Thick film interpretation

OVERVIEW OF THICK FILMS

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A thick film is prepared by placing a small drop of blood on a slide then spreading it in a circular motion. The thick layer acheived is then air-dried without fixation.

The principles are:

• The blood will therefore be many layers thick (around 6-20) compared with the single layer of a thin film
• The erythrocytes are unfixed so will be lysed during staining appearning only as debris.
• The Giemsa stain will therefore stain and distinguish the remaining white cells, parasites.
• This allows parasites to be detected with high sensitivity using fewer microscopic fields

All these features allow thick films to be highly sensitive for parasite detection, but also introduce difficulties for morphologists. Principally staining inconsistencies as stain variable penetrates the thick cell layer and disruption of parasites caused by drying and shrinkage, finally the technique disrupts or eliminates red cells that provide valuable clues to species. Many skills are transferable from thin to thick film specimens but the differences require microscopists to be aware of potential artefact and to have experience of thick blood films. Typical appearances of a case of P.falciparum with easily detected trophozoites are shown below.

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  Low magnification view for scale, 3 regions marked

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  Region A: a single disrupted trophozoite

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  Region B: 5 trophozoites in three group

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  Region C: 2 trophozoites in one group

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Note the differences in recognition - the typical ring form and vacuole of the parasite are not as easy to distinguish and chromatin dots may appear to separate from parasite cytoplasm while the absence of intact red cells takes away important clues to parasite size and distribution.

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Some strengths and weaknesses of each approach are summarised in the table below:

COMPARISON OF THICK AND THIN FILMS
Feature	Thick Film	Thin Film
Sensitivity for detection	Higher: detects low parasitaemia ~5–10 parasites/µL	Lower: generally needs ~50 parasites/µL for reliable detection)
Species Identification	Poor: RBC morphology lost and species-specific features may be difficult	Excellent: Parasite morphology and RBC characteristics are readily observed
Quantification of parasitaemia	Difficult: requires estimation so is imprecise	Easier: parasites can be counted per number of RBCs
Preparation and staining	Longer: requires air drying before careful staining to avoid artefact	Faster: films are fixed and stained immediately with clearer morphology

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