A simple guide to the main blood counts, common reasons for changes, and symptoms that should not wait for a repeat report.
A complete blood count, or CBC, looks at the cells in your blood. Your report may also call it a full blood count or haemogram. It can help investigate tiredness, fever or unusual bleeding and monitor an existing condition. It cannot diagnose every illness on its own. MedlinePlus: Complete blood count
What does each part measure?
- Haemoglobin, or Hb: The oxygen-carrying protein inside red blood cells.
- RBC count and haematocrit: The number of red cells and the proportion of blood they occupy.
- MCV: The average size of red blood cells. It gives clues that help doctors investigate anaemia.
- White blood cells, or WBC: Cells involved in defending your body. A differential count describes the different types.
- Platelets: Small blood components that help stop bleeding by forming clots. MedlinePlus: Complete blood count
Common reasons for abnormal results
Low haemoglobin: This may indicate anaemia. Iron deficiency is one possible cause, but the doctor needs to establish why it occurred. Heavy periods, pregnancy or bleeding from the stomach or bowel can contribute to iron deficiency. Other causes of anaemia include vitamin B12 or folate deficiency, inherited conditions and chronic illness. NHS: Iron deficiency anaemia; NHLBI: Causes of anaemia
Changed white-cell counts: A high count may accompany infection, inflammation or a medicine effect. Low counts can occur with some infections, medicines or conditions affecting blood-cell production. The type of cell affected and your symptoms matter more than a single label saying “high” or “low”. MedlinePlus: White blood count
Changed platelets: A low count can have several causes, including immune conditions and medicines. A high count may occur with infection, inflammation or iron-deficiency anaemia. Platelet number is only one part of assessing bleeding or clotting risk. MedlinePlus: Platelet tests
Low platelets alone do not confirm dengue. If dengue is suspected, your doctor considers symptoms and appropriate infection tests. MedlinePlus: Dengue fever testing
How to use your report
Read the units and laboratory reference intervals carefully. Compare older CBCs with your doctor. Dehydration, activity, medicines and menstruation can affect some measurements. A repeat count or additional tests may be needed; the right next step depends on the overall pattern. MedlinePlus: Complete blood count
Take a short symptom timeline to your appointment. Mention heavy periods, recent infections, bleeding, medicines and any previous abnormal counts. Useful questions are: “Which result matters most?” and “What needs checking before deciding on treatment?”
When should you seek help?
Arrange review for persistent tiredness, recurrent infections, unexplained bruising or unusual bleeding. If you have a known low white-cell count and develop fever or feel unwell, contact your medical team promptly. MedlinePlus: Platelet tests; NHS: Low white blood cell count
Seek emergency care for severe breathing difficulty, chest pain, collapse or heavy bleeding that will not stop. Do not wait for a fresh CBC before getting help. MedlinePlus: Recognising medical emergencies
Sources and further reading
- MedlinePlus: Complete blood count
- MedlinePlus: White blood count
- MedlinePlus: Platelet tests
- NHS: Iron deficiency anaemia
- MedlinePlus: Dengue fever testing
- NHS: Low white blood cell count
- MedlinePlus: Recognising medical emergencies
- NHLBI: Causes of anaemia
Editorial review and source check: 17 October 2026.
Medical disclaimer: This article provides general health information, not a diagnosis or a treatment plan. A qualified medical professional should interpret your results in the context of your health. Do not start, stop or change medicines based on this article.
© 2026 EndYourIllness. Original text. Third-party names and marks belong to their respective owners.
