Full blood count and blood film

• A full blood count (FBC) gives the amount of haemoglobin, the size of the red cells, a white cell count with a differential, and a platelet count. The same analyser usually also reports haematocrit, MCH, MCHC and the red cell distribution width (RDW).
• The blood film is what the numbers cannot do: it shows the shape, size and inclusions of cells, confirms true thrombocytopenia and may show malaria, parasites, reactive lymphocytes or blasts.
• Start with the haemoglobin (and the sex- and age-specific reference range), then the MCV to place the anaemia as microcytic, normocytic or macrocytic, then the trend and the other cell lines.
• Every reference range, threshold and first-line statement in this page is cited to an open source. Ranges differ between laboratories, methods and populations, so the range printed on the report always wins.
• Ferritin, a reticulocyte count, iron studies, vitamin B12 and folate, renal function and markers of haemolysis are not part of the FBC, but the FBC pattern tells you which of them to request next.

By the end of this page you should be able to:

1. State what haemoglobin, haematocrit, the red cell indices and RDW each measure.
2. Read a WBC with its differential, and recognise neutrophilia, lymphocytosis, eosinophilia, leukopenia and the platelet abnormalities.
3. Explain why a microcytic, normocytic and macrocytic pattern forms, and name the common causes of each.
4. Apply an interpretation approach to three short sample result sheets and conclude with the next step.
5. Recognise pseudothrombocytopenia and the other pre-analytical pitfalls.
6. Use the WHO 2024 haemoglobin cut-offs by age and sex.
7. Define the limits of the FBC and say when a blood film, a repeat sample or senior help is needed.

An FBC is one of the most common tests in medicine, and one of the most powerful if it is read as a story rather than as a list. Each line measures a different compartment: haemoglobin tells you how much oxygen-carrying pigment is present, the indices describe the average red cell, the white count and differential describe inflammation and immunity, and the platelet count describes primary haemostasis. The blood film then checks the analyser's assumptions, looking at the cells themselves.

The teaching sequence used here mirrors what is done at the bedside: what each test measures; the ranges and thresholds that define abnormal; the causes of high and low values; the named patterns that the combinations form; three invented sample result sheets worked through step by step; and the pitfalls that make a normal-looking report wrong. This page assumes no prior knowledge of haematology.

Keep three safety rules in mind throughout. The laboratory that performed the test is the authority for its own reference interval, because intervals are built on local method, population and analyser. ^[5] The count tells you what is abnormal; the clinical picture tells you why and whether it matters now. ^[5] And anaemia is a manifestation rather than a diagnosis: even mild anaemia deserves a cause. ^[1]

Haemoglobin (Hb). This is the oxygen-carrying protein in the red cell expressed as a concentration in blood, usually in grams per litre (g/L) or grams per decilitre (g/dL). It is the single most important number on the FBC, because it is the measure of anaemia and (together with the haematocrit) of erythrocytosis. ^{[1] [2]}

Haematocrit (Hct, packed cell volume). The fraction of the blood volume occupied by red cells, given as a percentage or a volume fraction. It is related to haemoglobin by the mean cell volume, and in many laboratories it is calculated rather than measured directly. It rises in dehydration or polycythaemia and falls in anaemia or haemodilution. ^[1]

Mean corpuscular volume (MCV). The average size of a red cell, in femtolitres (fL). It is the first sorting tool in anaemia: microcytic when MCV is under 80 fL, normocytic when 80 to 100 fL, and macrocytic when over 100 fL (in adults). ^{[1] [3]} Reticulocytes are larger than mature red cells, so heavy reticulocytosis can lift the MCV on its own. ^[1]

Mean corpuscular haemoglobin (MCH). The average haemoglobin per red cell, in picograms (pg). It usually follows the MCV and is of little clinical significance beyond describing the pattern; it should not be interpreted alone. ^[4]

Mean corpuscular haemoglobin concentration (MCHC). The concentration of haemoglobin inside each red cell, in g/dL or g/L, which is effectively how "red" the cell is. The term hypochromia is used when MCHC is under 30 percent. Spherocytes can raise the MCHC. ^[1]

Red cell distribution width (RDW). The degree of variation in red cell size (anisocytosis), usually given as a coefficient of variation in percent or as a standard deviation in fL. A high RDW can be the only abnormality when microcytic and macrocytic populations coexist, because the MCV reports only the mean. ^[1]

