Lipid profile biochemistry

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lipid profile cardiovascular risk assessment

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Lipid Profile - Biochemistry

1. What is a Lipid Profile?

A lipid profile (also called a lipid panel or fasting lipid panel) is a blood test that measures the circulating concentrations of lipids and lipoproteins. The standard panel includes:

Total cholesterol (TC)
Triglycerides (TG)
HDL-cholesterol (HDL-C)
LDL-cholesterol (LDL-C) - usually calculated
Non-HDL cholesterol (TC - HDL-C)
VLDL-cholesterol (derived)

2. Biochemical Components

Cholesterol

Cholesterol is a highly hydrophobic steroid alcohol consisting of four fused hydrocarbon rings (A-D, the steroid nucleus) with an eight-carbon branched hydrocarbon chain at C-17, a hydroxyl group at C-3, and a double bond between C-5 and C-6. Most plasma cholesterol is esterified (fatty acid attached at C-3), making it even more hydrophobic. Because of this, cholesterol must be transported in lipoprotein particles or solubilized by phospholipids.

Functions of cholesterol:

Structural component of all cell membranes (modulates fluidity)
Precursor of bile acids (for fat digestion)
Precursor of steroid hormones (glucocorticoids, mineralocorticoids, sex hormones)
Precursor of vitamin D

Major sources of liver cholesterol and routes of efflux

The liver is central to cholesterol homeostasis - Lippincott Illustrated Reviews: Biochemistry, 8th ed.

Cholesterol Synthesis (De Novo)

Synthesized in virtually all tissues, but primarily the liver, intestine, adrenal cortex, and gonads. All carbons come from acetyl-CoA; reducing equivalents from NADPH.

Key steps:

2 Acetyl-CoA → Acetoacetyl-CoA (thiolase)
Acetoacetyl-CoA + Acetyl-CoA → HMG-CoA (HMG-CoA synthase, cytosolic isoform)
HMG-CoA → Mevalonate - rate-limiting step - catalyzed by HMG-CoA reductase (uses 2 NADPH; irreversible). This is the target of statins.
Mevalonate → isoprene units → squalene → lanosterol → cholesterol (via ~20 additional steps)

Regulation of HMG-CoA reductase:

Inhibited by high intracellular cholesterol (SREBP pathway - downregulates transcription)
Inhibited by statins (competitive inhibitors)
Activated by insulin; inhibited by glucagon
Degraded when sterol levels are high

Triglycerides (TG)

Triacylglycerols are esters of glycerol with three fatty acids. They are the primary form of energy storage in adipose tissue and a major energy source for the body. Elevated TG (>150 mg/dL) is associated with metabolic syndrome, pancreatitis risk (at very high levels >500 mg/dL), and cardiovascular risk when TG-rich remnant lipoproteins accumulate.

3. Lipoprotein Structure

Because lipids are insoluble in water, they are packaged into lipoproteins - spherical macromolecular complexes with:

Hydrophobic core: triglycerides (TG) + cholesteryl esters (CE)
Amphipathic shell (monolayer): phospholipids + unesterified (free) cholesterol + apolipoproteins

Structure of a typical lipoprotein particle

Tietz Textbook of Laboratory Medicine, 7th ed.

The shell is stabilized by hydrogen bonding and van der Waals forces. Cholesteryl ester transfer protein (CETP) exchanges TGs and CEs between lipoproteins. Phospholipid transfer protein (PLTP) transfers phospholipids between lipoproteins.

4. Lipoprotein Classes

Lipoproteins are classified by hydrated density (ultracentrifugation) or electrophoretic mobility. Density increases as lipid content decreases and protein content increases.

Lipoprotein	Density (g/mL)	Diameter	Major Apolipoproteins	TG (%)	Cholesterol (%)	Main Function
Chylomicrons	<0.95	75-1200 nm	ApoB-48, ApoC-II, ApoE	85-88	3-7	Dietary lipid transport (intestine→tissues)
VLDL	0.95-1.006	30-80 nm	ApoB-100, ApoC-II, ApoE	45-65	18-22	Endogenous TG transport (liver→tissues)
IDL	1.006-1.019	25-35 nm	ApoB-100, ApoE	25-40	30-40	VLDL→LDL intermediate
LDL	1.019-1.063	18-25 nm	ApoB-100	5-10	~50	Cholesterol delivery to tissues
HDL	1.063-1.21	5-12 nm	ApoA-I, ApoA-II	Trace	18-22	Reverse cholesterol transport
Lp(a)	>1.05	26 nm	ApoB-100, Apo(a)	~5	~45	Independent CVD risk factor

