Counting methods for low concentrations of trypanosomes in blood: I. Drop method

Abstract

This paper introduces a new counting method for trypanosomes varying in concentrations from 25,000 to 10 per mm 3 of blood: the so-called drop method. A measured quantity (1 mm 3) of infected blood is dropped on a hemacytometer cover slip and a second hemacytometer cover slip is placed horizontally on the drop, so that the blood flows out evenly. Then the area of the flattened blood drop is measured by placing the preparation on plotting paper. Next, part of the preparation is examined under a microscope with a square of known area in the ocular eyepiece, and the trypanosomes are counted upto a given number of squares. The concentration is estimated by calculating the number of trypanosomes in the whole preparation (1 mm 3). Experiments carried out on rats infected with Trypanosoma rhodesiense showed that the drop method is rather accurate. However, drop method counts always proved to be lower than hemacytometer counts. It was figured out how this deviation can be adjusted; also a calculation for the 95% confidence limits of 1 drop method count is given.