Red cell count (RBC). The number of red cells per volume, in millions per microlitre (or 10^12 per litre). It is usually proportionate to haemoglobin, so a low haemoglobin with a relatively preserved red count is a useful pointer toward thalassaemia trait rather than iron deficiency. ^[5]

White cell count with differential. The total white cell count (WBC) and the proportion (or better, the absolute count) of neutrophils, lymphocytes, monocytes, eosinophils and basophils. The absolute count is what matters: a percentage of 60 percent neutrophils means little without knowing whether the total is 2 ×10^9/L or 20 ×10^9/L. ^{[4] [5]}

Neutrophils. These are the principal acute defenders against bacterial infection. Neutrophilia is a rise above the laboratory interval; it is common in infection, inflammation, surgery, trauma, physiological stress and corticosteroid use. Neutropenia is a low absolute neutrophil count (ANC); in adults it is generally defined as an ANC under about 1.5 ×10^9/L, and severe neutropenia as under 0.5 ×10^9/L. ^[5]

Lymphocytes. The cells of adaptive immunity. Acute lymphocytosis is most often reactive, especially viral, whereas a persistent or rising lymphocytosis needs review of previous counts and consideration of chronic infection, medication effects and lymphoproliferative disease. ^[5]

Monocytes. The phagocytic cells of the blood. Monocytosis has a broad differential (infection, inflammation, reactive states, haematological disease), and the important distinction for a first-time reader is transient or reactive versus persistent and unexplained. ^[5]

Eosinophils. Cells of allergy and parasitic infection. Eosinophilia is an eosinophil count above the laboratory interval. ^[3]

Basophils. The least common granulocyte, often grouped with the total granulocyte count.

Platelets. The cell fragments of primary haemostasis, reported in ×10^9/L (or thousands per microlitre). Platelet count is the FBC's read-out for bleeding and clotting risk and for bone marrow and consumptive disease. ^[6]

Reticulocytes. The young red cells still containing ribosomal RNA, reported as a percentage of red cells or as an absolute count. A reticulocyte count is the FBC-equivalent of a messenger from the marrow: it tells you whether the marrow is responding. It is low when production is deficient (iron deficiency, renal failure, marrow failure) and high when red cells are being destroyed or lost. ^[8]

What a blood film adds. The analyser counts and sizes; the film shows. A blood film (peripheral smear) stained with a polychrome stain such as Wright-Giemsa shows cell morphology, confirms true platelet counts, reveals shape changes (schistocytes, spherocytes, target cells, sickle cells, ovalocytes), inclusions (basophilic stippling, Howell-Jolly bodies, malaria parasites, Heinz bodies), immature granulocytes and reactive or abnormal lymphocytes. ^[1] It is the step that converts "thrombocytopenia on the analyser" into real thrombocytopenia or into pseudothrombocytopenia (platelet clumping). ^[7]

Ranges are assay-, age-, sex- and population-specific. For some analytes they change with posture, time of day, fasting and pregnancy. Only cholesterol, creatinine and HbA1c are internationally harmonised in practice; most other analytes, including PTH, vitamin D and ALP, are method-specific. The interval printed on your patient's own report is therefore authoritative; where laboratory and textbook disagree, the laboratory wins. ^{[1] [5]}

A separate and important set of thresholds is the definition of anaemia, which is a threshold (not merely a centile). The WHO 2024 guideline on haemoglobin cut-offs is the first substantial revision since 1968 and reaffirmed most of the longstanding values while revising some subgroup cut-offs. ^[2]

The Merck Manual gives the corresponding adult cut-offs (men Hb under 13 g/dL, women under 12 g/dL) with a perinatal age-specific table for infants and children. ^[1] WHO and the Merck summary agree for adults; where a guideline and a laboratory interval disagree, the laboratory interval is authoritative for that patient. ^{[1] [5]}

MCV cut-offs for adults treated as categorical thresholds: ^{[1] [3]}

• Microcytic: MCV under 80 fL.
• Normocytic: MCV 80 to 100 fL.
• Macrocytic: MCV over 100 fL.
• Hypochromia: MCHC under 30 percent.
• Severe anaemia by symptom correlation: pallor is common with haemoglobin under 70 g/L. ^[1]

Neutropenia thresholds applied in adults: ^[5]

• Neutropenia: ANC under about 1.5 ×10^9/L.
• Severe neutropenia: under about 0.5 ×10^9/L.

