PathologyLabTraining

Blood Film Preparation, Staining and Morphology: A Training Guide for Biomedical Scientists

11 min read · Published: 2025-01-08 · Updated: 2026-06-08

The peripheral blood film remains one of the most powerful, inexpensive and underrated investigations in haematology. A well-made, well-stained film examined by a competent biomedical scientist (BMS) can reveal acute leukaemia, malaria, microangiopathic haemolytic anaemia or a spurious analyser count in minutes. This guide walks NHS trainees and registered staff across Bands 2 to 8 through preparing, staining and systematically examining a blood film, recognising red cell, white cell and platelet morphology, and applying current ICSH and British Society for Haematology (BSH) guidance on when to make and review films.

Why the Blood Film Still Matters

Modern haematology analysers count and characterise cells with great precision, but they cannot replace human morphological assessment. The analyser produces numbers and flags; the film provides the picture. Even in fully automated laboratories using digital morphology systems such as CellaVision, a proportion of samples will always require a manual film and expert review.

Films are made to investigate analyser-generated flags, to follow up abnormal full blood count (FBC) parameters, and in response to specific clinical requests. The International Council for Standardization in Haematology (ICSH) and the BSH have published guidance to make this process consistent, reproducible and clinically meaningful. Working to a recognised standard is also an expectation under ISO 15189:2022, the international standard against which UKAS (United Kingdom Accreditation Service) assesses medical laboratories.

Preparing the Blood Film

A good film is the foundation of accurate morphology. Errors at the preparation stage cannot be corrected at the microscope.

Sample requirements:

The wedge (spread) technique — step by step:

1. Place a small drop of well-mixed blood (approximately 2-3 mm diameter) about 1 cm from the end of a clean, grease-free glass slide. 2. Hold a spreader slide at an angle of around 30-45 degrees and draw it back into the drop, allowing blood to spread along its edge by capillary action. 3. Push the spreader smoothly and steadily forward in a single, even motion. 4. Aim for a film roughly 3-4 cm long with a smooth, tongue-shaped tail.

Adjusting the film: A higher spreader angle, faster push or larger drop produces a thicker film; the opposite produces a thinner one. Anaemic (low haematocrit) samples need a steeper angle or faster spread, while polycythaemic samples need a shallower angle or slower spread to achieve an even monolayer.

Features of a well-made film:

Allow films to air-dry rapidly and completely. Slow drying in humid conditions causes red cell artefact (drying or "water" artefact resembling target cells or echinocytes).

Romanowsky Staining

Blood films are stained using Romanowsky stains, which combine a basic dye (azure B) and an acidic dye (eosin Y) to give the characteristic polychromatic effect. The ICSH reference method, based on purified azure B and eosin Y, underpins the modern understanding of how these dyes work, and most NHS laboratories use ready-made commercial variants such as May-Grünwald-Giemsa (MGG) or Wright-Giemsa, applied manually or by automated stainers.

The chemistry in brief:

Optimal staining shows:

| Structure | Expected appearance | |-----------|---------------------| | Red cell cytoplasm | Pink to orange-red | | Nuclei (lymphocytes, neutrophils) | Purple to blue-purple | | Neutrophil granules | Fine, lilac/neutral | | Eosinophil granules | Bright orange-red | | Basophil granules | Dark blue-purple | | Platelets | Purple granules, pale blue cytoplasm |

Common staining faults and causes:

Correct buffer pH (typically around 6.8) is critical for reproducible results, which is why pH-controlled buffered water is used rather than tap water. Staining quality is monitored through internal quality control and external quality assessment such as UK NEQAS schemes.

When to Make and Review a Film: ICSH and BSH Criteria

Not every FBC needs a film, but missing the right ones is a clinical safety issue. Two complementary frameworks guide practice.

Analyser flags and review criteria (ICSH/ISLH consensus): The International Consensus Group for Hematology Review published 41 rules defining when to take action — including blood film review — following automated FBC and white cell differential analysis. These cover both first-time samples and delta-check rules for repeat samples within a defined interval. Each NHS laboratory adapts these into validated, locally verified rules within its analyser middleware and standard operating procedures, as required under ISO 15189:2022.

Typical triggers for making a film include:

Significant abnormalities and onward referral (BSH): BSH guidance on significant haematological abnormalities found at the FBC and film helps standardise what constitutes a finding requiring urgent communication or specialist referral — for example, suspected acute leukaemia, blasts, marked unexplained cytopenias, or features of microangiopathic haemolysis. Knowing your local critical-result and escalation policy, and documenting actions, is part of safe practice.

