Lipoprotein:- Part 2 – Low-Density Lipoprotein (LDL), LDL-Cholesterol (Bad cholesterol)

August 2, 2026Chemical pathology Lab Tests

Table of Contents

Low-Density Lipoprotein (LDL)

What sample is needed for low-density lipoprotein (LDL)?

This is done in serum.
Plasma with EDTA can be used, but not with oxalate.
Get a sample after 12 hours of fasting.
The sample is stable for 1 to 3 days at 4 °C.

What are the indications for Low-density lipoprotein (LDL)?

To assess lipid profile.
To assess the risk of heart disease.
To monitor the therapy in case of abnormal findings.

How will you discuss the pathophysiology of lipoprotein?

Lipids are synthesized in the liver and intestine and then transported to many tissues for their metabolic function.
1. Lipids are insoluble, so they are transported as macromolecular complexes called lipoproteins.
2. A lipoprotein’s simple structure is explained in the following diagram.

Low-Density Lipoprotein (LDL): Lipoprotein structure

The outer covering lipoproteins are called Apoproteins, and these are classified into:
1. Apo-1.
2. Apo-II.
3. Apo- B.
4. Apo-D.
5. Apo-E.
Apolipoproteins are a hydrophilic component of lipoproteins.
Lipids like cholesterol and triglycerides are hydrophobic and need to be placed in water-soluble micellar structures (Apolipoproteins) to be transported in the plasma.

Low-Density Lipoprotein (LDL): Lipoprotein and Apoproteins

Lipoprotein electrophoresis

Fats are absorbed from the intestine as chylomicrons, which transport the dietary fats to the muscles and the adipose tissue.

Fat absorption

Low-density lipoprotein (LDL)

How will you define LDL (Low- density lipoprotein)?

This is the cholesterol carried in the circulation by LDL (Low-density lipoprotein).
LDL has a longer half-life of 3 to 4 days than its precursor, VLDL.
The liver and intestinal mucosal cells produce LDL.
LDL catabolism takes place in the liver and peripheral tissues.
LDL differs from VLDL because of its lower cholesterol content and lack of C or E apoproteins.

What are the contents of Low-density lipoprotein (LDL)?

Cholesterol = 45%
Triglycerides = 10%
Phospholipids = 20%
Protein = 25%

LDL (Low density lipoprotein)

Another source (Teitz Fundamentals of Clinical Chemistry):
1. Cholesterol 8%.
2. Cholesterol esters 42%.
3. Triglycerides 6%.
4. Phospholipids 22%.
5. Apoproteins 22%.

What are the functions of Low-Density Lipoprotein (LDL)?

This is called bad cholesterol.
Most of the cholesterol carried by the LDL is deposited in the lining of the blood vessels.
1. LDL molecules are taken into the cell through LDL receptors.
2. Once inside the cells, LDL is metabolized into its components.
3. One of the components, cholesterol, is partly used by the cells and partly goes out into the blood circulation.
4. LDL, through cholesterol, has an atherogenic role.

LDL (Low density lipoprotein) metabolism

LDL increases the risk of atherosclerosis in the coronary arteries (A high LDL level is atherogenic).
LDL also causes peripheral vascular disease.
LDL provides cholesterol to other body cells and leads to atherosclerosis of the vessels.

LDL (Low density Lipoprotein) role in the Coronary disease

LDL (Low-density lipoprotein) role in coronary disease

LDL levels can be lowered by diet, exercise, and statins.
LDL is mainly metabolized in the liver.

What is the normal low-density lipoprotein (LDL)?

Source 1

LDL-Cholesterol:

Age	Male mg/dL	Female mg/dL
Cord blood	20 to 56	21 to 58
5 to 9 years	63 to 129	68 to 140
10 to 14 years	64 to 133	68 to 136
15 to 19 years	62 to 130	59 to 137
20 to 24 years	66 to 147	57 to 159
25 to 29 years	70 to 165	71 to 164
30 to 34 years	78 to 185	70 to 156
35 to 39 years	81 to 189	75 to 172
40 to 44 years	87 to 186	74 to 174
45 to 49 years	97 to 202	79 to 186
50 to 54 years	89 to 197	88 to 201
55 to 59 years	88 to 203	89 to 210
60 to 64 years	83 to 210	100 to 224
65 to 69 years	98 to 210	92 to 221
>70 years	88 to 186	96 to 206

To convert into SI unit x 0.0259 = mmol/L

What is the risk of coronary artery disease based on Low-Density Lipoprotein (LDL)?

Risk for coronary heart disease	LDL level
Desirable LDL	<130 mg/dL (3.36 mmol/L)
Border-line risk of LDL	130 to 159 mg/dL (3.36 to 4.11 mmol/L)
High-risk LDL	≥160 mg/dL

Another source of Low-Density Lipoprotein (LDL) safe values:

Adults = <130 mg/dL
Children = <110 mg/dL

How will you calculate Low-density-lipoprotein (LDL)?

The following formula can calculate the LDL:

LDL = Total cholesterol – X – HDL
- X = (Triglycerides ÷ 5) – HDL
- LDL = Total cholesterol – ([triglycerides ÷ 5] – HDL)
Another formula is :
- LDL = Total cholesterol – HDL-chol – X
- X = Triglycerides ÷ 5
OR LDL = Total cholesterol — (HDL cholesterol + 1/5 x Triglycerides).

