Cholesterol Blood Test Explained: LDL, HDL, Triglycerides and More

A cholesterol blood test — also called a lipid panel, lipid profile, or fasting lipid test — measures the levels of different types of fat in the blood. These fats, known collectively as lipids, play important roles in the body but can contribute to cardiovascular disease when present in unhealthy amounts. The test is used to assess cardiovascular risk, guide treatment decisions, and monitor the response to lifestyle changes or medication.

What Is Cholesterol?

Cholesterol is a waxy, fat-like substance found in every cell in the body. It is essential for making cell membranes, producing hormones (including oestrogen and testosterone), synthesising vitamin D, and producing bile acids needed for digestion. The liver produces most of the cholesterol in the body; the rest comes from animal-based foods in the diet.

Cholesterol cannot dissolve in blood and must be transported by proteins called lipoproteins. Different types of lipoprotein carry cholesterol in different directions and have different effects on cardiovascular health.

Total Cholesterol

Total cholesterol is the combined measure of all cholesterol in the blood, including LDL, HDL, VLDL (very low-density lipoprotein), and other fractions. On its own, total cholesterol gives a broad overview but is limited as a risk indicator because it includes both “good” (HDL) and “bad” (LDL) cholesterol. Typical desirable levels: below 5.0 mmol/L in the UK; below 200 mg/dL in the US.

LDL Cholesterol (Low-Density Lipoprotein)

LDL is often called “bad” cholesterol because elevated levels are strongly associated with the build-up of fatty plaques in artery walls (atherosclerosis), which increases the risk of heart attack and stroke. LDL particles carry cholesterol from the liver to cells throughout the body. When there is more LDL than the body needs, the excess can deposit in the walls of blood vessels.

In the UK, a desirable LDL is generally below 3.0 mmol/L for the general population, and below 1.8–2.0 mmol/L for people at high cardiovascular risk or with established cardiovascular disease. In the US, guidelines typically target LDL below 100 mg/dL (below 70 mg/dL in high-risk patients). LDL is usually calculated from other measurements rather than directly measured (using the Friedewald equation), though direct LDL measurement is also available.

HDL Cholesterol (High-Density Lipoprotein)

HDL is often called “good” cholesterol because it carries cholesterol away from arteries back to the liver, where it is processed and removed from the body. Higher HDL levels are generally associated with lower cardiovascular risk.

A low HDL (below 1.0 mmol/L for men, below 1.2 mmol/L for women in the UK, or below 40 mg/dL for men and 50 mg/dL for women in the US) is considered a cardiovascular risk factor. HDL can be raised through regular aerobic exercise, stopping smoking, and moderate alcohol reduction. Some medications also affect HDL levels. Very high HDL (above 2.5–3.0 mmol/L) does not appear to provide additional cardiovascular protection and may in some contexts be associated with increased risk — though this relationship is complex.

Triglycerides

Triglycerides are the main form of fat stored in the body and are derived from dietary fats and from excess calories converted by the liver. They are transported in the blood by VLDL particles. Elevated triglycerides (hypertriglyceridaemia) are an independent risk factor for cardiovascular disease and are also associated with pancreatitis at very high levels.

Common causes of raised triglycerides include excess alcohol consumption, high intake of refined carbohydrates and sugars, obesity, type 2 diabetes, hypothyroidism, kidney disease, and certain medications. Fasting before the test (usually 9–12 hours) is traditionally required for accurate triglyceride measurement, as eating raises triglycerides temporarily, though non-fasting lipid panels are increasingly used in clinical practice. Typical desirable levels: below 1.7 mmol/L (UK) or below 150 mg/dL (US).

Non-HDL Cholesterol and the Total:HDL Ratio

Non-HDL cholesterol is calculated as total cholesterol minus HDL cholesterol. It includes LDL and other atherogenic (plaque-forming) lipoproteins such as VLDL and IDL (intermediate-density lipoprotein), making it a broader risk indicator than LDL alone. In the UK, non-HDL cholesterol below 4.0 mmol/L is generally targeted.

The total cholesterol:HDL ratio (TC:HDL) divides total cholesterol by HDL cholesterol. A lower ratio indicates a more favourable lipid profile. In UK guidelines, a TC:HDL ratio below 4 is generally considered desirable. This ratio is often used in cardiovascular risk calculators alongside other factors such as age, sex, blood pressure, smoking status, and diabetes.

What Affects Cholesterol Levels?

Cholesterol levels are influenced by a combination of genetic, dietary, and lifestyle factors. Genetics plays a significant role — familial hypercholesterolaemia (FH) is a common inherited condition (affecting approximately 1 in 250 people) that causes very high LDL levels from birth and significantly increases cardiovascular risk if untreated. Diet contributes, particularly saturated fat and trans fats, which raise LDL. Exercise raises HDL and can lower triglycerides. Obesity, smoking, alcohol excess, and physical inactivity all worsen the lipid profile. Medical conditions including hypothyroidism, diabetes, kidney disease, and liver disease can also alter lipid levels.

Interpreting Results: Context Matters

Individual cholesterol numbers should always be interpreted alongside broader cardiovascular risk assessment. A raised LDL in a young, non-smoking person with no family history of heart disease and normal blood pressure carries different implications than the same LDL in an older person with diabetes, high blood pressure, and a family history of early heart disease.

Clinicians use validated risk calculators (such as QRISK3 in the UK or Pooled Cohort Equations in the US) to estimate ten-year cardiovascular risk, taking into account multiple factors beyond lipids alone. Treatment decisions — whether lifestyle changes alone are sufficient, or whether medication such as a statin is indicated — are based on this overall risk assessment rather than on any single lipid value.

Cholesterol Targets During Treatment

For people already taking cholesterol-lowering medication (most commonly statins), repeat lipid panels are used to monitor the response to treatment and ensure targets are being met. In high-risk patients (e.g., those with established cardiovascular disease), LDL targets are more aggressive — typically below 1.8 mmol/L or a 50% reduction from baseline in UK guidelines (NICE NG238). For people on statins, the total cholesterol and non-HDL cholesterol are also monitored regularly.

References

1. National Library of Medicine. Cholesterol Levels: What You Need to Know. MedlinePlus. https://medlineplus.gov/cholesterol.html
2. NHS. Cholesterol. https://www.nhs.uk/conditions/high-cholesterol/
3. NICE Guideline NG238. Cardiovascular disease: risk assessment and reduction, including lipid modification. National Institute for Health and Care Excellence. 2023.
4. American Heart Association. Understanding Your Cholesterol Levels. https://www.heart.org/en/health-topics/cholesterol/about-cholesterol
5. Grundy SM, Stone NJ, Bailey AL, et al. 2018 AHA/ACC guideline on the management of blood cholesterol. Journal of the American College of Cardiology. 2019;73(24):e285–e350.
6. Defesche JC, Gidding SS, Harada-Shiba M, et al. Familial hypercholesterolaemia. Nature Reviews Disease Primers. 2017;3:17093.