ITP is diagnosed clinically with a platelet count under 100 ×10^9/L and a normal haemoglobin and white count, after other causes of thrombocytopenia are excluded. ^[6]

Values are read in pairs: high needs an explanation, low needs a different one, and many results only make sense read together with another line. The table is a map rather than a diagnosis; use it to decide what to ask next.

Microcytic cells occur when haemoglobin synthesis is impaired or when production is iron-limited. ^[1] Iron deficiency, thalassaemia and haemoglobin-synthesis defects are the common causes; in anaemia of chronic disease the MCV can be borderline microcytic. ^[1]

Normocytic cells reflect red cell loss or failed production without a defect in haemoglobin synthesis: acute blood loss, renal disease, marrow failure or mixed causes. A normal MCV never excludes iron deficiency, particularly when causes are mixed. ^[5]

Macrocytic cells occur when DNA synthesis is impaired (B12 or folate deficiency, chemotherapeutic or antifolate drugs) and in alcohol use disorder and liver disease, where membrane abnormalities enlarge the cell. Acute bleeding can briefly make the indices macrocytic because of early release of large reticulocytes. ^[1]

Neutrophilia is a common reactive finding: infection and inflammation are the usual drivers, but the same finding after surgery or in a stable postoperative patient may be expected. A raised WBC is not a diagnosis of infection and should not be equated with sepsis; clinical assessment and physiological observations remain central. ^[5]

Persistent or marked neutrophilia without a reactive explanation raises other possibilities, including medication effects and haematological disease, and becomes more concerning when it is unexplained, accompanied by other cytopenias, or associated with constitutional symptoms or organomegaly. ^[5]

Neutropenia needs care with both the number and the patient. Confirm the absolute neutrophil count rather than relying on percentages, decide whether it is acute or chronic and whether it is isolated or accompanied by other abnormalities, and treat neutropenia with fever or clinical deterioration as urgent. ^[5]

Lymphocytosis is divided clinically by duration: acute lymphocytosis is commonly reactive (viruses especially), whereas persistent or unexplained lymphocytosis requires trend review and exclusion of chronic infection, medication effects and lymphoproliferative disease. NICE guidance on suspected cancer applies where persistent or unexplained lymphocyte abnormalities occur with lymphadenopathy, hepatosplenomegaly, unexplained fever, recurrent infection, bruising or bleeding. ^[5]

Eosinophilia (a count above the laboratory interval) is the classic signature of allergy and parasitic infection, and it often improves or worsens with the underlying condition. Monocytosis is broad and is first sorted as transient or reactive versus persistent and unexplained, with splenomegaly, constitutional symptoms or other cell-line abnormalities pushing toward specialist review. ^[5]

Thrombocytopenia means platelets under about 150 ×10^9/L. Before treating the number, confirm it is real: EDTA-anticoagulated samples can clump platelets in vitro, producing a spurious thrombocytopenia without a true shortage of platelets. This is pseudothrombocytopenia, an artefact rather than a disease. ^[7] The blood film and a repeat count (often on a citrate or alternative tube) settle it. ^[7]

Thrombocytosis means platelets above the upper limit of the reference interval (above about 400 ×10^9/L by the MedlinePlus adult interval). It is most often reactive (infection, inflammation, bleeding, iron deficiency, post-splenectomy) and less commonly a primary myeloproliferative disorder. ^[3]

Severe ITP is usually defined as a platelet count under 20 ×10^9/L and warrants treatment, with ITP diagnosed at counts under 100 ×10^9/L together with a normal haemoglobin and white cell count after other causes are excluded. ^[6]

A raised haemoglobin or haematocrit should first be sorted by plasma volume: apparent erythrocytosis (dehydration, reduced plasma volume) versus true erythrocytosis. Persistent or unexplained erythrocytosis needs senior review, with attention to smoking, respiratory disease, hypoxia, medications and other clinical features. ^[5]

Patterns are what exam questions test. Read the haemoglobin first, then the MCV, then the other lines, in that order; the named combinations below then fall out naturally.

