Haematology Basics

The full blood count (FBC) is the most common haematology test ordered. It measures the quantity of cells in the blood and also describes some features of red cells without the need for making blood films.


The FBC uses flow cytometry which is where cells are piped into a single file and a laser is shined onto them. The characteristics of the cells can be determined by the patterns they produce when scattering the light.


Blood is made up of plasma (~60%) and cells (~40%). Plasma is not the same is serum, which is plasma with no clotting factors. Plasma contains proteins, the most common one being albumin, followed by gammaglobulins (antibodies). Water, electrolytes, hormones and glucose are also contained in plasma. The cellular component of blood is made up of red cells, white cells and platelets.


An FBC requires blood to be collected into a container with EDTA added to it, to ensure that the sample does not clot (EDTA chelates calcium to prevent clotting). Such containers are purple topped vacutainers(TM) or red topped monovettes(TM).


Sometimes abnormal cells in the blood such as blasts (leukaemic stem cells) or enzyme deficiencies can interfere with analysis and will mean that a manual count using a blood film and haemocytometer will have to be used.


White Blood Cells

White blood cells (WBCs) comprise the immune component of the blood. They can be either myeloid or lymphoid. Myeloid cells contain granules which have cytotoxic enzymes within them and phagocytose pathogens. Lymphocytes are either T, B or NK cells. They produce antibodies or are directly cytotoxic. In the FBC you are given a total white cell count (WCC) and a differential; which details the individual white blood cell levels.


Neutrophils: These are granuloctytes. They comprise 40-75% WBCs. They have multi-lobed nuclei. Normally there can be up to 5 lobes; more lobes can be seen in B12 and folate deficiency (see image)


Lymphocytes have a single large round nucleus and very little cytoplasm. You cannot tell a T cell from a B cell by looking at it, and importantly a FBC will not give you T or B Cell levels, this is a separate test that must be specifically requested.


Monocytes/Macrophages: Monocytes and macrophages are of the same lineage, a monocyte is in the circulation and a macrophage is a monocyte that has left the circulation and is resident in a tissue. Monocytes have a large C shaped nucleus.


Eosinophils have a bi-lobed nucleus and large granules containing histamine, which is stimulated to release by IgE, causing increased capillary permeability and pruritus. They are involved in the response to parasitic infections and allergies, so will be raised in these situations. Drugs such as penicillin can cause eosinophilia.


Basophils possess very stained cytoplasmic granules that almost obliterate the nucleus when examined microscopically. They are involved in hypersensitivity, but otherwise are of limited importance.


Megaloblastic Anaemia

This is a blood film form a patient with megaloblastic anaemia, possibly due to a folate/B12 deficie

Red Blood Cells

Erythrocytes or red blood cells (RBCs) are mainly responsible for transporting oxygen around the body as they contain haemoglobin (Hb). Hb also acts as a buffer in acid base balance. RBCs do not have a nucleus or any DNA, which means they cannot synthesise proteins, however they can actively metabolise glucose. They use this process to create 2,3-DPG a substance that makes Hb release oxygen more readily to tissues.


Survival:  120 day lifespan. They are phagocytosed in spleen (collecting in the Cords of Bilroth) and liver, which is known as the reticulo-endothelial system.


Transfusions are not assumed to last 120 days, as we have no idea how long RBCs have been in vasculature of donor. If they are very mature when transfused then their life span in the recipient is short.


In an FBC, several results look at RBCs:


  • Hb - Haemoglobin contained inside RBCs
  • Red Cell Count (RCC) - Number of RBCs in blood
  • Packed Cell Volume (PCV or Haematocrit [Hct.]) - The proportion of blood that is made up of RBCs if blood is allowed to settle.
  • Mean Cell Volume (MCV) - The size of cells
  • Mean Cell Haemoglobin (MCH) - The amount of Hb in each cell. It calculated by Hb divided by RCC.
  • Mean Cell Haemoglobin Concentration (MCHC) - This is Hb divided by PCV, it is not a particularly useful parameter.
  • Red Cell Distribution Width (RDW) - The amount of deviation in RBC size. This is useful if you have a mixed anaemia of small cells and large cells, as the average size may be normal, but RDW will be high.


 A reticulocyte count is not routinely included and has to be specially requested. Reticulocytes are immature red blood cells that have small amounts of DNA in them, their levels are increased when the marrow is overproductive e.g post haemorrhage. Their numbers are low in underproductive marrow e.g. aplastic anaemia.


Haemolytic Anaemia

Marrow smear from a patient with haemolytic anaemia, in which increased number soy normoblasts - ery


Platelets (Plts) are the second most numerous cell in the blood, and are derived from megakaryocytes. Normally they are discoid and anucleate. They survive for 10-12 days. Platelets are crucial to clotting as they form a primary haemostatic plug in response to vessel damage and provide a surface upon which the coagulation cascade occurs on to form a definitive thrombus.


Full blood counts measure their number, however other tests are required to examine function or morphology.


10 Steps in FBC Interpretation

1. Check patient details and the date and time of the sample, have the lab commented on the sample with any technical information that might have affected analysis?


2. Are the values normal in Hb, PCV, WCC, Plts?


3. Look at the differential WCC (even if overall WCC normal).


4. IF Hb is low, look at the MCV to determine if it is a microcytic or macrocytic anaemia


5. If MCV is normal but Hb is low, look at RDW to see if there is a mixed anaemia. A film may need to be made to reliably asses the blood.