LDL-cholesterol calculation formula

NOTE: These formulas only help if the fasting triglycerides value is <400 mg/ dl.

What is the significance of Low-Density Lipoprotein (LDL?

An elevated level is a major risk factor for atherosclerosis.

What are the causes of increased Low-Density Lipoprotein (LDL)?

Familial hyperlipidemia type 2.
Familial hypercholesterolemia.
Glycogen storage diseases.
The secondary causes are :
1. Hypothyroidism.
2. Nephrotic syndrome.
3. Multiple myelomas.
4. Dysglobulinemia.
5. Liver disease with obstruction.
6. Diabetes mellitus.
7. Chronic renal failure.
8. A diet high in cholesterol and saturated fats.
9. Alcohol intake.
10. Hepatoma.

What are the causes of decreased Low-Density Lipoprotein (LDL)?

Hypolipoproteinemia.
Hyperthyroidism.
Hyperlipoproteinemia type 1.
Chronic anemias.
Hepatocellular diseases.
Chronic pulmonary disease.
Reye’s syndrome.
Acute stress is like a burn.
Inflammatory joint disease.

What is the relationship between NCEP classification of total cholesterol and LDL in adults for coronary disease?

Category	Total cholesterol	LDL-cholesterol
Desirable	<200 mg/dL (5.15 mmol/L)	<130 mg/dL (3.36 mmol/L)
Borderline (High-risk)	200 mg/dL to 239 mg/dL (5.15 to 6.18 mmol/L)	130 mg/dL to 160 mg/dL (3.36 to 4.11 mmol/L)
High Risk	>240 mg/dL (6.2 mmol/L)	>160 mg/dL (4.14 mmol/L)

NCEP = National Cholesterol Education Program.

How will you explain Adult Treatment Panel III (ATP III) for the classification of cholesterol?

Total cholesterol mg/dL

Desirable level = <200 mg/dL
Borderline level = 200 to 239 mg /dL
High level = >240 mg/dL.

LDL mg/dL

Optimal level = <100 mg/ dL.
Near-optimal level = 100 to 129 mg/dL.
Borderline level = 130 to 159 mg/dL.
High level = 160 to 189 mg/dL.
Very high level = >190 mg/dL.

HDL mg/dL

Low level = <40 mg / dL.
High level = >60 mg / dL.

What are ATP III (Adult Treatment Panel) recommendations for Low-Density Lipoprotein (LDL)?

Risk Group	Risk Factors	Target or desirable level of LDL
Presence of coronary heart disease (CHD) or CHD-equivalent	If there is coronary heart disease Diabetes Noncoronary atherosclerotic vascular disease	<100 mg/dL
2 or more major risk factors	Major risk factors are: Hypertension (140/90) Smoking F/O premature coronary heart disease HDL <40	<130
>2 major risk factors	Above are the major risk factors	<160 mg/dL

How will you summarize the characteristics of the lipoproteins?

Characteristics	Chylomicron	HDL	LDL	VLDL
Plasma appearance	Creamy layer, slightly turbid	Clear	Clear or yellow-orange tint	Turbid to opaque
Size (diameter nm)	>70.0	4 to 10	19.6 to 22.7	25 to 70
Electrophoretic mobility	Origin	α – region	β – region	Pre – β region
Molecular weight	0.4 to 30 x 109	3.6 x 109	2.75 x 109	5 to 10 x 109
Synthesized in (Tissue of origin)	Intestine	Intestine and liver	Intravascular	Liver and intestine
Composition by weight in %
Cholesterol esterified	5%	38%	49%	11 to 14%
Cholesterol unesterified	2%	10%	13%	5 to 8%
Triglycerides	84%	9%	11%	44 to 60%
Phospholipids	7%	22%	27%	20 to 23%
Proteins	2%	21%	23%	4 to 11%
Triglycerides	Markedly raised	Normal	Normal/ Raised	Moderately to markedly raised
Clinical significance of	Pancreatitis and acute abdomen	Decreased risk of CAD	Increased risk of CAD	Increased risk of CAD
Functions	Transport dietary lipids to tissue	Carry cholesterol from tissue to liver	Carries cholesterol to tissue	Transport endogenous TG from the liver to adipose tissue

What is the relation of LDL (mg/dL) according to NCEP (National Cholesterol Education Program)?

Coronary heart disease risk	LDL value	LDL value	LDL goal
No coronary heart disease risk HDL is >35 mg/dL	<160 mg/dL, treat with diet	>160 mg/dL, treat with drugs	Try to keep <160 mg/dL
<2 risk factors	>160 mg/dL, treat with diet	>190 mg/dL, treat with drugs	Try to keep <160 mg/dL
>2 risk factors	>130 mg/dL, treat with diet	>160 mg/dL, treat with drugs	Try to keep <130 mg/dL
Coronary heart disease Other atherosclerotic disease	>100 mg/dL, treat with diet	>130 mg/dL, treat with drugs	Try to keep <100 mg/dL

Questions and answers:

Question 1: What is the risk of LDL for coronary artery disease?

Question 2: What will happen to LDL in hyperthyroidism?