• Microcytic anaemia. Haemoglobin low and MCV under 80 fL. The usual causes are iron deficiency, thalassaemia and related haemoglobin-synthesis defects; anaemia of chronic disease can be microcytic. RDW is often raised, and the red cell count is relatively preserved in thalassaemia trait. ^{[1] [5]}
• Normocytic anaemia. Haemoglobin low with MCV 80 to 100 fL. Think acute blood loss, renal disease, marrow failure, haemolysis or mixed causes. Iron deficiency may be present with a normal MCV. ^[5]
• Macrocytic anaemia. Haemoglobin low with MCV over 100 fL. Think B12 or folate deficiency, alcohol, liver disease, antifolate or hydroxyurea-type drugs and marrow disorders; acute bleeding can do this transiently via reticulocytes. ^[1]
• Mixed (normocytic MCV with high RDW). Microcytic and macrocytic processes cancelling each other: for example combined iron and B12 deficiency. ^[1]

• Neutrophilia. Usually a reactive signal of infection, inflammation or stress; the cause is usually the patient's clinical condition, not the number.
• Lymphocytosis. Acute and reactive is common; persistent or unexplained is a different problem. ^[5]
• Monocytosis. Transient or reactive first, persistent next. ^[5]
• Eosinophilia. Allergy or parasitic infection is the starting point; persistent and marked forms are rarer.
• Leukopenia. Nonspecific injury to white cell production or survival; viral infection, medications and bone marrow disorders are the common categories. ^[3]
• Neutropenia with fever or clinical deterioration. Treat as urgent, without waiting for the count to improve. ^[5]

• Thrombocytopenia. A count under about 150 ×10^9/L, first confirmed as real (pseudothrombocytopenia excluded), then split by production versus destruction versus sequestration.
• Thrombocytosis. Usually reactive; iron deficiency and inflammation are common drivers. ^[3]
• Severe thrombocytopenia under 20 ×10^9/L usually warrants treatment. ^[6]

The values below are invented for teaching and labelled illustrative; they are not real patient data. The ranges and rules used in the interpretation are claims from the cited sources.

Interpretation:

1. Haemoglobin 148 g/L is within the male interval of 130 to 180 g/L and above the WHO anaemia cut-off of less than 130 g/L for men, so there is no anaemia. ^{[2] [3]}
2. MCV 89 fL is normocytic and MCHC 33 g/dL is normochromic, with a normal RDW.
3. The white count and all the differential components sit inside a normal interval and the platelet count (245) is inside 150 to 400 ×10^9/L.
4. No cell line is abnormal and the haemoglobin is well above the anaemia threshold.
5. Conclusion: this is a normal FBC; no haematological action is required.

Interpretation:

1. Haemoglobin 96 g/L in a woman is below the WHO cut-off of less than 120 g/L for non-pregnant women, so there is anaemia. ^[2]
2. MCV 72 fL is under 80 fL: microcytic, with MCH low and MCHC under 30 percent (hypochromia), the iron-deficient shape. ^{[1] [3]}
3. RDW is raised, consistent with anisocytosis as iron becomes limiting. ^[1]
4. The white count is normal, but the platelet count is 410 ×10^9/L, above the upper limit of the 150 to 400 interval; reactive thrombocytosis in iron deficiency is expected. ^[3]
5. The pattern is microcytic hypochromic anaemia with reactive thrombocytosis. Iron deficiency is the likely cause, but it is a finding, not a diagnosis: iron studies (ferritin, transferrin saturation) and a search for the cause (gastrointestinal or menstrual loss) are the next steps. ^[1]
6. Conclusion: likely iron deficiency anaemia; next step is haematinics and investigation of the cause, not iron alone.

Interpretation:

1. Haemoglobin 118 g/L in a man is below 130 g/L, so there is mild anaemia. ^[2]
2. The MCV is 97 fL (normocytic) and all other red cell indices are assumed normal.
3. The WBC count is low, with neutrophils 0.9 ×10^9/L, under the 1.5 ×10^9/L threshold for neutropenia and approaching severe neutropenia under 0.5 ×10^9/L. ^[5]
4. Platelets 42 ×10^9/L is far below the normal 150 to 400 interval but above the 20 ×10^9/L level that usually warrants urgent treatment. ^[6]
5. There are three cytopenias at once, which is a red flag for a process affecting the marrow (aplastic, leukaemic or infiltrative disease) or a consumptive process; it should not be read as three separate problems. ^[5]
6. Conclusion: multiple-line cytopenia with severe thrombocytopenia and neutropenia; repeat the count on a fresh citrate tube to exclude pseudothrombocytopenia, and arrange urgent senior/haematology review, with a blood film (and marrow examination if clinically indicated).

