Page 38 - 2020 JSOM Winter

BINAXNOW RESULTS TABLE
Positive Positive Positive Negative
P. falciparum P. falciparum P. vivax
Mixed P. ovale
P. malariae
C

T1
T2

MICROSCOPY ◆ A thin blood smear is a drop of blood that is spread
When done correctly, microscopic examination of thick and across a large area of the slide. Thin blood smears help
thin blood smears is the most reliable test for malaria. Blood providers discover what species of malaria is causing the
smears are taken most often from a finger prick and a few infection.
drops of blood. Thick and thin blood smears allow direct vi- ◆ The two smears can work in tandem if a thick and thin
sualization of parasites and their reproductive derivatives— smear is made. This method allows the provider to ob-
schizonts in malaria. See U.S. Department of Defense. Special serve both thick and thin smears and find the blood
Operations Forces Medical Handbook. 2011. density that is most likely to yield visualization of the
parasite to the given observer.
A thick blood smear is a drop of blood on a glass slide. Thick ◆ If a high index of suspicion exists for malaria in a given
blood smears are most useful for detecting the presence of par- patient, but microscopy does not reveal an obvious ma-
asites, because they examine a larger sample of blood. (Often larial infection, serial thick and thin smears can be re-
there are few parasites in the blood at the time the test is done.) peated every 8 or 24 hours depending on the severity
of the case.

MAKING A THICK AND THIN SMEAR
Step 1 Step 2 Step 3 Step 4 Step 5
Bring a clean spreader Wait until the blood While holding the Wait until the smear is When the film is
slide, held at a 45° angle, spreads along the entire spreader slide at the same completely dry. Fix the completely dry, stain with
toward the drop of blood width of the spreader angle, push it forward thin film with 100% 7.5% Giemsa stain for
on the specimen slide. slide. rapidly and smoothly. (absolute) methanol. 15 minutes.

The thick and thin film should be air-dried, fixed with 100% highest concentration of RBCs in view, giving him or her the
(absolute) methanol, and allowed to dry before staining with highest probability of identifying the parasite. Move the slide
7.5% Giemsa stain for 15 minutes. Plasmodium parasites are along this plane of side-by-side RBCs to accurately rule out or
always intracellular, and they demonstrate, if stained correctly, in a malarial parasitic infection (Image 1). If the malaria par-
blue cytoplasm with a red chromatin dot (see images 1–4 be- asite is identified, begin appropriate anti-malaria/anti- parasite
low). Common errors in reading malaria films can be caused treatment immediately in conjunction with the antibiotic ther-
by platelets overlying a red blood cell, concern regarding miss- apies (minimum, better, best) mentioned above.
ing a positive slide, and misreading of artifacts as parasites.
The slide is best read by starting at the thin end of the slide MALARIA PARASITES
and moving it towards the thick side until the RBCs are side Images 2–4 demonstrate plasmodium within cells with the
by side, but NOT overlapping. This will give the observer the characteristic “signet ring” sign.

36 | JSOM Volume 20, Edition 4 / Winter 2020