6. If there are abnormalities in cells, ask yourself if it is isolated to one lineage or many e.g. pancytopenia (all lineages low).


7. If the WCC is elevated, is there any one leucocyte type increased or are multiple types affected?


8. Remember to relate any abnormalities to the clinical context, if the result are not in keeping with what you would expect, the sample may need to be repeated.


9. Would other blood tests confirm/refute FBC findings? For example liver function tests in macrocytic anaemia or Iron studies if microcytic.


10. Is a film needed, or does a bone marrow sample need to be taken, for example if malignancy is possible?



Normal Ranges:


  • Haemoglobin

Men: 13-18g/dL

Women: 11.5-16g/dL

  • MCV (Mean cell volume)

76-96 fl

  • White cells (total)

4-11 x 10*9/L

  • Neutrophils

40-75% (2-7 x10*9/L)

  • Lymphocytes

20-45% (1-4 x10*9/L)

  • Eosinophils

1-6% (0.04-0.4 x10*9/L)

  • Platelets

150-500 x 10*9/L 


Interpretation of these anomalies

It’s important to now look at these anomalies and ask ‘what COULD this mean?’ Note that this is an interpretation: nothing can be said definitively. You must look at this in context: examine the patient and interpret their FBC with regard to them.



Haemoglobin is usually the first value to be looked at when interpreting a blood count. The most common situation in which this is deranged is in anaemia, in which the value is less than 12 for women and 13 (or thereabouts) for men. The causes of anaemia are many, and elucidation of the case depends on other parameters, such as mean corpuscular volume. 

Anaemia can be divided into microcytic (and hypochromic), macrocytic (with or without megakaryocytes) and normocytic. Clinically, the commonest cause of a microcytic anaemia is iron deficincy, in which the anaemia is usually mild (Hb of 8-11) and has various causes, including nutritional deficiency, menstrual blood loss, acute blood loss, some malabsoptive states, rare transporter deficiencies, and the most important, gastro-intestinal bleeding. It is important to exclude GI bleeding before simply supplementing a patient's iron.

Other types of anaemia can be due to chronic disease (normocytic, normochromic, in which MCV is wihtin normal ranges, as is mean corpuscular haemoglobin) or to deficiency states, such as low B12 or Folic acid (megaloblastic anaemia). 

Conversely, the haemoglobin values may be above the normal. When high values are recorded, it is important to consider the patient's clinical status. High haemoglobin may be a result of dehydration, whereby low intravascular fluid results in an increase in the relative number of RBCs. If the patient's fluid status seems normal, their body habitus and lifestyle habits ought also to be considered - overweight, hypertensive male smokers are the archetypal sufferers of Gaisbock Syndrome, or apparent polycythaemia. Polycythaemia can also be secondary to altitude, to malignancy (e.g. erythorpoetin-secreting renal cell tumours), chronic hypoxia, such as in COPD. Finally, one ought also to consider Polycythaemia Rubra Vera, one of the myeloproliferative disorders. 

Microcytic Anaemia causes:

  1. Iron Deficiency
  2. Beta-thalassaemia trait
  3. Lead poisoning
  4. Hereditary spherocytosis
  5. Hereditary ellipsocytosis

Macrocytic Anaemia causes:

  1. B12 deficiency
  2. Folate deficiency
  3. Hypothyroidism
  4. Alcoholism
  5. Haemolytic anaemia (need blood film to presence of schistocytes; has various causes)


White Cell Count

Most often, this will be increased. The causes of this are reactive (as a response to infection or other insult to the body) and malignancy (any of the leukaemias or lymphomas)


There are some slightly more specific causes that lead to certain cell lines being elevated within the white cell differential.



This can occur in:

  • Atopy/allergy
  • Vasculitis (Churg-Strauss)
  • Parasitic infection
  • Hypereosinophilic syndrome
  • Pregnancy
  • Myelodysplastic Syndrome



    Increased platelets are seen in many situations. This increase may be reactive, or primary.


    Causes of a reactive thrombophilia are:

    • Post operatively
    • Infection
    • Inflammation
    • Infection


    The cause of primary thrombophilia is the Myelodysplastic Syndrome


      Neutrophilia and Neutropenia

      The differentials for neutrophilia are:

      • Infection
      • Vasculitis
      • Inflammation
      • Malignancy


        The differential for neutropenia (low neutrophil count) are:

        • Malignancy (with bone marrow infiltration)
        • Iatrogenic
        • Overwhelming infection



          Reactive neutrophil changes can be seen occurring: there are purple cytoplasmic granules (toxic gran

          Lymphocytosis and Lymphopenia

          Lymphocytosis is an excess of lymphosytes. This will be demonstrable on the differential WCC. It is caused by:

          • Acute viral infection
          • Chronic Lymphocytic Leukaemia
          • Acute Lymphocytic Leukaemia
          • Some lymphomas with very poorly differentiated cells


            Lymphopenia occurs in:

            • Iatrogenic causes: corticosteroid use and other medications; chemotherapy
            • Infection - viral (common cold, HIV)
            • Autoimmune disease - SLE, rheumatoid arthritis



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