A Mechanical Aid in Making Blood Films.

The apparatus which forms the subject of this paper was designed to eliminate some of the variable factors during the preparation of blood films by the slide method, and thus to reduce the errors of differential leucocyte counts. These errors can be grouped, according to their sources, into those arising during the preparation of the films due to a variety of technical factors, and into others inherent in every process of sampling, in which percentages of a total population are estimated from an analysis of small fractions. Efforts may justifiably be made to reduce the size of the first group of errors-; that of the second is irreducible. The distribution of leucocytes on blood films is irregular, there being a relative excess of cells along the edges and at the end, an excess to which the various cell classes contribute to a varying extent: the larger a leucocyte, the more likely is it to be carried to the edges or the end (Gyllensward, 1929). Since lymphocytes are the smallest leucocytes, there is thus a relative lymphocytosis in the centre, and a relative lymphopenia at the edges of the film. MacGregor, Richards, and Loh (1940) have shown that a " battlement " count, which takes in parts of the edges and the adjacent interior, largely obviates this source of error, and gives results closely approximating those of counts of all leucocytes in the film. Many factors, other than differences in cell size, may influence the quality of blood films and contribute to irregularities in the distribution of leucocytes. Thus the size of the drop of blood used, the speed of spreading, the width and nature of the spreading edge, the angle at which it is held, and the pressure applied, may all vary from observer to observer, and from occasion to occasion.t This problem has been analysed by Mainland, Coady, and Joseph (1936), who conclude that while it is impossible to incriminate any single factors, their combination leads to irregular spreading: " The variation . . . can be attributed to minor differences in cell size, and, perhaps, local variations in the current of the blood as it flows over the slide, which are uncontrollable factors." The most important result of the irregular leucocyte distribution is the fact tht a large number of cells have to be counted in order to ensure reasonable accur cy.