5. Apolipoprotein Functions

Apolipoprotein	Location	Function
ApoA-I	HDL	Activates LCAT; SR-B1 ligand; major structural protein of HDL
ApoB-48	Chylomicrons	Structural; secretion of chylomicrons from intestine
ApoB-100	VLDL, IDL, LDL, Lp(a)	LDL receptor ligand; cannot transfer between particles
ApoC-I	VLDL, HDL	Activates LCAT; inhibits CETP, LPL
ApoC-II	VLDL, chylomicrons	Activates lipoprotein lipase (LPL) - essential for TG hydrolysis
ApoC-III	VLDL, HDL	Inhibits LDL receptor and LRP clearance; stimulates LDL assembly; loss-of-function = lower CVD risk
ApoE	VLDL, IDL, HDL	Hepatic clearance ligand for LDL receptor (remnant uptake)

6. Lipoprotein Metabolism Pathways

Exogenous (Dietary) Pathway

Dietary fat absorbed in intestine → chylomicrons (with ApoB-48) secreted into lymphatics → thoracic duct → blood
LPL (activated by ApoC-II) on capillary endothelium hydrolyzes TG → fatty acids taken up by muscle/adipose
Chylomicron remnants (enriched in ApoE, cholesteryl esters) cleared by liver via LDL receptor-related protein (LRP)

Endogenous Pathway

Liver secretes VLDL (with ApoB-100, ApoC-II, ApoE) containing endogenous TG
LPL hydrolyzes VLDL TG → IDL (intermediate density lipoprotein)
IDL further processed by hepatic lipase (HL) → LDL (cholesterol-enriched, ApoB-100 only)
LDL taken up by tissues via LDL receptor (LDLR) → receptor-mediated endocytosis → intracellular cholesterol released
Excess LDL oxidized → taken up by macrophage scavenger receptors → foam cells → atherosclerotic plaques

Reverse Cholesterol Transport (HDL Pathway)

Nascent HDL (lipid-poor ApoA-I / pre-β HDL) secreted by liver and intestine
Picks up free cholesterol from peripheral cells via ABCA1 transporter
LCAT (Lecithin-Cholesterol Acyl Transferase, activated by ApoA-I) esterifies the cholesterol → mature spherical HDL (HDL3 → HDL2)
Cholesteryl esters transferred to VLDL/LDL via CETP, or directly taken up by liver via SR-B1 receptor
This pathway constitutes the main anti-atherogenic mechanism of HDL

7. Key Enzymes & Proteins in Lipid Metabolism

Enzyme/Protein	Function	Clinical Relevance
HMG-CoA reductase	Rate-limiting step in cholesterol synthesis	Target of statins
LPL (Lipoprotein lipase)	Hydrolyzes TG in VLDL/chylomicrons; requires ApoC-II	LPL deficiency → familial hyperchylomicronemia (Type I)
LCAT	Esterifies cholesterol on HDL	LCAT deficiency → fish-eye disease
CETP	Transfers CE from HDL to VLDL/LDL; exchanges TG	CETP inhibitors raised HDL but failed to reduce CVD in trials
LDL receptor (LDLR)	Hepatic/peripheral uptake of LDL (via ApoB-100)	Mutations → familial hypercholesterolemia (FH)
PCSK9	Degrades LDLR → fewer receptors → less LDL clearance	PCSK9 inhibitors (evolocumab, alirocumab) lower LDL markedly
Hepatic lipase (HL)	Converts IDL → LDL; remodels HDL
7-α-Hydroxylase	Rate-limiting step in bile acid synthesis	Upregulated by LXR (high cholesterol); downregulated by FXR (bile acids)
Niemann-Pick C1L1 (NPC1L1)	Intestinal cholesterol absorption	Target of ezetimibe

8. LDL Cholesterol Calculation

Since LDL-C is not measured directly in most labs, it is calculated using the Friedewald equation:

LDL-C = TC - HDL-C - (TG/5) (in mg/dL)

or in mmol/L: LDL-C = TC - HDL-C - (TG/2.2)

The TG/5 term estimates VLDL-C (assuming TG:VLDL-C ratio of 5:1)
Not valid when TG >400 mg/dL (ratio assumption breaks down), in Type III dyslipidemia (broad beta disease), or in chylomicronemia
In such cases, direct LDL-C measurement or the Martin-Hopkins equation (uses variable TG:VLDL ratio based on non-HDL-C and TG) is preferred

9. Reference Ranges and Clinical Interpretation

(Values from NCEP ATP III / AHA guidelines, consistent with Tietz 7th ed.)

