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Effects of anticoagulant on malarial parasites All anticoagulants have some effect on the morphology of malaria parasites and the red blood cell they inhabit. This effect depends on:
- The stage of the parasite
- The time taken for the blood to reach the laboratory
- The type of anticoagulant used
Blood films should be prepared as soon as possible after the blood is taken. Potassium ethylene diamine tetra-acetic acid (EDTA) is the anticoagulant of choice, but if the blood is left at room
temperature for several hours in EDTA, the following sequestrene effects may be seen.
1. Gametocytes may continue to develop and male gametocytes can exflagellate, liberating gametes into the plasma. These can be mistaken for organisms such as Borrelia. However, these
gametes can be seen singly or as an entangled group and possess a central nucleus ,which distinguishes them morphologically from Borrelia species. 2. Gametocytes of Plasmodium falciparum
which have a characteristic crescent shape may round up and then resemble those of P. malariae. However, they can be differentiated from the gametocytes of P. malariae in that the latter
contain large amounts of scattered black pigment (haemozoin), the
chromatin is present as a compact mass in females and diffuse in males and they occupy most or all of the red cell with the red cell membrane sometimes not visible. Those of P. falciparum
exhibiting an EDTA effect are larger, heavily pigmented with yellow-brown rod shaped pigment and the red cell membrane is not visible.
3. Accolé forms (i.e. trophozoites seen on the edge of the red blood cell), which are characteristic of P. falciparum, may be seen in P. vivax
because of attempted reinvasion of the red blood cell by merozoites. These can be seen as round, compact structures sitting on the cell membrane. Trophozoites of P. falciparum
in an EDTA blood several hours old may continue growing and may displace the red cell membrane. This may resemble a bubble on the red cell membrane. True accolé forms of P. falciparum
are in line with the margin of the red cell. 4. Mature trophozoites of P. vivax
may condense when exposure becomes prolonged and in cases of very long exposure, red blood cells containing gametocytes and mature schizonts may be totally destroyed along with the contained parasites. The malaria pigment, haemozoin, always remains and can provide a clue to the presence and, to an expert eye, identity of the parasite.
5. The morphology of the red blood cell may be altered by shrinkage or crenation.
These effects can be seen below.
In summary, films from blood collected in EDTA should be prepared as soon as it comes into the laboratory in order to minimise anticoagulant effect on malaria parasites. 
A rounded up gametocyte of
Plasmodium falciparum showinh haemazoin pigment 
An exflagellating gametocyte of Plasmodium falciparum |
