Almost everyone has been in this situation. You have a blood test. The results come back — sometimes with a note from your GP, sometimes just numbers on a patient portal. You stare at them. Some are flagged as high or low. Some are in units you have never seen before. Most make no sense.
This guide is my attempt to translate the most common blood tests into something a non-doctor can actually use.
One important caveat: blood test results must always be interpreted in the context of your symptoms, your medical history, and your clinical picture. A result that looks abnormal may be entirely normal for you. A result within the normal range can still be clinically significant. This guide gives you context, not a diagnosis.
Full Blood Count (FBC) — the basics of your blood
The FBC is usually the first test ordered. It measures the cells in your blood.
Haemoglobin (Hb): The protein in red blood cells that carries oxygen.
- Normal range: Men 130–170 g/L, Women 120–160 g/L
- Low: Anaemia — fatigue, breathlessness, pallor. Many causes: iron deficiency, B12/folate deficiency, chronic disease, blood loss, sickle cell, others
- High: Polycythaemia — can increase clotting risk. Causes include smoking, lung disease, or rarely blood disorders
Mean Cell Volume (MCV): The size of red blood cells.
- Low MCV with low Hb: Iron deficiency anaemia or thalassaemia trait
- High MCV with low Hb: B12 or folate deficiency (megaloblastic anaemia)
White Cell Count (WCC/WBC): Immune cells. A raised count usually indicates infection or inflammation. Very high counts can indicate blood disorders. Low counts can indicate bone marrow suppression.
Platelets: Involved in blood clotting. Low platelets increase bleeding risk. Very high platelets can increase clotting risk.
Kidney function tests (U&E / Renal Profile)
Creatinine: A waste product filtered by the kidneys. High creatinine suggests the kidneys are not filtering effectively. Normal range varies by age, sex, and muscle mass — Black patients may have slightly higher creatinine due to greater muscle mass, which can cause overestimation of kidney impairment using some calculation methods.
eGFR (estimated Glomerular Filtration Rate): Calculated from creatinine, age, and sex. Estimates how well your kidneys are filtering.
- Above 90: Normal kidney function
- 60–89: Mildly reduced (often normal for older people)
- 45–59: Mildly-moderately reduced — monitor
- 30–44: Moderately-severely reduced — specialist review
- Below 30: Severely reduced — specialist management
Important note: The traditional eGFR calculation included a "race correction" factor that gave Black patients a higher estimated eGFR, potentially masking kidney disease. Most UK laboratories have now removed this race correction. If your results are from several years ago, they may have used the old formula.
Urea: Another waste product. Elevated urea alongside elevated creatinine suggests kidney impairment. Elevated urea with normal creatinine can indicate dehydration.
Potassium: Must be in a tight range. High potassium (hyperkalaemia) can cause dangerous heart rhythm abnormalities. Low potassium (hypokalaemia) causes muscle weakness. Potassium levels are affected by kidney function, certain medications (ACE inhibitors, diuretics), and diet.
Sodium: Regulates fluid balance. Abnormal sodium levels (high or low) require clinical assessment.
Liver function tests (LFTs)
ALT (Alanine Aminotransferase): Enzyme found mainly in liver cells. Elevated ALT indicates liver cell damage. Common causes: fatty liver disease, alcohol, viral hepatitis, medications.
AST (Aspartate Aminotransferase): Similar to ALT. Also found in muscle cells, so elevated after heavy exercise or muscle injury.
GGT (Gamma-Glutamyl Transferase): Sensitive marker for alcohol use and liver disease. Elevated GGT is one of the earliest signs of alcohol-related liver damage.
ALP (Alkaline Phosphatase): Found in liver, bone, and placenta. Elevated in liver disease, bone disorders, or normal pregnancy.
Bilirubin: The yellow pigment from broken-down red blood cells, processed by the liver. High bilirubin causes jaundice. Mildly elevated unconjugated bilirubin is common in Gilbert's syndrome — a benign, harmless condition affecting 5% of people.
Albumin: Protein made by the liver. Low albumin indicates poor liver synthetic function, malnutrition, or chronic illness.
Thyroid function tests
TSH (Thyroid Stimulating Hormone): The most important first-line thyroid test. Produced by the pituitary gland to control thyroid hormone output.
- High TSH: Underactive thyroid (hypothyroidism) — fatigue, weight gain, cold intolerance, constipation
- Low TSH: Overactive thyroid (hyperthyroidism) — weight loss, palpitations, heat intolerance, anxiety
T4 (Free thyroxine): The main thyroid hormone. Ordered alongside TSH to confirm thyroid status.
HbA1c — blood sugar over time
I covered this in the diabetes article, but it belongs here too.
HbA1c measures average blood glucose over 2–3 months.
- Below 42 mmol/mol: Normal
- 42–47: Prediabetes
- 48 or above: Diabetic range
Cholesterol (lipid profile)
Total cholesterol: The combined cholesterol in your blood. Target generally below 5.0 mmol/L.
LDL cholesterol ("bad cholesterol"): Deposits cholesterol in arterial walls — causes atherosclerosis. Target below 3.0 mmol/L for general population, below 1.8 or lower for high cardiovascular risk patients.
HDL cholesterol ("good cholesterol"): Transports cholesterol away from arteries. Higher is better. Above 1.0 mmol/L for men, 1.2 for women.
Triglycerides: Fats in the blood, affected by diet, alcohol, diabetes. Below 1.7 mmol/L is optimal.
Total:HDL ratio: A combined risk measure — below 4 is generally considered good.
Case study: Simba's confusing results
Simba, 45, received his blood test results on the NHS app and called me in a panic. His creatinine was flagged as above the reference range. His ALT was slightly elevated. He was convinced he had kidney failure and liver disease.
His actual picture: his creatinine was 105 — the upper limit of normal for his laboratory was 100. A difference of 5 units, clinically insignificant for his age and body composition. His eGFR was 82 — entirely normal. His ALT was 48, slightly above the 45 upper limit — he had started a new intensive gym programme 3 weeks earlier. Elevated ALT from muscle activity is common and harmless.
Nothing was wrong. But the numbers, without context, had caused him significant distress for 3 days.
This is why I prefer to discuss blood test results with patients directly rather than just releasing them on a portal without context.
Sources: NHS Full Blood Count reference guide; NICE guidance on CKD staging and eGFR (NG203, 2021); British Thyroid Foundation patient information; British Heart Foundation lipid targets; NKF/ASN Task Force on Race in eGFR equations 2021.