A blood test report is, in theory, full of useful information. In practice, it lands in your inbox as a table of numbers next to a narrow column of reference ranges, with no explanation of why a result matters, what caused it, or what to do next.
Whether you are a clinician reviewing a patient's results for the first time, or a patient trying to understand what your GP has ordered, reading blood work is a skill. It takes time to build, but it is completely learnable.
- Why reference ranges are not the whole story
- The full blood count (FBC): what it covers
- Liver function tests (LFTs): what the markers mean
- Thyroid function tests (TFTs): understanding TSH, T4, and T3
- Urea and electrolytes (U&Es): kidney health in numbers
- HbA1c: the diabetes marker explained
- Lipid profiles: cholesterol and cardiovascular risk
- Iron studies and bone profiles
- How to build this knowledge quickly
Why reference ranges are not the whole story
Every laboratory prints a reference range alongside each result. That range represents the values found in 95% of healthy adults in their testing population. Which means that, by design, 1 in 20 healthy people will have a result outside the range on any given test.
A result outside the range is a prompt to investigate further, not a diagnosis. A result inside the range does not rule out disease. Context matters: the patient's age, sex, medication history, clinical picture, and trend over time all shape how you interpret a number.
The best readers of blood work use the reference range as a starting point, not a final answer.
The full blood count (FBC): what it covers
The full blood count is the most commonly ordered blood test in UK clinical practice. It measures the cells in the blood across three main lines: red cells, white cells, and platelets.
Haemoglobin and red cell indices
Haemoglobin (Hb) is the protein in red blood cells that carries oxygen. A low Hb defines anaemia. The normal range differs by sex: roughly 130 to 170 g/L for adult males and 120 to 160 g/L for adult females.
When haemoglobin is low, the mean corpuscular volume (MCV) tells you the size of the red cells, which narrows the cause significantly. Low MCV points to iron deficiency or thalassaemia. High MCV points to vitamin B12 or folate deficiency.
Red blood cell count (RBC) is the actual number of red cells. It often moves in the same direction as haemoglobin, but the two can diverge in thalassaemia trait, where RBC can be normal or high while haemoglobin is low.
White cell count and differential
The white blood cell count (WBC) covers all circulating immune cells. A raised WBC most commonly reflects infection, inflammation, or steroid treatment. A very high WBC should raise the question of haematological malignancy.
The differential breaks WBC into subtypes. Neutrophils rise with bacterial infection. Lymphocytes rise with viral infection. A high neutrophil-to-lymphocyte ratio is associated with poor outcomes in several conditions and is increasingly used as a marker of systemic inflammation.
Platelets
Platelets are the clotting cells. A low platelet count, called thrombocytopaenia, increases bleeding risk. Causes include immune thrombocytopaenic purpura (ITP), bone marrow suppression, liver disease, and sepsis. A raised count, thrombocytosis, can be reactive to infection or iron deficiency, or can reflect a primary bone marrow disorder.
Liver function tests (LFTs): what the markers mean
Despite the name, a standard liver function test panel includes markers of liver cell damage, bile flow, and synthetic function. Not all of them are pure measures of liver function.
Markers of liver cell damage
ALT (alanine aminotransferase) is the most specific marker of hepatocyte injury. It is released when liver cells are damaged or destroyed. A mild rise, up to three times the upper limit of normal, is common with fatty liver disease, alcohol use, and medications including statins and paracetamol in overdose. A sharply elevated ALT, ten times or more above normal, points to acute hepatitis, ischaemic hepatitis, or significant drug-induced liver injury.
AST (aspartate aminotransferase) is less specific than ALT. It is also found in muscle, cardiac tissue, and red blood cells. An AST:ALT ratio greater than 2:1 is the classic pattern in alcoholic hepatitis.
Markers of bile flow
ALP (alkaline phosphatase) rises with biliary obstruction and infiltrative liver disease. It is also raised in bone disease and, importantly, in children and adolescents due to normal bone growth. A raised ALP in isolation should prompt measurement of GGT to confirm hepatic origin.
GGT (gamma-glutamyl transferase) is a sensitive marker of alcohol intake and biliary disease. It is the first marker to rise with regular alcohol consumption, often before any structural liver damage occurs. A raised GGT alongside a raised ALP confirms biliary disease. A raised GGT with normal ALP is the typical pattern of early alcohol-related liver change.
