What do the common blood tests actually measure?
Full blood count, kidney and liver function, HbA1c, cholesterol, thyroid and iron — what each panel is looking at, what a reference range is, and why borderline results are so common.
- Difficulty
- intermediate
- Time
- 9 min
- Read
- 5 min
- Safety
- caution
Short answer
A full blood count counts the cells. U&Es look at kidney function and salts. LFTs look at the liver. HbA1c reflects average blood sugar over roughly the previous three months. A lipid panel measures cholesterol fractions. TFTs check the thyroid. Ferritin measures iron stores. A reference range is where most healthy people fall — not a boundary between well and ill.
Blood test results arrive as a wall of abbreviations and numbers with a flag beside anything outside a range, and the flags are what people fix on. Understanding what each panel is actually looking at makes the conversation with a clinician a much better one, without pretending you can read your own results.
Safety
Step by step
- Understand what a reference range is.It is the range within which most healthy people's results fall — typically defined so that a proportion of perfectly healthy people sit outside it by definition. It is a population statistic, not a diagnosis line, which is why a slightly flagged result is so often unremarkable.
- Full blood count — the cells.Counts and characteristics of red cells, white cells and platelets. Haemoglobin and related measures speak to anaemia; the white cell count and its subtypes to infection and inflammation; platelets to clotting. It is the most commonly ordered test there is.
- Urea and electrolytes — kidneys and salts.Sodium, potassium, urea and creatinine, usually with an estimated filtration rate calculated from them. It reflects how the kidneys are working and the body's salt and fluid balance, and it is routinely monitored for people on certain medicines.
- Liver function tests — the liver, and more.A group of enzymes and proteins including ALT, ALP, bilirubin and albumin. The name is slightly misleading: they are markers that change with liver stress, bile flow and nutrition rather than a measure of how well the liver is doing its job.
- HbA1c — average blood sugar.It measures the proportion of haemoglobin that has sugar attached, which reflects average blood glucose over roughly the preceding two to three months. It is used both to diagnose diabetes and to monitor it, and it is not affected by whether you ate breakfast.
- Lipid panel — cholesterol fractions.Total cholesterol, LDL, HDL and triglycerides, often reported with a ratio. It is interpreted alongside blood pressure, age, smoking, family history and other factors in an overall cardiovascular risk estimate rather than on its own.
- Thyroid function — TSH and thyroid hormones.TSH is the pituitary's instruction to the thyroid, so it moves in the opposite direction to thyroid activity, which surprises people. It is usually the first test, with T4 and sometimes T3 added.
- Ferritin, B12 and folate — stores and vitamins.Ferritin reflects iron stores and rises with inflammation, which complicates it. B12 and folate are checked with anaemia, tiredness, and certain neurological symptoms.
- Ask the right question about your own results.Not "is this number normal" but "what does this mean in my situation, and what happens next". Results are interpreted against your history, your symptoms, your medicines and your previous results, none of which a range knows about.
Tips
- Ask whether a test needs fasting when it is booked. Lipids and glucose sometimes do; most do not, and turning up having eaten when you should not have wastes the appointment.
- Ask when and how you will get the results, and what happens if you hear nothing. "No news is good news" is a common assumption and a poor system — results do go astray.
- Where you have access to your own record through the NHS App or your GP's online service, you can often see results before the appointment. Reading them without the context is not always comfortable, and it is worth deciding in advance whether you want to.
Common mistakes
- Treating a flagged result as a diagnosis — Reference ranges are constructed so that a proportion of healthy people fall outside them. Slightly out-of-range results are extremely common and most of them mean nothing on their own.
- Comparing your result to a friend's, or to a different laboratory's range — Ranges differ between laboratories and between methods, which is why a result is always reported with the range that laboratory uses. Comparing across them is meaningless.
- Searching a single value online and drawing a conclusion — Every value has multiple possible explanations, most of them benign, and the search results are weighted towards the alarming ones. It is the fastest route to unnecessary distress.
- Assuming normal results mean nothing is wrong — Plenty of conditions do not show in routine bloods. Normal results are useful information and they do not close down a symptom that is still there.
If it doesn't work
A result is flagged and nobody has contacted you
Cause: It may have been reviewed and judged unremarkable, or it may have been missed — Fix: Ring the practice and ask what the clinician's comment on the result was. Practices record a comment against each result and reception can usually read it to you.
You have been asked to repeat a test
Cause: Very common — confirming a borderline result, or checking a change over time — Fix: It is usually reassurance-seeking rather than alarm. Ask what they are looking for and when, so you know whether the timing matters.
You do not understand the report
Cause: Reports are written for clinicians — Fix: Ask. A pharmacist can explain what a test measures, and the clinician who ordered it can explain what yours means. Both are better than a search engine.
Questions people ask
Can I ask for a copy of my results?
Yes. You have a right of access to your health records, and in England the NHS App gives many people direct access to test results as they arrive.
Why do I need repeat blood tests on my medication?
Several common medicines require monitoring of kidney function, liver enzymes, thyroid levels or blood counts. It is routine safety monitoring rather than a sign of a problem.
What does 'borderline' mean?
That the result sits close to the edge of the range, where the distinction between normal and abnormal is least meaningful. It usually means repeat it, look at the trend, or interpret it alongside everything else.
Do private blood test panels help?
They produce a lot of numbers without the clinical context that makes numbers useful, and abnormal results from broad screening panels in people without symptoms frequently lead to further tests that find nothing. Discuss with a GP what would actually be informative for you.