Microscopic Diagnosis of Malaria

Microscopic Diagnosis of Malaria

The direct microscopic visualization of the parasite on the thick and/or thin blood smears has been the "gold standard" for malaria diagnosis.

Thick Blood smear
Thick smears consist of a thick layer of dehemoglobinized (lysed) red blood cells (RBCs). Thus, thick smears allow a more efficient detection of parasites (increased sensitivity). However, they do not permit an optimal review of parasite morphology.
Allow the smear to dry thoroughly. Insufficiently dried smears (and/or smears that are too thick) can detach from the slides during staining. You can accelerate the drying by using a fan or hair dryer. Do not fix thick smears with methanol or heat. If there will be a delay in staining smears, dip the thick smear briefly in water to hemolyse the RBCs.

Thin Blood Smear:
Thin smears consist of blood spread in a layer such that the thickness decreases progressively toward monolayer. Fix the smear by dipping them in absolute methanol.

Microscopic examination:
First screen the thick/thin smear at a low magnification (10× or 20× objective lens), to detect large parasites(microfilaria) then examine the smear using oil immersion objective. NCCLS recommend examination of at least 300 oil immersion fields for the determination of "No Parasite Seen".

Thin Smear Showing Plasmodium Falciparum

Diagnostic Points for Plasmodium falciparum

1. Red Cells are not enlarged.
2. Rings appear fine and delicate and there may be several in one cell.
3. Some rings may have two chromatin dots.
4. Presence of marginal or applique forms.
5. It is unusual to see developing forms in peripheral blood films.
6. Gametocytes have a characteristic crescent shape appearance. However, they do not usually appear in the blood for the first four weeks of infection.
7. Maurer's dots may be present.

Diagnostic Points for P. vivax

1. Red cells containing parasites are usually enlarged.
2. Schuffner's dots are frequently present in the red cells as shown above.
3. The mature ring forms tend to be large and coarse.
4. Developing forms are frequently present.