How to Read Your Blood Test Results: A Complete UK Guide

You've had a blood test. The results have arrived — a page of abbreviations, decimal points, and ranges that look more like a spreadsheet than a health report. Your GP says everything is "fine," but you want to understand what the numbers actually mean. You're not alone.

Millions of blood tests are performed in the UK every year, and most patients receive their results with little or no explanation. This guide breaks down every common marker you're likely to see — in plain English, with UK reference ranges — so you can read your results with confidence.

How Blood Tests Are Reported

Every blood test result follows the same basic format: the marker name, your value, and a reference range. The reference range represents the values seen in 95% of healthy adults. If your result falls within that range, it's typically considered normal. If it falls outside, it's flagged — usually with an "H" (high) or "L" (low).

Results are reported in units specific to each marker. In the UK, we use SI units — millimoles per litre (mmol/L), nanomoles per litre (nmol/L), grams per litre (g/L), and so on. These differ from American units, so be careful if you're comparing your results to US-based information online.

Your results may arrive via the NHS App, a letter, or a phone call from your surgery. Private providers typically send results through a secure online portal with doctor commentary attached.

Understanding Reference Ranges

Reference ranges are not one-size-fits-all. They vary by age, sex, laboratory, and even time of day (cortisol, for example, is naturally highest in the morning). A result just outside the range isn't automatically a problem, and a result just inside isn't automatically reassuring.

Think of reference ranges as guardrails, not finish lines. What matters is context: your symptoms, your medical history, trends over time, and how markers relate to each other. A single out-of-range result rarely tells the whole story.

Full Blood Count (FBC)

The full blood count is the most commonly ordered blood test in the UK. It measures the cells circulating in your blood.

• Haemoglobin (Hb) — the oxygen-carrying protein in red blood cells. Low levels indicate anaemia. Normal: 130–170 g/L (men), 120–150 g/L (women).
• White blood cells (WBC) — your immune army. Elevated levels can indicate infection or inflammation. Normal: 4.0–11.0 × 10⁹/L.
• Platelets — responsible for blood clotting. Very low levels increase bleeding risk; very high levels may indicate inflammation or bone marrow issues. Normal: 150–400 × 10⁹/L.
• MCV (mean corpuscular volume) — the average size of your red blood cells. High MCV suggests B12 or folate deficiency; low MCV suggests iron deficiency. Normal: 80–100 fL.
• Ferritin — your iron stores. Often tested alongside FBC. Low ferritin is the earliest indicator of iron deficiency, even before haemoglobin drops. Normal: 30–400 µg/L (men), 30–150 µg/L (women). Optimal is generally above 50 µg/L.

Liver Function Tests (LFTs)

Your liver processes toxins, produces proteins, and manages bile. Liver function tests measure enzymes and proteins that indicate how well it's coping.

• ALT (alanine aminotransferase) — the most liver-specific enzyme. Elevated ALT strongly suggests liver cell damage. Normal: 7–56 IU/L.
• AST (aspartate aminotransferase) — found in the liver, heart, and muscles. Less specific than ALT. Normal: 10–40 IU/L.
• GGT (gamma-glutamyl transferase) — sensitive to alcohol use and bile duct problems. Normal: 8–61 IU/L (men), 5–36 IU/L (women).
• Albumin — a protein made by the liver. Low levels may indicate chronic liver disease or poor nutrition. Normal: 35–50 g/L.
• Bilirubin — a breakdown product of red blood cells. Mildly elevated bilirubin is often harmless (Gilbert's syndrome affects ~5% of the population), but significantly raised levels can indicate liver or bile duct problems. Normal: 3–17 µmol/L.

Kidney Function Tests

Your kidneys filter waste, balance fluids, and regulate blood pressure. Two key markers reflect their performance.

• Creatinine — a waste product from muscle metabolism, filtered by the kidneys. High creatinine suggests reduced kidney function. Normal: 59–104 µmol/L (men), 45–84 µmol/L (women).
• eGFR (estimated glomerular filtration rate) — calculated from creatinine, age, and sex. It estimates how efficiently your kidneys filter blood. Above 90 mL/min is normal. Below 60 mL/min sustained over three months indicates chronic kidney disease.
• Urea — another waste product. Elevated levels can reflect dehydration, high protein intake, or kidney impairment. Normal: 2.5–7.8 mmol/L.

Lipids and Cholesterol

Lipid panels measure the fats circulating in your blood — a cornerstone of cardiovascular risk assessment.

• Total cholesterol — the sum of all cholesterol types. Ideally below 5.0 mmol/L, though context matters.
• LDL cholesterol — the "bad" cholesterol that deposits in artery walls. Ideally below 3.0 mmol/L; below 2.0 mmol/L for high-risk patients.
• HDL cholesterol — the "good" cholesterol that removes LDL from arteries. Higher is better. Ideal: above 1.0 mmol/L (men), above 1.2 mmol/L (women).
• Triglycerides — fats linked to diet, alcohol, and metabolic health. Ideally below 1.7 mmol/L. Fasting samples are more accurate.
• Total cholesterol:HDL ratio — a useful summary measure. Below 4.0 is ideal.

If your provider tests ApoB (apolipoprotein B), this is increasingly recognised as a superior marker to LDL for predicting cardiovascular risk. Ideal: below 0.9 g/L; below 0.7 g/L for higher-risk individuals.