Total Cholesterol (mg/dL)

Category	Value
Desirable	<200
Borderline high	200-239
High	≥240

LDL-Cholesterol (mg/dL)

Category	Value
Optimal	<100
Near optimal	100-129
Borderline high	130-159
High	160-189
Very high	≥190
Very high risk goal	<70 (or even <55 for very high-risk patients)

HDL-Cholesterol (mg/dL)

Category	Value
Low (risk factor)	<40 (men), <50 (women)
Normal	40-59
Protective	≥60

Triglycerides (mg/dL)

Category	Value
Normal	<150
Borderline high	150-199
High	200-499
Very high (pancreatitis risk)	≥500

Non-HDL Cholesterol

= TC - HDL-C. Represents all atherogenic particles (VLDL, IDL, LDL, Lp(a)). Goal for high-risk patients: <130 mg/dL (30 mg/dL higher than LDL-C goal)

10. Frederickson-WHO Classification of Dyslipidemias

Type	Lipoprotein Elevated	Lipid Abnormality	Genetic Cause
I	Chylomicrons	↑TG, ↑TC	LPL deficiency / ApoC-II deficiency
IIa	LDL	↑TC (TG normal)	Familial hypercholesterolemia (LDLR mutation)
IIb	LDL + VLDL	↑TC + ↑TG	Combined hyperlipidemia
III	IDL (β-VLDL)	↑TC + ↑TG	ApoE2 homozygosity
IV	VLDL	↑TG (TC normal/mildly ↑)	Familial hypertriglyceridemia
V	VLDL + Chylomicrons	↑↑TG	Combined

11. Pre-Analytical Factors

Fasting: 9-12 hours required for accurate TG and Friedewald LDL-C (non-fasting OK for TC, HDL-C, non-HDL-C)
Posture: Cholesterol can be 5-10% higher standing vs. supine (hemoconcentration)
Drugs: Oral contraceptives ↑TG; thiazides ↑TC+TG; beta-blockers ↓HDL, ↑TG
Conditions affecting results: Hypothyroidism (↑LDL), nephrotic syndrome (↑LDL), liver disease (abnormal LpX), pregnancy, acute illness (↓TC acutely due to acute phase response)
Specimen: Serum or EDTA plasma acceptable; avoid prolonged tourniquet; no hemolysis

12. Advanced Lipid Markers

Marker	Significance
ApoB-100	Counts atherogenic particle number; superior to LDL-C when TG high or small dense LDL present; one ApoB per LDL/VLDL/IDL particle
ApoA-I	Surrogate for HDL particle function; some guidelines recommend with ApoB
Lp(a)	Independent CVD risk factor; elevated in FH; genetic determinant; not responsive to statins (responds to PCSK9 inhibitors, inclisiran)
Small dense LDL	More atherogenic than large LDL; penetrates arterial wall more readily; associated with TG-rich states
Non-HDL-C	Better predictor than LDL-C in hypertriglyceridemia; recommended by 2022 guidelines alongside ApoB

Sources:

Tietz Textbook of Laboratory Medicine, 7th ed., Chapter 36 (Lipoproteins)
Lippincott Illustrated Reviews: Biochemistry, 8th ed., Chapter 18 (Cholesterol and Lipoprotein Metabolism)
Henry's Clinical Diagnosis and Management by Laboratory Methods, Chapter on Lipid Disorders

Recent evidence note: A 2024 network meta-analysis (PMID 39521985) confirms PCSK9 inhibitors and inclisiran as efficacious alternatives to statins+ezetimibe for LDL lowering, consistent with the PCSK9 pathway described above.

Standard lipid profile

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Standard Lipid Profile

A standard (or routine) lipid profile measures the major circulating lipid fractions used to assess cardiovascular risk and diagnose dyslipidemia. As defined by Tietz 7th ed.: "a standard lipid profile that will identify most hyper- and dyslipidemias should include plasma cholesterol and TG together with LDL-C, HDL-C, and non-HDL-C."

