In simple terms
Cholesterol is a waxy fat the body needs but cannot dissolve in blood. It travels wrapped inside particles called lipoproteins.
LDL stands for low-density lipoprotein. LDL cholesterol is the portion those particles carry, and it is often called bad cholesterol because it is the main source of cholesterol buildup and blockage in the arteries.
How it works
A blood test called a lipoprotein panel, or lipid panel, measures it. The test normally follows 9 to 12 hours without food or drink other than water.
Units differ by country. The United States reports results in milligrams per deciliter, while the United Kingdom and most of Europe use millimoles per liter, so the same blood gives two very different looking numbers.
Thresholds differ too. MedlinePlus lists a healthy LDL level as below 100 mg/dL for adults aged 20 and over, and below 110 mg/dL for people aged 19 or younger. HEART UK lists below 3.0 mmol/L for healthy adults, and 2.0 mmol/L or below for people who already have cardiovascular disease.
Because national guidelines are built around overall risk, the number is read alongside age, blood pressure, smoking and family history rather than on its own.
Why it matters
When LDL is high, cholesterol collects on artery walls. The arteries narrow, blood flow falls, and the risk of coronary artery disease rises.
Some of what pushes LDL up can be changed, including saturated fat intake, weight, physical activity and smoking. Some cannot, including age, sex and family history.
Because guidelines are revised as trial evidence builds, the target attached to a given result can change over time and between countries.
Where you’ll see it
- Routine blood test results and workplace health checks.
- Food packaging and national heart health campaigns.
- Screening programs that invite adults for a cholesterol test.
- News coverage of cholesterol-lowering drug trials.
Example
A results letter reports LDL cholesterol of 4.1 mmol/L. That sits above the level HEART UK lists for healthy adults, and a clinician would read it together with blood pressure, smoking status and family history.
Often confused with
Total cholesterol is everything measured in the sample. LDL is one part of it. HDL, carried by high-density lipoproteins, is the part that moves cholesterol away from the arteries.
Key facts
- LDL is often called bad cholesterol because it is the main source of cholesterol buildup and blockage in the arteries.1
- A lipoprotein panel measures LDL and normally requires 9 to 12 hours without food beforehand.1
- MedlinePlus lists a healthy LDL level as below 100 mg/dL for adults aged 20 and over, and below 110 mg/dL for people aged 19 or younger.1
- HEART UK lists below 3.0 mmol/L for healthy adults, and 2.0 mmol/L or below for people who already have cardiovascular disease.2
- Diet, weight, physical activity and smoking can change LDL levels, while age, sex and family history cannot.1
This entry explains a medical term. It is not medical advice. If you are concerned about your health, speak with a qualified health professional.
Related concepts
In the news
Quick checkWhy is LDL called bad cholesterol?Show answer
Because LDL particles are the main source of the cholesterol that builds up on artery walls, narrowing them and restricting blood flow.
Sources
- MedlinePlus, US National Library of Medicine. Cholesterol Levels: What You Need to Know. Undated (accessed 18 September 2026)
- HEART UK. What are my cholesterol test results?. Undated (accessed 18 September 2026)
Editorially reviewed by Specialty Digest Editorial TeamLast reviewed September 21, 2026Researched and drafted with AI assistanceReport an issue