Common blood tests explained: CBC, HbA1c, lipids, thyroid

Complete blood count (CBC)

Also called a full blood count, a CBC is a group of tests. It measures your red blood cells, which carry oxygen, your white blood cells, which help fight infection, and your platelets, which help blood clot. It also reports haemoglobin, the protein in red cells that carries oxygen, and haematocrit, the share of your blood made up of red cells.

It is often part of a routine check-up. Doctors also use it to help find or follow conditions that affect blood cells, such as infections and anaemia.

HbA1c

HbA1c shows your average blood sugar over the past two to three months. It works by measuring how much of your haemoglobin has glucose attached. Doctors use it to screen for and diagnose type 2 diabetes and prediabetes, and to monitor diabetes. WHO says it can be done at any time of day and does not need fasting.

Lipid profile

A lipid profile or lipid panel looks at fats in your blood. It usually reports:

• LDL cholesterol, often called "bad" cholesterol, the main source of blockages in the arteries
• HDL cholesterol, often called "good" cholesterol, which helps remove LDL from the blood
• Total cholesterol, the combined amount in your blood
• Triglycerides, another type of fat in the blood

It helps you and your doctor assess your risk of heart disease. You may be asked to fast for 9 to 12 hours first, so ask before you book.

Thyroid and vitamin D tests

A TSH test measures thyroid-stimulating hormone, which is made by the pituitary gland and tells your thyroid how much hormone to make. Doctors use it to check how well the thyroid is working, and to look for an overactive or underactive thyroid. It can also be used to check treatment.

A TSH test cannot show what is causing a thyroid problem. If the result is abnormal, doctors often order T4 and T3 tests as well. Biotin (vitamin B7) can affect the result, so tell your doctor if you take it.

This test measures 25-hydroxyvitamin D, the form your liver makes from vitamin D. It is the most accurate way to see whether you have enough. MedlinePlus says routine testing is not recommended for everyone. It is usually ordered when a doctor suspects a bone or other health problem linked to very low vitamin D, or when someone has a higher risk of low levels.

Getting ready, and reading your results

A CBC and a vitamin D test usually need no preparation. You may need to fast if other tests are run on the same sample, so follow the instructions you are given. Tell the doctor or lab about all your medicines, vitamins and supplements, and do not stop any medicine unless your doctor tells you to.

We have left out number ranges here on purpose. An abnormal result does not automatically mean you have a medical condition. Things like diet, activity, medicines, periods and hydration can affect some results, and what is normal can depend on your age and health. Ask a doctor to explain what yours mean for you.

A blood test is not a way to deal with symptoms that worry you now. Call 998 for severe chest pain, severe shortness of breath, or sudden weakness or confusion.

You can search the lab test list on medicAI, where each test page explains what it checks. Staff from a licensed partner laboratory collect your sample at home, and you can see a doctor by video to talk your results through.