Understanding Common Blood Tests

Full blood count (FBC)

The full blood count is one of the most frequently requested tests. It measures the different types of cell in your blood. Red blood cells carry oxygen around the body, and a low level (often measured as haemoglobin) suggests anaemia, which can cause tiredness and breathlessness. White blood cells fight infection, and their number can rise with infection or inflammation, or fall in certain conditions. Platelets help the blood to clot, and abnormal levels can affect bleeding and clotting. An FBC can point to a wide range of issues, from a simple iron deficiency to signs that need further tests. Because so many things can shift these numbers, doctors always read an FBC alongside your symptoms and other results rather than in isolation.

Kidney tests (U&Es) and liver tests (LFTs)

Urea and electrolytes, usually shortened to U&Es, check how well your kidneys are working and the balance of salts such as sodium and potassium in your blood. They include markers like creatinine and an estimate of kidney function (eGFR). Abnormal results can indicate dehydration, kidney problems, or the effects of some medicines, and these tests are often used to monitor long-term conditions. Liver function tests, or LFTs, measure substances that show how the liver is working, including enzymes that can rise when the liver is irritated or damaged, and proteins the liver makes. Raised LFTs can have many causes, from a minor viral illness or alcohol to gallstones or liver disease. As always, the results guide further assessment rather than giving an instant diagnosis.

Thyroid tests

Thyroid function tests check the gland in your neck that controls your metabolism — the rate at which your body uses energy. The main test measures a hormone called TSH, which the brain uses to tell the thyroid how hard to work, often alongside a thyroid hormone called T4. A high TSH usually suggests an underactive thyroid (hypothyroidism), which can cause tiredness, weight gain, feeling cold and low mood. A low TSH often suggests an overactive thyroid (hyperthyroidism), which can cause weight loss, a fast heartbeat, anxiety and feeling hot. Thyroid problems are common, especially in women, and are usually very treatable once diagnosed. These tests are also used to check that thyroid treatment is set at the right level over time.

HbA1c and CRP

HbA1c is a test used to diagnose and monitor diabetes. It reflects your average blood sugar level over the past two to three months, rather than at a single moment, by measuring how much sugar has attached to the haemoglobin in your red blood cells. A raised HbA1c can indicate diabetes or a higher risk of developing it, and for people who already have diabetes it helps show how well blood sugar is being controlled. CRP, or C-reactive protein, is a marker of inflammation in the body. It rises with infections, injuries and inflammatory conditions, and can help doctors judge whether an infection or flare-up is present and how it is responding to treatment. Neither test names a specific cause on its own, but both are valuable pieces of the picture.

Making sense of your results

A few principles help when reading blood test results. Each result is compared with a reference range — the range seen in most healthy people — but a result just outside the range is common and often not important. Ranges can also vary slightly between laboratories, and normal values differ with age, sex and pregnancy. What matters most is the overall pattern, how results compare with your previous tests, and how they fit with your symptoms. That is why doctors interpret blood tests as a whole rather than reacting to a single number. If you have questions about your results, ask the clinician or practice that ordered them; in England you can often view results through the NHS App, but it is always best to discuss anything worrying with a health professional.