Components of the Standard Panel

Parameter	How Obtained	Unit
Total Cholesterol (TC)	Directly measured	mg/dL or mmol/L
Triglycerides (TG)	Directly measured	mg/dL or mmol/L
HDL-Cholesterol (HDL-C)	Directly measured	mg/dL or mmol/L
LDL-Cholesterol (LDL-C)	Calculated (usually) or directly measured	mg/dL or mmol/L
VLDL-Cholesterol (VLDL-C)	Estimated (TG ÷ 5)	mg/dL or mmol/L
Non-HDL Cholesterol	Calculated: TC - HDL-C	mg/dL or mmol/L

1. Total Cholesterol (TC)

Method: Enzymatic colorimetric assay - cholesterol esterase cleaves cholesterol esters, cholesterol oxidase oxidizes free cholesterol, and the H₂O₂ produced reacts with a chromogen (Trinder reaction) to give a measurable color.

Reference ranges (NCEP / AHA):

Category	mg/dL	mmol/L
Desirable	<200	<5.18
Borderline high	200-239	5.18-6.19
High	≥240	≥6.22

TC alone is a poor predictor because it includes both atherogenic (LDL) and anti-atherogenic (HDL) fractions
Always interpreted alongside HDL-C and LDL-C

2. Triglycerides (TG)

Method: Enzymatic - glycerol released by lipase hydrolysis is measured via glycerol kinase and glycerol-3-phosphate oxidase, generating H₂O₂ detected colorimetrically.

Reference ranges:

Category	mg/dL	mmol/L
Normal	<150	<1.70
Borderline high	150-199	1.70-2.25
High	200-499	2.26-5.64
Very high	≥500	≥5.65

Fasting sample required for accurate TG measurement (non-fasting TG can be 20-30% higher after a meal due to postprandial chylomicrons)
TG >500 mg/dL: risk of acute pancreatitis
TG >400 mg/dL: Friedewald LDL-C calculation is invalid

3. HDL-Cholesterol (HDL-C)

Method: Homogeneous (direct) enzymatic assays are standard. These use surfactants or antibodies to selectively react with non-HDL lipoproteins, leaving only HDL cholesterol accessible to the cholesterol assay. Older precipitation methods (polyethylene glycol, dextran sulfate-Mg²⁺, phosphotungstic acid-Mg²⁺) sediment non-HDL lipoproteins; cholesterol is then measured in the supernatant.

Reference ranges (NCEP ATP III):

Category	mg/dL	mmol/L
Low (independent risk factor)	<40 (men) / <50 (women)	<1.03 / <1.30
Normal	40-59	1.03-1.55
High (protective)	≥60	≥1.55

HDL-C is inversely correlated with cardiovascular risk
HDL-2 subclass is considered more cardioprotective than HDL-3
Very high HDL-C (>100 mg/dL) may paradoxically not be protective (dysfunctional HDL, CETP deficiency)

4. LDL-Cholesterol (LDL-C)

A. Friedewald Calculation (most common)

LDL-C = TC − HDL-C − (TG ÷ 5) (mg/dL)

LDL-C = TC − HDL-C − (TG ÷ 2.175) (mmol/L)

The TG÷5 term estimates VLDL-C (based on the assumption that VLDL TG:cholesterol ratio = 5:1).

Limitations of Friedewald:

Not valid when TG >400 mg/dL (ratio assumption breaks down, LDL-C underestimated)
Not valid if chylomicrons are present (non-fasting sample)
Not valid in Type III hyperlipidemia / β-VLDL (IDL is wrongly included in "VLDL")
Not valid in cholestasis (LpX present)
Accuracy also decreases at very low LDL-C (<70 mg/dL), common in treated patients

B. Martin-Hopkins Equation (improved)

Uses an adjustable TG:VLDL-C factor based on the individual's non-HDL-C and TG levels (lookup table). Accuracy vs. ultracentrifugation: 92% (vs. 85% for Friedewald). Particularly superior for LDL-C <70 mg/dL and in hypertriglyceridemia.

C. Direct (Homogeneous) LDL-C Assay

Specific surfactants or antibodies block non-LDL particles; cholesterol is measured enzymatically in the remaining LDL. Used when TG >400 mg/dL, in non-fasting samples, or post-treatment monitoring at low LDL-C.

