Full blood count and blood film interpretation

At a glance

• The FBC is the commonest blood test in medicine. Read it in the same order every time: is the sample valid, which lines are abnormal, what do the indices say, how does it compare with old results, and what does the film add?
• Use absolute counts, not percentages.
• A film is needed when the analyser flags, a count is unexplained, or the patient is unwell. It often names the diagnosis (blasts, schistocytes, spherocytes, smear cells).
• Disease detail: Anaemia, Thrombocytopenia, Neutropenia, Leucocytosis, Thrombocytosis, Polycythaemia.

What the analyser measures

Parameter	How it's obtained	Note
Haemoglobin	Measured (lysis, spectrophotometry)	Falsely high with lipaemia, very high white count
Red cell count, MCV, platelet count	Measured (impedance or optical)	MCV rises in samples left overnight
Haematocrit	Calculated (RCC x MCV) on most analysers
MCH, MCHC	Calculated	MCHC = Hb / Hct
RDW	Calculated spread of red cell size	High in iron deficiency, mixed deficiency, reticulocytosis
White count and 5-part differential	Measured (flow, light scatter, fluorescence)	Flags blasts, immature granulocytes, atypical lymphocytes
Reticulocytes, immature platelet fraction	Measured by RNA stain	Marrow response

Typical adult reference ranges

Ranges vary by lab and population; use the one on the report.

Test	Typical adult range
Haemoglobin	Men 130 to 180 g/L; women 115 to 165 g/L (WHO anaemia: under 130 men, under 120 non-pregnant women)
MCV	80 to 100 fL
MCHC	310 to 360 g/L
White cells	4.0 to 11.0 x 10^9/L
Neutrophils	2.0 to 7.5 x 10^9/L
Lymphocytes	1.0 to 4.0 x 10^9/L
Eosinophils	Under 0.5 x 10^9/L
Platelets	150 to 400 x 10^9/L
Reticulocytes	About 20 to 100 x 10^9/L

Step by step

1. Sample. Was it clotted, haemolysed, lipaemic, drawn from a drip arm, or delayed? Is the result plausible next to the last one?
2. Which lines? One lineage suggests a peripheral cause (bleeding, immune destruction, drugs). Two or three lineages suggest the marrow, the spleen or a systemic process (sepsis, DIC, TMA).
3. Red cell indices. MCV sorts anaemia; the reticulocyte count says whether the marrow is responding (see Anaemia).
4. Absolute differential. Name the cell that is high or low.
5. Trend. Old FBCs turn an alarming result into a stable one, or the reverse.
6. Film. Ask for one, and read the morphology comment.

Spurious results

Artefact	What you see	Fix
EDTA-dependent platelet clumping	Low platelets, no bleeding, clumps on film	Repeat in citrate tube
Cold agglutinins	High MCV, low RCC, MCHC over 370 g/L	Warm sample to 37 degrees
Lipaemia, high bilirubin	High Hb and MCHC	Plasma replacement or manual correction
Very high white count	Falsely high Hb and MCV; fragments counted as platelets	Film and manual counts
Nucleated red cells	Overcounted lymphocytes on older analysers	Corrected white count
Delayed processing	MCV and Hct rise as cells swell	Fresh sample
Drip-arm dilution	All lines low; biochemistry shows the infusate (very high glucose with dextrose, high sodium and chloride with saline, high potassium if it is in the bag)	Redraw from the other arm

• Pitfall: transfusing platelets for EDTA pseudothrombocytopenia. Look at the film first.