Bilirubin is a breakdown product of haemoglobin. Raised bilirubin causes jaundice when it exceeds roughly 35 µmol/L. Pre-hepatic causes include haemolysis. Hepatic causes include hepatitis and cirrhosis. Post-hepatic causes include bile duct obstruction from gallstones or malignancy.
Markers of synthetic function
Albumin is produced by the liver. A low albumin in the context of chronic liver disease reflects reduced synthetic capacity. It is also a marker of malnutrition and significant systemic illness.
Total protein is the sum of albumin and globulins. Elevated total protein with low albumin suggests a raised globulin fraction, which can point to chronic infection, autoimmune liver disease, or haematological conditions.
Thyroid function tests (TFTs): understanding TSH, T4, and T3
Thyroid function is assessed using a cascade of hormones. The pituitary gland produces TSH (thyroid stimulating hormone) in response to circulating thyroid hormone levels. When thyroid hormone is low, TSH rises to stimulate production. When thyroid hormone is high, TSH is suppressed.
This makes TSH the most sensitive first-line marker of thyroid dysfunction. A normal TSH, in the range of 0.4 to 4.5 mU/L in most UK laboratories, virtually excludes significant thyroid disease in most clinical contexts.
If TSH is abnormal, free T4 (FT4) confirms the severity and direction of thyroid dysfunction. A raised TSH with low FT4 confirms overt hypothyroidism. A raised TSH with normal FT4 indicates subclinical hypothyroidism, which may or may not require treatment depending on symptoms, antibody status, and patient age.
Free T3 (FT3) is most useful in diagnosing T3 toxicosis, a form of hyperthyroidism where T4 is normal but T3 is elevated, and in monitoring patients on T3 replacement therapy. It is not a routine first-line test.
Urea and electrolytes (U&Es): kidney health in numbers
The urea and electrolytes panel is the core assessment of kidney function and fluid and electrolyte balance.
The kidney markers
eGFR (estimated glomerular filtration rate) is the key measure of kidney filtering capacity. It is calculated from creatinine, age, sex, and, in some equations, ethnicity. A normal eGFR is above 60 mL/min/1.73m². Two readings below 60 taken at least 90 days apart define chronic kidney disease (CKD).
Creatinine is a waste product of muscle metabolism filtered by the kidneys. It is sensitive to muscle mass, which means a fit young man with early kidney disease may have a creatinine within normal range. Similarly, an elderly woman with little muscle mass may have a seemingly normal creatinine despite significant CKD.
Urea rises with dehydration, high protein intake, and gastrointestinal bleeding, as well as with reduced kidney function. A raised urea with normal creatinine often reflects dehydration rather than kidney disease.
Electrolytes
Sodium (Na) is the main extracellular electrolyte. Abnormalities reflect fluid balance as much as absolute sodium content. Hyponatraemia, sodium below 135 mmol/L, is the most common electrolyte abnormality in hospital inpatients.
Potassium (K) is critical for cardiac conduction. Both high potassium (hyperkalaemia) and low potassium (hypokalaemia) can cause dangerous arrhythmias. ACE inhibitors and angiotensin receptor blockers commonly cause hyperkalaemia. Diuretics commonly cause hypokalaemia.
HbA1c: the diabetes marker explained
HbA1c (glycated haemoglobin) reflects average blood glucose over the preceding two to three months. Glucose binds irreversibly to haemoglobin, so a higher average glucose over that period produces a higher HbA1c reading.
UK laboratories report HbA1c in mmol/mol (the IFCC unit). Many international references still use percentage values (the NGSP unit), so you will encounter both.
The diagnostic thresholds in the UK are:
- Below 42 mmol/mol (6.0%): normal
- 42 to 47 mmol/mol (6.0 to 6.4%): prediabetes, or high risk of type 2 diabetes
- 48 mmol/mol (6.5%) or above: diabetes, on two separate occasions
HbA1c is not valid in conditions that affect red cell turnover, including haemolytic anaemia, haemoglobin variants, and iron deficiency anaemia. In these situations, a fasting glucose or oral glucose tolerance test is needed instead.
