You have received your blood test results. They came through an NHS app, a letter from your GP, or a printout at the surgery. And now you are looking at a column of abbreviations and numbers - some flagged in red, some with arrows pointing up or down - and either a reassuring letter saying everything is normal (without explaining what anything means) or a request to discuss some results with your GP (which feels alarming).
Blood tests are among the most informative tools in medicine. They can detect conditions years before symptoms develop, monitor how well treatments are working, and provide early warning of organ problems that would otherwise be silent. Understanding what they measure removes unnecessary anxiety and helps you participate more meaningfully in conversations with your doctor.
This guide covers the most commonly performed blood tests in UK general practice.
Full Blood Count (FBC)
The full blood count is one of the most frequently requested blood tests. It measures the cells in your blood.
Haemoglobin (Hb): The oxygen-carrying protein in red blood cells. Low haemoglobin indicates anaemia. Normal ranges: approximately 130-170 g/L in men, 120-150 g/L in women (ranges vary slightly between laboratories).
Causes of low haemoglobin include iron deficiency (the most common), B12 or folate deficiency, chronic disease, kidney disease, and less commonly haematological conditions. The type of anaemia is determined by looking at other FBC parameters alongside haemoglobin.
MCV (Mean Corpuscular Volume): The average size of red blood cells. Small red blood cells (low MCV, below 80 fL) most commonly suggest iron deficiency or thalassaemia trait. Large red blood cells (high MCV, above 100 fL) most commonly suggest B12 or folate deficiency, excess alcohol, liver disease, or hypothyroidism.
White Cell Count (WBC or WCC): The total number of white blood cells. Elevated white cell count (leucocytosis) suggests infection, inflammation, or - less commonly - haematological disease. Low white cell count (leucopenia) suggests immunosuppression, severe infection, or effects of certain medications.
Neutrophils: The most numerous white blood cells and the primary defenders against bacterial infection. Elevated neutrophils (neutrophilia) suggest bacterial infection, inflammation, or steroid use. Low neutrophils (neutropenia) significantly increases infection risk.
Lymphocytes: White blood cells involved in viral immunity and immune memory. Elevated in viral infections. Low in some immunodeficiency states.
Platelets: Cell fragments involved in clotting. Very low platelet counts (thrombocytopenia, below 50 x10^9/L) risk abnormal bleeding. Very high platelet counts (thrombocytosis, above 600 x10^9/L) may indicate inflammation, iron deficiency, or rarely haematological disease.
Urea, Electrolytes and Creatinine (U&Es)
Tests kidney function and electrolyte balance.
Sodium: The main extracellular electrolyte, involved in fluid regulation. Abnormal levels reflect problems with fluid balance, kidney function, or hormonal control.
Potassium: Critical electrolyte for heart and muscle function. Even mild abnormalities outside the range 3.5-5.5 mmol/L can cause dangerous heart rhythm problems. Potassium is affected by kidney function, diuretic medications, and certain blood pressure drugs.
Urea: A nitrogen-containing waste product produced from protein metabolism in the liver and filtered by the kidneys. Elevated urea suggests reduced kidney function, dehydration, or high dietary protein. Mildly elevated urea with normal creatinine often simply reflects dehydration or high meat intake.
Creatinine: A waste product of muscle metabolism, filtered at a more constant rate than urea. A better indicator of kidney function than urea alone.
eGFR (estimated Glomerular Filtration Rate): Calculated from the creatinine result (and in some older formulae, also from age, sex, and formerly from race - though the race correction has been removed from UK practice). Expresses kidney filtering capacity as a percentage of normal. An eGFR above 90 is normal. Below 60 on repeated measurements indicates chronic kidney disease. See our dedicated kidney disease guide for full interpretation.
Liver Function Tests (LFTs)
ALT (Alanine Aminotransferase): An enzyme found predominantly in liver cells. Released into the bloodstream when liver cells are damaged. The most specific test for liver cell damage. Elevated in alcoholic liver disease, non-alcoholic fatty liver disease (NAFLD), viral hepatitis, and drug-induced liver injury.
AST (Aspartate Aminotransferase): Similar to ALT but also found in heart and muscle tissue, so less specific to the liver. An AST to ALT ratio above 2:1 suggests alcoholic liver disease.
ALP (Alkaline Phosphatase): Elevated with bile duct obstruction, primary biliary cholangitis, and bone disease. Can be mildly elevated in normal pregnancy.
GGT (Gamma-Glutamyl Transferase): Very sensitive to alcohol - even moderate alcohol intake elevates GGT. Also elevated in NAFLD and bile duct problems.
Bilirubin: The yellow breakdown product of haemoglobin. Elevated bilirubin causes jaundice (yellow skin and whites of the eyes). Can be elevated due to liver disease, bile duct obstruction, or excessive breakdown of red blood cells (haemolytic anaemia).
Albumin: A protein synthesised by the liver. Low albumin suggests reduced liver synthetic function, malnutrition, or protein loss (from kidneys or gut). A useful indicator of how well the liver is actually working rather than how inflamed it is.
Thyroid Function Tests (TFTs)
TSH (Thyroid Stimulating Hormone): Produced by the pituitary gland to control thyroid activity. The most sensitive test for thyroid dysfunction. High TSH (above 4.5 mIU/L) indicates an underactive thyroid (hypothyroidism - the thyroid is not producing enough hormone, so the pituitary produces more TSH to stimulate it). Low TSH (below 0.4 mIU/L) indicates an overactive thyroid (hyperthyroidism).
Free T4 (thyroxine): The main circulating thyroid hormone. Measured to confirm hypothyroidism when TSH is elevated, or hyperthyroidism when TSH is low.
Hypothyroidism is extremely common in women, affecting approximately 2% of the adult female population. Symptoms include fatigue, weight gain, cold intolerance, constipation, low mood, and dry skin. Highly treatable with levothyroxine once diagnosed.
HbA1c
See the type 2 diabetes guide for full interpretation. In brief: below 42 mmol/mol is normal, 42-47 is prediabetes, 48 and above is diabetes.
Cholesterol and Lipid Profile
See the high cholesterol guide for full interpretation. In brief: total cholesterol alone is an inadequate measure. The LDL:HDL ratio and total:HDL ratio, interpreted alongside QRISK3 cardiovascular risk score, are more clinically meaningful.
Ferritin
Ferritin is the body's iron storage protein and the most sensitive test for iron deficiency, detecting depletion before haemoglobin falls. Many laboratories report ferritin above 10-15 mcg/L as normal - but most women feel symptomatic (fatigue, hair loss, brain fog) until ferritin is above 50-70 mcg/L. If you have symptoms of iron deficiency but have been told your blood tests are normal, ask specifically whether ferritin was checked and what the result was. See our iron deficiency guide for more.
Vitamin D
25-hydroxyvitamin D is the test to request. Below 25 nmol/L is deficiency; 25-50 is insufficiency; above 75 is optimal. See our vitamin D guide for full interpretation.
C-Reactive Protein (CRP)
A non-specific inflammation marker that rises within hours of infection, tissue injury, or inflammation. A very high CRP (above 100 mg/L) suggests significant bacterial infection or severe inflammation. A mildly elevated CRP (10-50 mg/L) can indicate many things and needs to be interpreted in clinical context.
Sources: NHS - Understanding Blood Tests; Labtestsonline.org - Blood Test Reference Ranges; British Society for Haematology guidelines; NICE guidelines on testing in primary care; Royal College of Pathologists - G193 guidance on laboratory reference ranges.
