LDL Cholesterol Test
Aged 40 as part of a routine cardiovascular health check, or if you are already thought to be at risk of cardiovascular disease for another reason, or to monitor your response to treatments which lower LDL cholesterol (LDL-C).
A blood sample taken from a vein in the arm, or occasionally from a fingerprick
Laboratory tests for LDL-C typically require a 12 hour fast; only water is permitted. Follow any instructions you are given.
Cholesterol is carried in the blood by proteins called “lipoproteins”. There are a number of different types of lipoproteins, which are named after how dense they are. LDL stands for ‘low-density lipoprotein’, and any cholesterol carried by LDL is known as LDL cholesterol, or LDL-C. LDL is considered to be undesirable because it deposits excess cholesterol in the walls of blood vessels and contributes to atherosclerosis, also known as 'hardening of the arteries' or ‘furring up of the arteries’, and ultimately cardiovascular disease, such as heart disease and strokes. Hence LDL-C is often termed 'bad' cholesterol.
LDL-C is usually tested as part of a lipid profile. This includes the total cholesterol as well as a “breakdown” of how much of that cholesterol is carried on LDL (ie LDL-C), and how much is carried on ‘high density lipoprotein’ (ie HDL cholesterol). Interestingly, HDL cholesterol seems to have a protective effect on the heart, and is therefore often termed ‘good’ cholesterol. Triglycerides are also measured as part of the lipid profile.
In fact, LDL-C is usually not measured directly, but calculated using an equation which uses the other components of the lipid profile. Occasionally LDL-C can be measured directly.
How is the sample collected for testing?
The test for LDL-C uses a blood sample. Most often, the blood sample is collected by venepuncture (using a needle to collect blood from a vein in the arm). Occasionally a fingerprick test can be used, although this is not commonly available in GP practices or hospitals in the UK.
Is any test preparation needed to ensure the quality of the sample?
A test for LDL-C typically requires a 12-hour fast, with only water permitted during this time. Your doctor might recommend a slightly shorter or longer fast. Strictly speaking, if LDL-C is being measured directly, rather than calculated using an equation, fasting is not necessary – however, fasting will be required for other components of the lipid profile which are usually measured at the same time. Follow any instructions you are given.
How is it used?
1) To identify people at high risk of developing cardiovascular disease
People who eventually go on to develop cardiovascular disease often have had a high LDL-C for many years before they develop any symptoms from it. Therefore, measuring LDL-C can be useful to help predict your chances of developing cardiovascular disease, allowing you to do something about it before it causes you any problems.
In fact, in the UK, the first line lipid test to identify people with ‘high cardiovascular risk’ is not LDL-C, but another parameter derived from the lipid profile called the total cholesterol : HDL cholesterol ratio. The LDL-C would almost always be checked at the same time, and does provide additional information to the total cholesterol : HDL cholesterol ratio. For example, if it is very high, this can sometimes indicate specific conditions such as ‘familial hypercholesterolaemia’.
2) In people known to be at high risk of developing cardiovascular disease, to monitor their response to treatment
If you have been found to be at increased risk of heart disease, you will usually be advised to make some lifestyle improvements, such as improving your diet, and you may also be offered treatments which can lower your LDL-C, such as statins. In either case, the LDL-C would then usually be monitored to find out if the treatments or changes you have made to your diet are working.
When is it requested?
LDL-C, as part of a lipid profile, is commonly tested by your GP when you reach the age of 40, as part of a routine cardiovascular health check.
It may be checked at other times if you are thought to be at increased risk of cardiovascular disease for another reason.
Finally, it will likely be checked at regular intervals if you have been prescribed treatments to help lower your LDL-C, such as statins.
What does the test result mean?
Elevated concentrations of LDL-C increase your risk of developing cardiovascular disease. However, as mentioned above, in the UK it is usually a different parameter called the total cholesterol : HDL cholesterol ratio which is used to identify people at high risk. This is used in conjunction with other factors such as age, gender, smoking, blood pressure, being overweight, to estimate your chances of developing heart disease in the future. If this indicates you might be at high risk, the LDL-C is then used as a ‘target’ for you to aim for. Your doctor would then usually recommend dietary changes and possibly medical treatments to help reduce your LDL-C down to this target.
In the UK, the target LDL-C for people at high risk of cardiovascular disease is usually ‘no more than 2.0 mmol/L’. A fall of 30% from the pre-treatment level is also used as an alternative target. Bear in mind that the lower your LDL-C, the lower your chances of cardiovascular disease, so there are additional (albeit small) benefits to an even lower LDL-C than 2.0 mmol/L. Furthermore, it means that if your LDL-C was very high to start off with (when getting down to 2.0 mmol/L may not be possible), any reduction you get with dietary improvements or medications will be beneficial.
If you are not considered to be at high risk of cardiovascular disease, there is no official target for LDL-C. However, many doctors would consider no more than 3.0 mmol/L as desirable.
Is there anything else I should know?
There is condition which runs in families called “familial hypercholesterolaemia” in which LDL-C is very high – usually over 4.9 mmol/L in adults. Some people, though not all, who have an LDL-C this high, have familial hypercholesterolaemia. This is an inherited condition which is important to pick up, as if it isn’t treated there is a high risk of heart disease at a young age. Furthermore, as it is inherited, other family members might be affected. Anyone suspected of having familial hypercholesterolaemia should be referred to a specialist lipid clinic.
LDL-C should be measured when a person is healthy. LDL-C is temporarily low during acute illness, immediately following a heart attack, or during stress (like from surgery or an accident). You should wait at least six weeks after any illness to have LDL-C measured.
In women, cholesterol is high during pregnancy. Women should wait at least six weeks after the baby is born to have LDL-C measured.
What treatments are recommended for high LDL levels?
The first step in treating high LDL-C is adoption of a healthy diet. In particular, people are often advised to reduce saturated fat intake, and increase intake of foods which are known to lower LDL-C, including vegetables, oats, soya, nuts, and some spreads fortified with ‘plant stanols’ or ‘plant sterols’. For advice on how to lower cholesterol through diet and lifestyle, visit the HEART UK website.
If dietary changes alone do not adequately lower LDL-C, drugs may be prescribed. A class of drug called ‘statins’ is usually used first line. Statins have been around for many years and there is excellent evidence that they are effective in lowering LDL-C, and have a low rate of side effects.
How much will LDL cholesterol change as a result of lifestyle changes, like exercise or diet?
The total amount of lowering from diet alone will vary, but on average is about 10%.
What is the formula used to calculate LDL-C?
The formula most often used by laboratories is called Freidewald's formula. It uses the results from the components of the lipid profile that are measured directly, and is below. Units are in mmol/L and the formula can only be applied if total triglycerides are less than 4.5 mmol/L.
LDL cholesterol = Total cholesterol – HDL cholesterol – (Total triglycerides/2.18)
Is it recommended for children to have an LDL-C test?
Usually, it is not necessary for children to have an LDL-C test. However there are important exceptions. If either of the parents is diagnosed as having familial hypercholesterolaemia (see above) there is a chance they could pass it on to their children. Therefore children of people with familial hypercholesterolaemia often have their LDL-C checked to help decide if they have inherited the condition and need treatment. There is now good evidence that many treatments to lower LDL-C can be safely used before the age of 18 – however this should only be done in a specialist lipid clinic.