How to Read Common Blood Tests (FBC & U&E)
Two blood tests come up on almost every patient: the full blood count (FBC) and urea & electrolytes (U&E). Learning to read them systematically — and to spot the common patterns — is one of the most useful skills you'll build. Reference ranges vary by lab, so always use the range printed on the report.
The full blood count (FBC)
- Haemoglobin (Hb) — low Hb is anaemia. Use the red cell size (MCV) to classify it: low MCV (microcytic) suggests iron deficiency; high MCV (macrocytic) suggests B12/folate deficiency or alcohol; normal MCV (normocytic) suggests chronic disease or acute blood loss.
- White cell count (WCC) — high in infection or inflammation; very high or very low warrants urgent thought. A high neutrophil count suggests bacterial infection; high lymphocytes often viral.
- Platelets — low (thrombocytopenia) raises bleeding risk; high can occur with inflammation or, rarely, bone-marrow disease.
Urea & electrolytes (U&E)
- Sodium (Na) — abnormalities usually reflect water balance more than salt. Both high and low sodium have wide differentials.
- Potassium (K) — critically important: both high (hyperkalaemia) and low (hypokalaemia) can cause dangerous arrhythmias. Always act on a markedly abnormal potassium.
- Urea & creatinine — markers of kidney function. A rising creatinine indicates acute kidney injury; the pattern of urea vs creatinine can hint at the cause (e.g. dehydration vs intrinsic renal problems).
A simple way to approach any result
- Check the patient's details and the reference range on the report.
- Scan for any critically abnormal value (e.g. potassium) and act on it first.
- Interpret each abnormal value in the clinical context — a result only means something alongside the patient.
- Compare with previous results to see the trend.
Stuck on a result pattern?
Ask Aboy AI "what causes a microcytic anaemia?" or "how do I interpret a high potassium?" and get a clear, cited answer.
Try Aboy AI freeFrequently asked questions
What does a low MCV mean?
A low mean cell volume (microcytic anaemia) most commonly suggests iron deficiency, and also thalassaemia and some chronic diseases.
Which blood result should I always act on quickly?
A markedly abnormal potassium — both high and low potassium can cause life-threatening cardiac arrhythmias and need prompt management.
This article is for education only and is not medical advice. Interpret blood results in clinical context and with qualified supervision.