The film

Finding	Think of
Pencil cells, hypochromia	Iron deficiency
Target cells	Thalassaemia, liver disease, hyposplenism, HbC
Spherocytes	Warm autoimmune haemolysis, hereditary spherocytosis, burns
Schistocytes (fragments)	TTP, HUS, DIC, malignant hypertension, mechanical valve
Bite and blister cells	Oxidative haemolysis, G6PD deficiency
Teardrop cells with leucoerythroblastic change	Myelofibrosis, marrow infiltration
Rouleaux	Myeloma, high immunoglobulins, inflammation
Howell-Jolly bodies	Hyposplenism
Oval macrocytes, hypersegmented neutrophils (6 lobes, or over 5% with 5)	B12 or folate deficiency, drugs
Hypogranular neutrophils, pseudo-Pelger cells	MDS
Blasts, Auer rods	Acute leukaemia. Auer rods mean myeloid
Smear (smudge) cells	CLL
Toxic granulation, Döhle bodies, left shift	Infection, inflammation, G-CSF
Atypical (reactive) lymphocytes	EBV, CMV, HIV seroconversion, toxoplasmosis, drugs
Malaria parasites	Febrile traveller: thick and thin films plus rapid antigen test

• Schistocytes: ICSH 2021 counts them as a proportion of red cells. Over 1%, as the main abnormality, supports thrombotic microangiopathy in adults. A few fragments are normal, and automated fragmented red cell counts can only screen.

Immune and drug effects on the white count

• Corticosteroids: neutrophilia within hours, with lymphopenia and eosinopenia. A white count of 15 x 10^9/L in a patient on prednisolone may not mean infection.
• Eosinophilia over 0.5 x 10^9/L: allergy, asthma, drugs (DRESS), parasites (Strongyloides in people from endemic areas, including some remote Aboriginal communities and refugees), EGPA, Hodgkin lymphoma, adrenal insufficiency. Over 1.5 x 10^9/L persistently: look for organ damage (Leucocytosis).
• Lymphopenia: HIV, steroids, lymphoma, SLE, sarcoidosis, chemotherapy, viral infection. A low CD4 count, not the lymphocyte count, guides HIV prophylaxis.
• Neutropenia in people of African or Middle Eastern ancestry may be the Duffy-null associated neutrophil count: no excess infection risk, and it shouldn't exclude people from trials or prompt dose cuts (Hibbs 2024).

Worked examples

1. Two microcytic patients

	Patient A	Patient B
Hb (g/L)	92	118
RCC (x 10^12/L)	3.9	6.1
MCV (fL)	70	64
RDW	High	Normal
Mentzer (MCV/RCC)	18	10.5

• A: iron deficiency pattern. Ferritin and a search for the cause (Iron studies).
• B: mild anaemia with a high red cell count and a Mentzer index under 13: thalassaemia trait. Haemoglobin HPLC, and check ferritin, since iron deficiency lowers HbA2 (Thalassaemia).

2. Pancytopenia

A 66-year-old has Hb 84 g/L, MCV 104 fL, white cells 1.9 x 10^9/L (neutrophils 0.6), platelets 41 x 10^9/L. The film shows hypogranular neutrophils and 3% blasts.

• Three lines down with dysplasia and blasts points to the marrow: MDS or evolving AML. Check B12, folate and copper, then bone marrow with cytogenetics (Myelodysplastic syndromes).

3. An isolated low platelet count

A well 30-year-old has platelets of 38 x 10^9/L on a pre-operative FBC, normal last year. Haemoglobin and white count are normal. The film comments on large platelet clumps.

• EDTA pseudothrombocytopenia. Repeat in a citrate tube. No treatment, and note it in the record so it isn't rechecked at every admission.

Special populations

• Pregnancy: haemoglobin under 110 g/L is anaemia (under 105 g/L in the second trimester, WHO 2024). White count rises to about 15 x 10^9/L. Platelets fall mildly.
• Older people: a mild normocytic anaemia still needs explaining. MDS and CKD are common causes.
• Aboriginal and Torres Strait Islander people: iron deficiency, hookworm and Strongyloides eosinophilia in some remote communities; HTLV-1 related lymphocytosis in central Australia.

Pitfalls

• Reading a percentage differential instead of the absolute count.
• Missing a falsely high MCHC that signals cold agglutinins or lipaemia.
• Treating EDTA pseudothrombocytopenia.
• Not asking for a film when two or three lines are abnormal.
• Calling a steroid neutrophilia sepsis, or a Duffy-null neutrophil count drug-induced neutropenia.
• Ignoring a high RDW with a normal MCV (combined iron and B12 deficiency).