Home Economics American Trypanosomiasis Chagas Disease, Second Edition: One Hundred Years of Research
Other direct tests
The dry smear as for differential count has a much lower sensitivity, and will be positive only when a large number of parasites are present, equivalent to more than one parasite per field with the fresh smear. It is not recommended because of that. The thick smear, as used for malaria diagnosis, also has lower sensitivity but is better than the dry smear. The morphology of the parasite is generally not well preserved, but is an excellent option when used in the field in malaria regions. If negative for hematozoan, the presence of a flagellate could be diagnostic. Many otherwise nondiagnosed cases were found by malaria control program personnel, after proper training, in Brazil and other countries.
Two are most used: the Strout technique and microhematocrit.
The Strout51 technique is very simple. Centrifuge tubes, a centrifuge, and a microscope will suffice. Blood (3—5 mL) is collected without anticoagulant and left to clot, at room temperature, or quicker, at 37°C. Once the clot is formed (15—60 min) the blood exudate is transferred with a pipette to a centrifugal tube and spun down at low speed (i.e., 50 g, 500 rpm according to the radius of the centrifuge) for 5 min. This will allow for the separation of most (but not all) red blood cells that remain at the bottom of the tube. Take all the supernatant (approximately 1 mL) and transfer to another centrifuge tube and spin hard (i.e., 500g, around 2000 rpm) for 10 min. This will clear the suspension, having a clear supernatant. Take nearly all the supernatant and store for serology. Now, with the last drop remaining at the bottom of the tube, resuspend it and apply 10 pL to a glass slide and cover slip, with the same methodology as the fresh blood smear explained above. Look on the microscope. Parasites are there. The rationale of this method is that all the parasites escape from the clot to the serum. By separating most of the red blood cells with the first spin, we get rid of them. By the second spinning, parasites are forced to remain at the bottom of the tube. Hint: Do not delay in preparing the smear after the centrifuge stops, otherwise the parasites will swim to the sera and will be lost.
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