Full Blood Count
Full Blood Count with Differential (FBC)
What it is
The Full Blood Count (FBC) — also called Complete Blood Count (CBC) — is an automated panel that quantifies the cellular components of blood across three major lineages: red cells, white cells, and platelets. The red cell indices include haemoglobin concentration (the oxygen-carrying capacity of the blood), haematocrit (the proportion of blood volume occupied by red cells), red cell count, and calculated indices describing red cell size (MCV — mean corpuscular volume) and haemoglobin content per cell (MCH, MCHC). The white cell differential quantifies the five major leucocyte populations: neutrophils (the primary responders to bacterial infection), lymphocytes (the adaptive immune cells — T and B cells), monocytes (innate immune cells and precursors to macrophages), eosinophils (involved in parasitic infection and allergic responses), and basophils. Platelets — the small cell fragments responsible for primary haemostasis — are counted and their size (MPV, mean platelet volume) measured. Together, the FBC provides a quantitative snapshot of bone marrow output, oxygen delivery capacity, immune cell distribution, and haemostatic function. It is the most ordered laboratory test in clinical medicine for good reason: its breadth of information relative to cost is unmatched.
Why we measure it
The FBC is included in the quarterly Pulse draw because bone marrow output and blood cell composition change continuously in response to nutrition, inflammation, chronic disease, and haematological pathology — often long before clinical symptoms appear. Anaemia — defined as haemoglobin below the sex-specific reference limit — affects approximately 25% of the global population and is the most common finding on FBC. It is caused by a wide range of conditions: iron deficiency (the most common cause globally, often preceding symptoms of fatigue by months), vitamin B12 or folate deficiency (producing macrocytic anaemia), chronic disease or inflammation (anaemia of chronic disease), and, less commonly, haematological malignancy. The MCV — red cell size — is the primary discriminator: microcytic anaemia (small red cells) points to iron deficiency or thalassaemia; normocytic anaemia to chronic disease or acute blood loss; macrocytic anaemia to B12/folate deficiency or liver disease. The white cell differential adds a further surveillance layer: a rising neutrophil count without infection suggests occult inflammation or haematological stress; persistent lymphopenia raises immune competence concerns; unexplained eosinophilia prompts investigation of parasitic or allergic aetiology; and an abnormal differential — particularly one that does not resolve — can be an early signal of haematological malignancy long before it becomes symptomatic. Quarterly FBC ensures these patterns are seen as trends rather than single data points.
Why every 3 months
Blood cell counts respond to changes in nutrition, infection, stress, and medication on a timescale of days to weeks. Many of the conditions the FBC screens for — iron deficiency anaemia, B12 deficiency, early myelodysplasia — evolve gradually over months. Quarterly measurement provides the resolution to catch a slowly falling haemoglobin, a rising MCV, or an evolving differential pattern before it reaches symptomatic thresholds. It also allows efficient correlation with other quarterly markers: a falling haemoglobin alongside elevated ferritin points to anaemia of chronic inflammation; a rising MCV alongside low B12 points to megaloblastic anaemia — patterns only visible when the markers are tracked together at the same cadence.
What movement means
The FBC is a multi-component panel; there is no single threshold that characterises the result. The key signals to track are: haemoglobin and MCV together (anaemia type and severity), the white cell differential (pattern and trajectory), and platelet count. The ranges below cover the clinically most actionable components. Each laboratory provides its own sex- and age-stratified reference ranges.
Haemoglobin — Anaemia (Male)
< 130 g/L (< 13.0 g/dL)
WHO-defined anaemia threshold for adult males. MCV determines the type: low MCV (< 80 fL) suggests iron deficiency or thalassaemia; normal MCV suggests chronic disease or acute loss; high MCV (> 100 fL) suggests B12/folate deficiency or liver disease.
WHO Haemoglobin Concentrations for the Diagnosis of Anaemia, 2011
Haemoglobin — Anaemia (Female)
< 120 g/L (< 12.0 g/dL)
WHO-defined anaemia threshold for adult non-pregnant females. Iron deficiency is the most common cause in premenopausal women. Ferritin (already in the Pulse draw) confirms iron store depletion before haemoglobin falls.
WHO Haemoglobin Concentrations for the Diagnosis of Anaemia, 2011
White Cell Count — Leucopenia
< 4.0 × 10⁹/L
Low total white cell count. Warrants differential review — neutropenia (< 1.8 × 10⁹/L neutrophils) is the clinically significant component, associated with increased infection risk. Can reflect viral infection, medication effect, or bone marrow suppression.
Standard laboratory reference ranges
Platelets — Thrombocytopaenia
< 150 × 10⁹/L
Below the lower reference limit. Mild thrombocytopaenia (100–150 × 10⁹/L) warrants monitoring and investigation of cause; severe thrombocytopaenia (< 50 × 10⁹/L) carries significant bleeding risk and requires urgent haematology review.
Standard laboratory reference ranges
In the protocol
References
- 1.
World Health Organization. “Haemoglobin Concentrations for the Diagnosis of Anaemia and Assessment of Severity.” WHO Vitamin and Mineral Nutrition Information System. 2011.
- 2.
Camaschella C. “Iron-Deficiency Anaemia.” New England Journal of Medicine. 2015.
- 3.
Weiss G, Goodnough LT. “Anaemia of Chronic Disease.” New England Journal of Medicine. 2005.
- 4.
Tefferi A, Vardiman JW. “Myelodysplastic Syndromes.” New England Journal of Medicine. 2009.