Blood film
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[[Image:Blood_film_01.jpg|right|thumb|200px|Blood films, [[Giemsa]] stained]]
A '''blood film''' or '''peripheral blood smear''' is a [[microscopic slide|slide]] made from a drop of [[blood]], that allows the cells to be examined microscopically. Blood films are usually done to investigate [[hematology|hematological]] problems (disorders of the blood itself) and, occasionally, to look for [[Apicomplexa|parasites]] within the blood such as [[malaria]] and [[filariasis|filaria]].
==Preparation==
Blood films are made by placing a drop of blood on one end of a slide, and using a ''spreader slide'' to disperse the blood over the slide's length. The aim is to get a region where the cells are spaced far enough apart to be counted and differentiated.
The slide is left to air dry, after which the blood is [[fixation (histology)|fixed]] to the slide by immersing it briefly in [[methanol]]. The fixative is essential for good staining and presentation of cellular detail. After fixation, the slide is [[staining (biology)|stained]] to distinguish the cells from each other.
===Common blood film staining methods===
* [[Romanowsky stain]]
** [[Giemsa stain]]
** [[Wright's stain]]
** [[Jenner's stain]]
** [[Leishman stain]]
** [[Field's stain]]
===Disorders===
Characteristic red blood cell abnormalities are [[anemia]], [[sickle cell anemia]] and [[spherocytosis]]. Sometimes the microscopic investigation of the red cells can be essential to the diagnosis of life-threatening disease (e.g. [[Thrombotic thrombocytopenic purpura|TTP]]).
White blood cells are classified according to their propensity to stain with particular substances, the shape of the nuclei and the granular inclusions.
[[image:bloodfilmcll.jpg|left|thumb|200px|A [[microscopic]] view of an abnormal blood film.]]
* [[Neutrophil granulocyte]]s usually make up close to 80% of the [[white count]]. They have multilobate nuclei and lightly staining granules. They assist in destruction of foreign particles by the [[immune system]] by [[phagocytosis]] and [[intracellular killing]].
* [[Eosinophil granulocyte]]s have granules that stain with [[eosin]] and play a role in [[allergy]] and parasitic disease. Eos have a multilobate [[nucleus]].
* [[Basophil granulocyte]]s are only seen occasionally. They are polymorphonuceated and their granules stain dark with alkaline stains, such as [[haematoxylin]]. They are further characterised by the fact that the granules seem to overlie the nucleus. Basophils are similar if not identical in cell lineage to mast cells, although no conclusive evidence to this end has been shown. Mast cells are "tissue basophils" and mediate certain immune reactions to allergens. * [[Lymphocyte]]s have very little [[cytoplasm]] and a large [[cell nucleus|nucleus]] (high NC ratio) and are responsible for [[antigen]]-specific immune functions, either by [[antibodies]] ([[B cell]]) or by direct [[cytotoxicity]] ([[T cell]]). The distinction between B and T cells cannot be made by light microscopy.
* [[Plasma cell]]s are mature [[B lymphocyte]]s that engage in the production of one specific [[antibody]]. They are characterised by light [[basophilic]] staining and a very eccentric [[cell nucleus|nucleus]].
* Other cells are white cell [[wiktionary:Precursor|precursor]]s. When these are very abundant it can be a feature of [[infection]] or [[leukemia]], although the most common types of leukemia ([[chronic myeloid leukaemia|CML]] and [[chronic lymphocytic leukaemia|CLL]]) are characterised by mature cells, and have more of an abnormal appearance on light microscopy (it should be noted that additional tests can aid the diagnosis).
==Use in diagnosing malaria==
[[Image:Plasmodium_falciparum_02.jpg|thumb|280px|Blood smear from a ''P. falciparum'' [[Malaria culture|culture]] (K1 strain). Several red blood cells have ring stages inside them. Close to the center there is a schizont and on the left a trophozoite.]]
The preferred and most reliable diagnosis of malaria is microscopic examination of blood films, because each of the four major parasite species has distinguishing characteristics. Two sorts of blood film are traditionally used. Thin films are similar to usual blood films and allow species identification, because the parasite's appearance is best preserved in this preparation. Thick films allow the microscopist to screen a larger volume of blood and are about eleven times more sensitive than the thin film, so picking up low levels of infection is easier on the thick film, but the appearance of the parasite is much more distorted and therefore distinguishing between the different species can be much more difficult.<ref name="warhurst1996">{{cite journal | author=Warhurst DC, Williams JE | title=Laboratory diagnosis of malaria | journal=J Clin Pathol | year=1996 | volume=49 | pages=533–38 |pmid=8813948 | doi=10.1136/jcp.49.7.533}}</ref> From the thick film, an experienced microscopist can detect parasite levels down to as low as 0.0000001%. Microscopic diagnosis can be difficult because the early trophozoites ("ring form") of all four species look identical and it is never possible to diagnose species on the basis of a single ring form; species identification is always based on several trophozoites. Please refer to the chapters on each parasite for their microscopic appearances: ''[[Plasmodium falciparum|P. falciparum]], [[Plasmodium vivax|P. vivax]], [[Plasmodium ovale|P. ovale]], [[Plasmodium malariae|P. malariae]]''.
The biggest pitfall in most laboratories in developed countries is leaving too great a delay between taking the blood sample and making the blood films. As blood cools to room temperature, male gametocytes will divide and release microgametes: these are long sinuous filamentous structures that can be mistaken for organisms such as ''Borrelia''. If the blood is kept at warmer temperatures, [[wiktionary:schizont|schizonts]] will rupture and merozoites invading erythrocytes will mistakenly give the appearance of the accolé form of ''P. falciparum''. If ''P. vivax'' or ''P. ovale'' is left for several hours in EDTA, the build up of acid in the sample will cause the parasitised erythrocytes to shrink and the parasite will roll up, simulating the appearance of ''P. malariae''. This problem is made worse if [[anticoagulant]]s such as [[heparin]] or [[citrate]] are used. The anticoagulant that causes the least problems is [[EDTA]]. [[Romanovski's stain]] or a variant [[staining (biology)|stain]] is usually used. Some laboratories mistakenly use the same stain as they do for routine haematology blood films ([[pH]] 7.2): malaria blood films must be stained at pH 6.8, or Schüffner's dots and James's dots will not be seen.
==See also==
* [[codocyte|codocyte (target cell)]]
==References==
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[[Category:Blood tests]]
[[Category:Pathology]]
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