Systematic Examination of the Film

A disciplined, repeatable routine prevents missed findings. Examine every film the same way.

1. Low power (x10): Assess overall quality, staining, cell distribution and platelet aggregates. Scan the tail and edges for large abnormal cells, parasites, rouleaux or fibrin. Select the correct reading area. 2. High power (x40): Form an overview of red cell, white cell and platelet morphology and confirm cellularity. 3. Oil immersion (x100): Perform detailed morphology, the differential count and inclusion identification.

A morphology comment should always be correlated with the FBC indices and, where possible, the clinical details. Morphology is interpreted in context, not in isolation.

Recognising Red Cell, White Cell and Platelet Morphology

Red cells (erythrocytes) — assess size, shape, colour and inclusions:

White cells (leucocytes) — perform the differential and note:

Platelets — assess number and morphology:

Standardised Morphology Grading

To make reporting consistent between staff and laboratories, the ICSH recommendations for the standardisation of nomenclature and grading of peripheral blood cell morphological features provide an agreed vocabulary and a semi-quantitative grading scheme. Findings may be reported as present/absent, by simple description, or graded.

The ICSH approach grades abnormalities by the percentage of affected cells, broadly as:

| Grade | Approximate proportion of affected cells | |-------|------------------------------------------| | Normal / within reference | Below ~5% | | Mild (1+) | ~5-25% | | Moderate (2+) | ~25-50% | | Marked (3+) | Above ~50% |

For reliable percentages, a sufficient number of cells must be assessed; counting around 1000 red cells gives a precise estimate of a given abnormality, consistent with statistical distribution principles. Some findings warrant separate, more rigorous quantitation — for example, ICSH recommendations on schistocytes set a confident reporting threshold of around 1%, with the schistocyte percentage having defined diagnostic value in thrombotic microangiopathy. Using consistent nomenclature and grading supports comparability, EQA performance, and clear communication with requesting clinicians.

Quality, Competence and Documentation

Morphology is a registrant skill that must be maintained. Under HCPC (Health and Care Professions Council) standards and IBMS (Institute of Biomedical Science) frameworks, staff performing and authorising film reports work within defined competencies. Good practice includes:

Continuing professional development — reviewing teaching slides, attending morphology updates and discussing cases with colleagues — keeps recognition skills sharp throughout a career.

Frequently Asked Questions

What angle should I use to spread a blood film?

A spreader angle of about 30-45 degrees suits most normal samples. Increase the angle (or push faster) for anaemic samples with a low haematocrit, and decrease it (or push more slowly) for polycythaemic samples. The aim is always an even monolayer with a feathered tail in the reading area.

Why is buffer pH so important in Romanowsky staining?

The azure B and eosin Y dyes are pH-sensitive. A buffer that is too alkaline pushes the film towards blue (basophilic) staining, while a too-acidic buffer makes it appear too pink (eosinophilic). A controlled, buffered pH around 6.8 gives reproducible colour balance, which is why pH-stabilised buffer rather than tap water is used.

Which analyser flags should prompt a manual blood film?

Common triggers include flags for blasts, atypical or variant lymphocytes, immature granulocytes, nucleated red cells, platelet clumps, plus new or unexplained cytopenias, marked leucocytosis or thrombocytosis. Each laboratory builds locally validated rules from the ICSH/ISLH consensus 41-rule framework, embedded in its analyser middleware and SOPs.

How does ICSH grading of morphology work?

ICSH recommends consistent nomenclature and a semi-quantitative grade based on the proportion of affected cells, typically mild (1+, ~5-25%), moderate (2+, ~25-50%) and marked (3+, above ~50%), with normal generally below ~5%. Assessing around 1000 cells gives a reliable percentage. Some abnormalities, such as schistocytes, have specific quantitation thresholds.

What causes spurious thrombocytopenia on a film?

Platelet clumping and platelet satellitism, often induced by EDTA, can cause the analyser to under-report platelets. The film reveals the clumps. A sample collected into citrate (with appropriate correction) usually resolves the discrepancy and gives a true count.

Do digital morphology systems remove the need for manual film skills?

No. Systems such as CellaVision pre-classify cells and improve efficiency, but a competent BMS must verify and reclassify cells, interpret findings in clinical context, and handle samples the system cannot resolve. Manual morphology skill and EQA participation remain essential.

Further training

Continue building your haematology and quality skills with these PathologyLabTraining resources and the wider NHS Laboratory Training pillar:

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