Reference ranges (ACC/AHA 2018):

Category	mg/dL	mmol/L
Optimal	<100	<2.59
Near optimal	100-129	2.59-3.34
Borderline high	130-159	3.35-4.12
High	160-189	4.14-4.89
Very high	≥190	≥4.92
Very high-risk goal	<70	<1.81
Extreme-risk goal (ACS/FH)	<55	<1.42

LDL-C is the primary treatment target for cardiovascular risk reduction.

5. VLDL-Cholesterol (VLDL-C)

Estimated as: VLDL-C = TG ÷ 5 (mg/dL) or TG ÷ 2.175 (mmol/L)

Normal range: 2-30 mg/dL

Elevated VLDL-C indicates overproduction of TG-rich lipoproteins (endogenous hypertriglyceridemia), often seen in metabolic syndrome, diabetes, obesity, and alcohol excess.

6. Non-HDL Cholesterol

Non-HDL-C = TC − HDL-C

This captures cholesterol in all atherogenic particles: VLDL + IDL + LDL + Lp(a). It is particularly useful when TG is elevated (Friedewald becomes unreliable) and is now recommended alongside LDL-C by 2018 ACC/AHA and 2019 ESC/EAS guidelines.

Treatment goals (30 mg/dL higher than LDL-C goal):

Risk Category	Non-HDL-C Goal
High risk	<130 mg/dL
Very high risk	<100 mg/dL
Extreme risk	<85 mg/dL

7. Specimen Requirements & Pre-Analytical Considerations

Factor	Recommendation
Fasting	9-12 hours required for TG and Friedewald LDL-C; non-fasting acceptable for TC, HDL-C, non-HDL-C
Specimen	Serum (plain tube) or EDTA plasma; values differ ~3%
Posture	Sit for ≥5 min before collection; standing raises cholesterol 5-10% (hemoconcentration)
Stability	Stable at 4°C for 4 days; −70°C for longer storage
Avoid	Prolonged tourniquet use, hemolysis, lipemic samples
Plasma appearance	Creamy top layer = chylomicrons; turbid = elevated VLDL; clear despite elevated LDL or HDL

Conditions affecting results:

Hypothyroidism → ↑LDL-C (reduced LDL receptor expression)
Nephrotic syndrome → ↑LDL-C, ↑TG
Liver disease → ↓cholesterol synthesis; LpX may form in cholestasis
Pregnancy → physiological ↑TG, ↑TC, ↑LDL-C (especially 3rd trimester)
Acute illness → ↓TC acutely (acute phase response)
Drugs → OCP ↑TG; thiazides ↑TC+TG; beta-blockers ↓HDL-C; statins ↓LDL-C 30-50%

8. Expanded Lipid Profile

The 2018 ACC/AHA and 2019 ESC/EAS guidelines recommend adding the following in selected patients:

Test	Indication
ApoB-100	Better particle count; preferred when TG high or discordance with LDL-C; 1 molecule per atherogenic particle
Lp(a)	Measure once in all adults; major independent risk factor; not reduced by statins
ApoA-I	Functional HDL assessment
LDL particle number (LDL-P)	Nuclear MR spectroscopy; superior predictor when LDL-C is low
hsCRP	Inflammatory risk marker; used in borderline-risk patients (Reynolds/PCE score)

9. Screening Recommendations

Adults ≥20 years: Fasting lipid panel every 4-6 years (ACC/AHA); more frequent if risk factors present
Children: Universal screening at age 9-11 years (AAP); selective screening if family history of premature CVD or FH
Children with FH: Statin therapy typically initiated at 8-10 years if LDL-C remains high; homozygous FH - begin in the first year of life

10. Treatment Thresholds (Guidelines Summary)

Guideline	LDL-C Threshold for Treatment
US 2018 ACC/AHA	≥190 mg/dL (primary); most ASCVD (secondary); diabetics; PCE 10-yr risk ≥7.5%
Europe 2019 ESC/EAS	≥190 mg/dL primary; ASCVD with LDL-C ≥55 mg/dL (secondary)
Canada 2016 CCS	LDL-C ≥193 mg/dL; most ASCVD; diabetics

Sources:

Tietz Textbook of Laboratory Medicine, 7th ed., Chapter 36
Henry's Clinical Diagnosis and Management by Laboratory Methods, Chapter on Lipid Disorders
Quick Compendium of Clinical Pathology, 5th ed., Section 1.8

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