Lipid profile of blood
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Lipid Profile of Blood
The lipid profile (also called a lipid panel or fasting lipid panel) is a blood test that measures the major circulating lipid fractions to assess cardiovascular risk. Here is a thorough, textbook-grounded overview.
1. Why It Is Ordered
Lipid levels are obtained to evaluate cardiovascular risk - specifically the risk of coronary heart disease (CHD) and atherosclerosis. The ATP III of the National Cholesterol Education Program (NCEP, 2001) recommended lipid screening as a primary tool for cardiovascular disease risk reduction.
2. The Four Major Lipoprotein Classes
Because cholesterol, triglycerides, and phospholipids are hydrophobic and insoluble in blood, they must be packaged into water-soluble macromolecules called lipoproteins - a hydrophobic lipid core surrounded by a shell of polar phospholipids and apolipoproteins.
| Lipoprotein | Density | Main Lipid Carried | Key Apolipoprotein | Origin |
|---|---|---|---|---|
| Chylomicrons | Lowest | Dietary triglycerides | ApoB-48 | Intestine (dietary fat) |
| VLDL | Very Low | Endogenous TG | ApoB-100, ApoC-II, ApoE | Liver |
| IDL | Intermediate | TG + cholesterol | ApoB-100, ApoE | Catabolism of VLDL |
| LDL | Low | Cholesterol esters | ApoB-100 | Catabolism of IDL/VLDL |
| HDL | High | Cholesterol esters | ApoA-I, ApoA-II | Liver + intestine |
- VLDL metabolism produces IDL, which is further metabolised to LDL
- Chylomicron metabolism produces chylomicron remnants that are cleared by the liver
3. Standard Lipid Panel - What Is Measured
The ATP III standard lipid panel consists of direct measurement of:
- Total cholesterol (TC)
- HDL cholesterol (HDL-C)
- Triglycerides (TG)
And one calculated value:
- LDL cholesterol via the Friedewald formula:
LDL-C = Total Cholesterol - HDL-C - (Triglycerides / 5)
- VLDL-C is estimated as TG/5 (i.e., 20% of triglycerides)
- Non-HDL cholesterol = TC - HDL-C (includes VLDL + IDL + LDL; useful in hypertriglyceridemia)
The test requires a 9-hour fast (some guidelines accept non-fasting TC + HDL as a valid CHD risk screen, without needing TG).
4. Reference Values / Classification (NCEP ATP III)
| Component | Value (mg/dL) | Classification |
|---|---|---|
| Total Cholesterol | < 200 | Desirable |
| 200-239 | Borderline high | |
| ≥ 240 | High | |
| LDL Cholesterol | < 100 | Optimal |
| 100-129 | Near/above optimal | |
| 130-159 | Borderline high | |
| 160-189 | High | |
| ≥ 190 | Very high | |
| HDL Cholesterol | < 40 | Low (risk factor) |
| ≥ 60 | Protective (negative risk factor) | |
| Triglycerides | < 150 | Normal |
| 150-199 | Borderline high | |
| 200-499 | High | |
| > 500 | Very high (pancreatitis risk) |
Source: NCEP Expert Panel, JAMA 2001 - cited in Berek & Novak's Gynecology and Textbook of Family Medicine 9e
5. Clinical Significance of Each Fraction
Total Cholesterol
~60-70% is carried as LDL-C; ~20-30% as HDL-C. Seasonal variation exists (2.5% higher in winter than summer).