Lipid profiles: cholesterol and cardiovascular risk
A lipid profile measures the fats circulating in the blood. The individual markers carry different levels of cardiovascular risk.
Total cholesterol is the sum of all cholesterol fractions. It is a blunt tool on its own. A normal total cholesterol in a person with very low HDL and high triglycerides can still carry significant risk.
LDL (low-density lipoprotein) is the primary target of statin therapy. LDL particles deposit cholesterol in artery walls, driving atherosclerosis. For primary prevention in most adults, the current NICE threshold for consideration of statin therapy is a 10-year cardiovascular risk of 10% or more, not a specific LDL level. For secondary prevention after a heart attack or stroke, the target is LDL below 1.4 mmol/L.
HDL (high-density lipoprotein) is often called "good cholesterol" because it transports cholesterol away from the arteries. Low HDL, below 1.0 mmol/L in men or below 1.2 mmol/L in women, independently increases cardiovascular risk.
Triglycerides are stored fats. Raised triglycerides are associated with insulin resistance, obesity, alcohol use, and thyroid disease. Very high triglycerides, above 10 mmol/L, carry a risk of acute pancreatitis.
Non-HDL cholesterol is total cholesterol minus HDL. It is increasingly preferred over LDL as a treatment target because it captures all the atherogenic lipoprotein particles, including IDL and VLDL. A non-HDL above 4.0 mmol/L warrants clinical attention.
Iron studies and bone profiles
Iron studies
Ferritin is the storage protein for iron. It is the most useful first-line marker of iron deficiency. A ferritin below 30 µg/L indicates depleted stores, even if haemoglobin remains normal. Symptoms including fatigue, cognitive impairment, and restless legs can occur before anaemia develops.
Ferritin is an acute phase protein, meaning it rises with inflammation and infection. A normal or elevated ferritin does not exclude iron deficiency in the context of active inflammation.
Serum iron and TIBC (total iron-binding capacity) together indicate how iron is being transported. Low iron with high TIBC is the classic pattern of iron deficiency. High iron with low TIBC, high transferrin saturation, suggests iron overload, including hereditary haemochromatosis.
Bone profile
The bone profile covers markers of bone metabolism and the minerals that support it.
Adjusted calcium corrects total calcium for albumin levels. Low adjusted calcium, below 2.2 mmol/L, causes symptoms ranging from muscle cramps to tetany and seizures. Causes include vitamin D deficiency, hypoparathyroidism, and chronic kidney disease.
Vitamin D (25-OH) is the storage form of vitamin D. Deficiency is common in the UK, particularly in darker skin tones, people with limited sun exposure, and older adults in care homes. Below 25 nmol/L is deficient. 25 to 50 nmol/L is insufficient. Supplementation is recommended for most adults in the UK through winter months.
PTH (parathyroid hormone) should be interpreted alongside calcium. A raised PTH with low calcium indicates secondary hyperparathyroidism, often from vitamin D deficiency. A raised PTH with high calcium indicates primary hyperparathyroidism, which can cause kidney stones, osteoporosis, and hypercalcaemic symptoms.
How to build this knowledge quickly
Reading about individual markers is useful. Having a space where you can actively test your understanding, compare it against real reference ranges, and build on it systematically is considerably more useful.
That is what the free blood test explainer tool is designed to do. It covers all eight panels discussed in this guide: FBC, LFTs, TFTs, U&Es, HbA1c, lipid profile, iron studies, and bone profile.
You can use it two ways. In Explain my results mode, you enter your own values and get an instant plain-English interpretation of each marker, with a visual range bar showing where your result sits. In Learn blood tests mode, you work through each panel marker by marker, with a self-check quiz at the end that awards points and tracks your progress across sessions.
The tool is free, works on any device, and requires no signup. Whether you are revising for a clinical exam, exploring a patient's results, or simply trying to understand what your own GP has ordered, it is built to make blood work genuinely accessible.
Start with the free blood test explainer tool.
The information in this article is for educational purposes. It does not constitute clinical advice or replace assessment by a qualified healthcare professional. If you have concerns about your blood test results, speak to your GP, pharmacist, or relevant clinician.
