ApoB100
- LDL apoprotein
- ApoB-containing lipoprotein marker
- Atherogenic lipoprotein index
Units of Measurement
mmol/L, µmol/L, g/L, mg/dL, mg/100mL, mg%, mg/mL
Description
Apolipoprotein B (ApoB) is the primary structural protein of all atherogenic lipoproteins:
- LDL
- VLDL
- IDL
- Lipoprotein(a)
- Chylomicron remnants
Each atherogenic particle contains exactly 1 ApoB molecule →
ApoB directly reflects the number of atherogenic particles, unlike LDL-C, which reflects the cholesterol content of those particles.
ApoB is one of the strongest predictors of cardiovascular disease (CVD) and is now recommended as a primary risk marker in major lipid guidelines.
Physiological Role
1. Lipoprotein Assembly
ApoB-100 (liver) and ApoB-48 (intestine) stabilize lipoproteins.
2. Atherogenesis
ApoB particles penetrate arterial walls → promote plaque formation.
3. LDL Receptor Binding
ApoB-100 binds LDL receptors to mediate uptake.
Clinical Significance
High ApoB (Most Important)
Indicates high atherogenic particle number, even if LDL-C is normal.
Conditions:
- Atherosclerosis / CAD
- Familial hypercholesterolemia
- Metabolic syndrome
- Type 2 diabetes
- Mixed dyslipidemia (high TG + high ApoB)
- Hypothyroidism
- Nephrotic syndrome
High ApoB reflects small dense LDL particles, which are highly atherogenic.
Low ApoB
Seen in:
- Malnutrition
- Hyperthyroidism
- Severe liver disease
- Abetalipoproteinemia (rare)
- MTP deficiency
Reference Intervals
Adult Reference Ranges
| Sex | ApoB (g/L) | mg/dL | Interpretation |
| Men | 0.60 – 1.20 g/L | 60 – 120 mg/dL | Normal |
| Women | 0.55 – 1.15 g/L | 55 – 115 mg/dL | Normal |
Cardiovascular Risk Thresholds
- Optimal ApoB: < 0.80 g/L (80 mg/dL)
- High risk: > 1.00 g/L (100 mg/dL)
- Very high risk: > 1.20 g/L (120 mg/dL)
- Extreme (FH): > 1.40 g/L (140 mg/dL)
Therapeutic Targets (AHA/ACC + ESC/EAS)
- Primary prevention: < 0.90 g/L
- High risk: < 0.80 g/L
- Very high risk: < 0.70 g/L
- Diabetes/insulin resistance: < 0.80 g/L
Unit Meanings
| Unit | Meaning |
| mmol/L | millimole per liter |
| µmol/L | micromole per liter |
| g/L | grams per liter |
| mg/dL | milligrams per deciliter |
| mg/100mL | mg% |
| mg% | mg per 100 mL |
| mg/mL | milligrams per milliliter |
ApoB vs LDL-C — Why ApoB Is Superior
| Parameter | ApoB | LDL-C |
| Measures | Particle number | Cholesterol content |
| Predicts ASCVD better | ✔ | — |
| Unaffected by TG/insulin resistance | ✔ | ✘ |
| Better in metabolic syndrome | ✔ | ✘ |
| Direct measurement | ✔ | Calculated |
| Useful when LDL-C appears normal | ✔ | ✘ |
If ApoB is high, ASCVD risk is high - even with normal LDL-C.
Diagnostic Uses
1. ASCVD Risk Assessment
Best predictor when:
- TG high
- LDL-C normal (discordance)
- Diabetes / metabolic syndrome
2. Monitoring Lipid-Lowering Therapy
Tracks response to:
- Statins
- PCSK9 inhibitors
- Ezetimibe
- Fibrates (if TG high)
3. ApoB/ApoA-1 Ratio
ApoB/ApoA-1 Ratio=Best global atherogenic index\text{ApoB/ApoA-1 Ratio} = \text{Best global atherogenic index}ApoB/ApoA-1 Ratio=Best global atherogenic index
- < 0.6: Low risk
- 0.6–0.8: Moderate
- > 0.8: High risk
4. Genetic Dyslipidemia
- Familial hypercholesterolemia
- Familial combined hyperlipidemia
5. Non-HDL-C Discordance Cases
ApoB resolves diagnostic uncertainty.
Analytical Notes
- Measured via immunonephelometry or immunoturbidimetry
- Fasting sample recommended in hypertriglyceridemia
- Inflammatory conditions may reduce ApoB slightly
- Stable across day-to-day variability
Clinical Pearls
- ApoB is a better predictor of MI than LDL cholesterol.
- Always check ApoB in diabetes, metabolic syndrome, high TG.
- ApoB remains elevated when LDL-C looks normal due to small-dense LDL.
- ApoB is the single most accurate blood marker of atherogenic burden.
- ApoB-guided therapy prevents undertreatment of high-risk patients.
Interesting Fact
Each atherogenic particle carries one ApoB protein, so ApoB is essentially a lipoprotein particle count, something LDL-C cannot provide.
References
- Tietz Clinical Chemistry & Molecular Diagnostics, 8th Edition - Lipids & Apolipoproteins.
- IFCC - Apolipoprotein Standardization Working Group.
- AHA/ACC 2018-2022 Cholesterol Guidelines.
- ESC/EAS 2019 Dyslipidemia Guidelines.
- Mayo Clinic Laboratories - ApoB Test.
- ARUP Consult - Lipid Disorders & Apo Testing.
- NIH / MedlinePlus - ApoB Overview.
- Clinical Lipidology & CVD Prevention Articles.