What is measured

Cholesterol

A blood test can measure cholesterol levels. This can be measured through either a 'pin prick' test or, more usually, by a blood test. Cholesterol levels remain constant over 24 hours, so it is possible to get a simple cholesterol result without you having to stop eating for a period before the test. However, a more detailed result that identifies tryglceride (a fat in the blood), overall levels of cholesterol,
high-density lipid protein (HDL – the form of cholesterol that protects against a build-up of fats on the artery walls) and low-density lipid protein (LDL – the form of cholesterol that increases the risk of fats building up on the artery walls) from a blood test carried out after you have not eaten for eight hours.

A normal cholesterol level is 3.9 to 6.6 mmol L-1. Recommended levels for a person with coronary heart disease are as follows.

• Total cholesterol – less than 4.8 mmol L-1
• LDL cholesterol – less than 3 mmol L-1
• HDL cholesterol – more than 1 mmol L-1
• Tryglycerides – less than 2 mmol L-1

When assessing your cholesterol levels, the doctor should take account of the number of risk factors for coronary heart disease you have, whether or not you are taking drugs to lower your cholesterol, and whether you have recently had surgery or an invasive procedure (one that involved your skin being cut or punctured).

Cardiac enzymes

If you have had severe prolonged chest pain, you may need a blood test to measure cardiac enzymes. When the heart muscle is damaged (for example, during a heart attack), enzymes leak into the blood from the heart cells. One of the enzymes that is measured is creatine kinase. That enzyme is a sign of muscle injury. It peaks during the first day of the injury and returns to normal after two days.

Lactic dehydogenase is another enzyme that is released by the cells of the heart in response to damage. It is released between 12 to 48 hours after the damage and reaches a peak after 48 to 72 hours. It then takes seven days for levels to return to normal. However, it is also found in other areas of the body and so can lead to a false positive result when used without other tests.

Blood tests play a vital role in helping a medical team diagnose acute coronary syndrome. There are now special blood tests to detect specific enzymes that are released when the heart muscle is damaged as a result of a heart attack or severe unstable angina. One such test is the troponin blood test. This particular test is very specific and highly sensitive. It helps indicate the level of damage caused by a heart attack or severe unstable angina.

The troponin test is usually asked for when a patient first comes into hospital and then after six hours and after 12 hours. Troponin levels are normally very low, so even a slight increase in troponin can be a sign of cardiac damage. Troponin levels remain raised for up to seven to 10 days after a heart attack.

Electrolytes

The electrolytes test measures the following in the blood.

• Sodium – a mineral that is responsible for maintaining the balance of fluid within the body. The normal limit is 133 to 144 mmol L-1.
• Potassium – a change in the level can affect the electrical activity of the heart. The normal limit is 3.3 to 5.3 mmol L-1.

Urea and creatine

These are waste products from the kidneys. A raised level suggests a change in kidney function, which may be due to a number of causes. These include dehydration and kidney failure. The normal level is 2.5 to 6.5 mmol L-1 for urea and 60 to 120 µmol L-1 for creatine.