• Pseudothrombocytopenia. EDTA platelet clumping produces a false low count. Confirm with the film and a repeat count before acting or diagnosing. ^[7]
• Assuming the reference interval is universal. Neonatal, childhood, pregnancy, high-altitude and sex-specific ranges differ; the interval printed on the report is the authority. ^{[1] [3]}
• Treating the MCV as a diagnosis. Iron deficiency can be normocytic; thalassaemia has mixed causes; reticulocytosis can push the MCV up. Ferritin and iron studies add the missing information. ^{[1] [5]}
• Assuming normocytosis excludes significant disease. Renal failure, marrow disease and mixed deficiencies often present normocytically.
• Reading percentages instead of absolute counts. A differential "70 percent neutrophils" has different meaning at different totals; the absolute count is what decides. ^{[4] [5]}
• Equating leukocytosis with infection. Stress, trauma, surgery, corticosteroids and medications all raise the WBC; converse rare infections can give a normal or low WBC. ^[5]
• Forgetting the film. Shape, inclusions, parasites and true versus clumped platelets are only visible on film. ^{[1] [7]}
• Correcting sodium or transfusing the haemoglobin. Decisions about treatment follow the clinical picture, not the number; transfusion thresholds are guidance, not rules. ^{[1] [5]}
• Missing the trend. A single FBC is a snapshot; the change from baseline is usually more informative than the value. ^[5]
• Not checking the tube. EDTA, haemolysis and delayed processing all distort counts. Repeat when the result does not fit.
• Reading reticulocytes without correcting for anaemia. In anaemia the reticulocyte percentage should be divided by the degree of anaemia (corrected reticulocyte count or index) before judging the marrow response. ^[8]

• The FBC reports oxygen-carrying capacity, cell size, the white cell compartment and platelets; the blood film shows morphology.
• The MCV divides anaemia into microcytic (under 80 fL), normocytic (80 to 100 fL) and macrocytic (over 100 fL). ^{[1] [3]}
• Anaemia is Hb under 130 g/L in men and under 120 g/L in women by WHO 2024 cut-offs; children's cut-offs are lower and trimester-specific in pregnancy. ^[2]
• Microcytic anaemia usually means iron deficiency or thalassaemia; normocytic anaemia means loss, renal or marrow disease; macrocytic anaemia means B12/folate deficiency, alcohol, liver disease or drugs.
• Haemoglobin numbers must always be read against the laboratory's own interval and the patient's clinical picture.
• MCHC under 30 percent is hypochromia; RDW high means anisocytosis or a mixed population. ^[1]
• Neutropenia is an ANC under about 1.5 ×10^9/L, severe under about 0.5 ×10^9/L; combine with clinical assessment, not the number alone. ^[5]
• Lymphocytosis is commonly acute and reactive, but persistent or unexplained needs review and may fall under suspected-cancer pathways. ^[5]
• Thrombocytopenia is a count under about 150 ×10^9/L; first establish that it is not pseudothrombocytopenia (EDTA clumping). ^{[3] [7]}
• Thrombocytosis is most often reactive. Platelets under 20 ×10^9/L usually warrant treatment. ^[6]
• Eosinophilia points to allergy or parasitic infection; monocytosis needs the transient-versus-persistent judgement.
• The blood film shows morphology, inclusions, parasites and true platelet numbers.
• Counts are percentages unless made absolute; the absolute count decides.
• Ranges, thresholds and first-line statements differ by source and laboratory; the printed interval on the report always wins.

Related topics, as plain text until they are linked in this build: anaemia approach and iron deficiency; megaloblastic (sideroblastic and megaloblastic) anaemia; thalassaemias; polycythaemia; neutropenia; neutropenic fever; immune thrombocytopenia; essential thrombocythaemia and myelofibrosis; aplastic anaemia; haemolytic anaemias; common laboratory reference values; reading a laboratory report; serum electrolytes result interpretation; bone profile; abnormal liver function tests.