Blood Sugar (Glucose and HbA1c)

These markers screen for diabetes and pre-diabetes.

• Fasting glucose — your blood sugar after an overnight fast. Normal: 3.9–5.5 mmol/L. Pre-diabetic: 5.6–6.9 mmol/L. Diabetic: 7.0 mmol/L or above.
• HbA1c (glycated haemoglobin) — reflects your average blood sugar over 2–3 months. It doesn't require fasting. Normal: below 42 mmol/mol. Pre-diabetic: 42–47 mmol/mol. Diabetic: 48 mmol/mol or above.

HbA1c is now the preferred diagnostic test for type 2 diabetes in the UK because it captures long-term trends rather than a single snapshot.

Thyroid Function

Thyroid tests are among the most frequently requested panels, particularly for women and anyone experiencing fatigue, weight changes, or mood disturbance.

• TSH (thyroid-stimulating hormone) — the master regulator. High TSH suggests an underactive thyroid (hypothyroidism); low TSH suggests an overactive thyroid (hyperthyroidism). Normal: 0.27–4.2 mIU/L.
• Free T4 (thyroxine) — the main hormone your thyroid produces. Low FT4 with high TSH confirms hypothyroidism. Normal: 12–22 pmol/L.
• Free T3 (triiodothyronine) — the active thyroid hormone. Not always tested by the NHS but important for a complete picture. Normal: 3.1–6.8 pmol/L.

A "normal" TSH doesn't always mean your thyroid is optimised. Many patients with TSH between 2.5 and 4.2 report fatigue and other symptoms — this is where a doctor review adds clinical value.

Iron Studies

Beyond ferritin, a full iron panel gives a complete picture.

• Serum iron — the amount of iron circulating in your blood. Normal: 10–30 µmol/L.
• TIBC (total iron-binding capacity) — measures how much transferrin (an iron-carrying protein) is available. High TIBC often accompanies iron deficiency. Normal: 45–80 µmol/L.
• Transferrin saturation — the percentage of transferrin that is carrying iron. Low saturation indicates deficiency; very high saturation may suggest haemochromatosis (iron overload). Normal: 20–50%.

Vitamins

Vitamin deficiencies are remarkably common in the UK and often missed.

• Vitamin D (25-hydroxyvitamin D) — essential for bones, immunity, and mood. Deficient: below 25 nmol/L. Insufficient: 25–50 nmol/L. Adequate: 50–75 nmol/L. Optimal: 75–150 nmol/L.
• Vitamin B12 — critical for nerve function and red blood cell production. Deficient: below 150 pmol/L. Borderline: 150–250 pmol/L. Normal: above 250 pmol/L.
• Folate (vitamin B9) — works alongside B12 in cell production. Normal: above 7 nmol/L. Low folate with low B12 causes megaloblastic anaemia.

Hormones

Hormone panels vary by age and sex, but some key markers include:

• Testosterone — relevant for both men and women, though ranges differ dramatically. Men: 8.6–29 nmol/L. Women: 0.3–1.7 nmol/L. Symptoms of deficiency can occur even within "normal" ranges.
• Oestradiol (E2) — the primary oestrogen. Varies significantly with the menstrual cycle. Interpreting oestradiol requires knowing where you are in your cycle.
• SHBG (sex hormone-binding globulin) — binds to testosterone and oestradiol, reducing their availability. High SHBG effectively lowers your free hormone levels even when total levels look normal.

What "Normal" Really Means

Here's the part most people miss: "normal" is a statistical concept, not a clinical one. The reference range covers 95% of the healthy population. That means 5% of perfectly healthy people will fall outside it, and some people within the range may still have a clinical problem.

"Normal" also doesn't mean "optimal." A vitamin D level of 26 nmol/L is technically above the deficiency threshold, but most clinicians would recommend supplementation. An HbA1c of 43 mmol/mol is in the pre-diabetic range — technically not diabetic, but far from ideal.

Trends matter more than snapshots. A creatinine level of 100 µmol/L is unremarkable on its own, but if it was 70 six months ago, that rising trend deserves attention.

When to Worry

Most out-of-range results are mild, explainable, and not urgent. But certain patterns warrant prompt medical attention:

• Very low haemoglobin (below 100 g/L) — significant anaemia that may need investigation
• eGFR below 30 — advanced kidney impairment
• ALT or AST above 3x the upper limit — possible liver injury requiring urgent review
• Potassium above 6.0 mmol/L or below 3.0 mmol/L — can affect heart rhythm and is potentially dangerous
• TSH above 10 mIU/L — likely requires thyroid replacement therapy
• Fasting glucose above 11 mmol/L — probable diabetes requiring immediate management

If any of these appear on your results, contact your doctor promptly — don't wait for a routine appointment.

Getting a Second Opinion

If your GP says everything is "normal" but you still feel unwell, you have options. Request a copy of your results (you're legally entitled to them), and look at the actual numbers rather than accepting a blanket "all fine." Compare your results to the ranges above, look for trends, and consider whether "normal" is truly optimal for you.

Private blood testing gives you access to a wider panel of markers — including ApoB, full thyroid profiles, and advanced hormone panels — that the NHS may not routinely offer. More importantly, private results typically come with a detailed doctor review that explains what the numbers mean in the context of your health, not just population averages.

Understanding your blood test results isn't about replacing your doctor. It's about becoming an informed participant in your own healthcare. The numbers are yours — you deserve to understand them.

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