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APPENDIX: Microscopic Procedures for Diagnosing Malaria
To establish the diagnosis of malaria, a blood smear must be prepared from
fresh blood obtained by pricking the finger (Figures A--1 and A--2).* The thin smear is
fixed in methanol before staining; the thick smear is stained unfixed. Many hospitals
have a Wright-Giemsa stain available, which is acceptable; however, Wright stain
alone will not reliably show Plasmodium parasites. For best results, the smear should
be stained with a 3% Giemsa solution (pH of 7.2) for 30--45 minutes. In
P. falciparum infections, the parasite density should be estimated by counting the percentage
of red blood cells infected --- not the number of parasites --- under an oil
immersion lens on a thin film.
Thick blood smears are more sensitive in detecting malaria parasites because
the blood is concentrated, allowing a greater volume of blood to be examined.
However, thick smears are more difficult to read, and thin smears may be preferred
by laboratories that have limited experience.
Plasmodium parasites are always intracellular, and they demonstrate, if stained correctly, blue cytoplasm with a
red chromatin dot. Common errors in reading malaria smears are caused by
platelets overlying a red blood cell, concern about missing a positive slide, and
misreading artifacts as parasites. Persons suspected of having malaria but whose blood
smears do not show the presence of parasites should have blood smears
repeated approximately every 12--24 hours for 3 consecutive days. If smears remain
negative, then the diagnosis of malaria is unlikely.
For rapid diagnosis, make thick and thin smears on separate slides. Air dry the
thin film, fix it with methyl alcohol, and immediately stain it. If no parasites are found
on the thin film, wait until the thick film is dry and examine it for organisms that
might not have been detected on the thin preparation.
* In Figures A--1 and A--2, the hands are shown ungloved to better illustrate their
placement during the procedures. However, wearing gloves while processing blood specimens
is recommended to prevent transmission of bloodborne pathogens
(MMWR 1988;37:377--82, 387--8 and
MMWR 1987;36[No. S2]).
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