LDL Cholesterol - "Bad Cholesterol"
- Direct association with CHD risk
- ~60-70% of plasma cholesterol
- Carries ApoB-100, which is the ligand for the LDL receptor
- Target for statin therapy; the primary therapeutic target in cardiovascular risk reduction
- Familial hypercholesterolemia: LDL-C ≥ 190 mg/dL triggers medication eligibility across all major guidelines (ACC/AHA 2018, ESC/EAS 2019, NICE 2016, CCS 2016)
HDL Cholesterol - "Good Cholesterol"
- Mediates reverse cholesterol transport: carries cholesterol back to the liver
- Carries ApoA-I, which activates LCAT (lecithin-cholesterol acyltransferase)
- Strong, independent inverse relationship with CHD: every 1 mg/dL decrease in HDL increases CAD risk by 2-3%
- Low HDL (< 40 mg/dL) is an independent risk factor
Triglycerides
- Carried mainly in chylomicrons (exogenous) and VLDL (endogenous)
- Elevated TG > 500 mg/dL: risk of acute hypertriglyceridemic pancreatitis
- Fasting required because a meal raises TG and thereby underestimates calculated LDL-C
- Diurnal variation: lowest in the morning, highest in the afternoon
Non-HDL Cholesterol
- Captures all atherogenic particles (VLDL + IDL + LDL + Lp(a))
- More informative than LDL alone when TG is elevated
- Calculated as: TC - HDL-C
6. Expanded Lipid Panel (When Indicated)
The 2019 ESC/EAS and 2018 ACC/AHA guidelines recommend measurement of:
- Lp(a) - lipoprotein(a): once in every adult's lifetime; modest independent association with stroke and CHD risk
- ApoB: reflects total atherogenic particle burden; recommended by European and Canadian guidelines
- Direct LDL measurement (homogeneous assay): more accurate than Friedewald when TG > 400 mg/dL
The AHA does not recommend lipoprotein subfractions or particle size in asymptomatic adults for routine risk assessment, as they do not improve predictive capacity beyond the standard panel.
7. Friedewald Formula - Limitations
The formula is invalid in three situations:
- Chylomicrons present (non-fasting state, or lipoprotein lipase deficiency)
- Triglycerides > 400 mg/dL (underestimates LDL)
- Dysbetalipoproteinemia (Type III hyperlipidemia) - also underestimates LDL
In these cases, direct LDL measurement is required.
8. Sources of Variation
| Factor | Effect |
|---|---|
| Non-fasting sample | TG elevated → LDL underestimated |
| Recent MI/stroke/surgery | TC + LDL decrease for weeks to months |
| Acute illness/inflammation | TG increase, LDL decreases |
| Unstable diet | Wait 3 weeks for stable diet before testing |
| Time of day | Morning specimens preferred (TG diurnal variation) |
| Season | TC ~2.5% higher in winter |
| Obesity/weight loss | TG most affected (↓ 40% with weight loss); TC/LDL change < 10% |
| Alcohol (moderate) | HDL ↑, LDL ↓, TG ↑ |
| Position + tourniquet time | Can alter results |
9. Drugs That Affect Lipid Components
| Drug | TC | LDL | HDL | TG |
|---|---|---|---|---|
| Thiazide diuretics | ↑ | ↑ | -- | ↑ |
| Beta-blockers | -- | -- | ↓ | ↑ |
| Alpha-blockers | ↓ | ↓ | ↑ | ↓ |
| Unopposed estrogens | ↓ | ↓ | ↑ | ↑ |
| Progestogens | -- | ↑ | ↓ | ↓ |
| Isotretinoin | ↑ | ↑ | ↓ | ↑ |
| Protease inhibitors | ↑ | -- | -- | ↑ |
| Tamoxifen | ↓ | ↓ | -- | ↑ |
Source: Textbook of Family Medicine 9e, citing Mantel-Tecewisse et al., Drug Saf 2001
10. Secondary Causes of Dyslipidemia
- Hypothyroidism - raises TC, LDL, Lp(a), ApoB; lowers HDL (atherogenic profile)
- Diabetes mellitus - raised TG, lowered HDL, small dense LDL
- Nephrotic syndrome - raised TC and LDL
- Obstructive liver disease - altered lipid pattern
- Chronic kidney disease - elevated LDL/HDL ratio associated with CKD progression risk
- Obesity - raises TG, lowers HDL
- Excess alcohol - raises TG
11. Screening Recommendations
- Adults: NCEP ATP III recommends routine lipid screening for cardiovascular risk reduction
- Children: American Academy of Pediatrics recommends universal screening between ages 9-11
- Testing should be avoided during acute illness (TG rise, LDL falls artificially)
- After MI: cholesterol decreases within 24 hours and remains depressed up to 12 weeks; wait 2-3 months after major illness before measuring
Sources:
- Textbook of Family Medicine, 9e, pp. 234
- Berek & Novak's Gynecology, p. 1067-1068
- Fischer's Mastery of Surgery, 8e, p. 3103
- Tietz Textbook of Laboratory Medicine, 7e, p. 1185
- Basic Medical Biochemistry: A Clinical Approach, 6e, pp. 1197
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