Counting of Cells in Urine

Counting of blood cells in urine is most accurately performed by means of a haemocytometer. The errors of the method are related to technique, observer inaccuracy and uneven distribution of cells in the haemocytometer. Leucocytes and non‐squamous epithelial cells in urine, when present in small numbers, have been shown to be distributed in the haemocytometer approximately according to a Poisson distribution. When leucocytes are present in large numbers “mucus” and clumping of cells may interfere with the random distribution. In quantitative estimation of cells in urine the common practice is to count the cells on a certain number of squares and to express the results as the number of cells per unit volume of urine (mm3, ml), or by the excretion rate (number of cells excreted per minute, hour, 12 or 24 hours). The standard error of this estimate, expressed by the confidence interval, must be based on the number of cells counted, and not on this number multiplied by a factor. It is assumed that the simplest and most convenient method of quantitative estimation of cells in urine is to count the cells in 1 mm3 of uncentrifuged urine. When the number of cells per unit volume is low, the confidence limits for the true number of cells per volume urine in a Poisson distribution may be obtained from statistical tables. When the number of cells is larger than about 40 per mm3, an estimate of the standard error of the count of a single sample cannot be obtained, since the distribution may not be random. In routine work information of sufficient accuracy may be achieved by examination of a convenient volume containing 50–100 cells, the number of cells per mm3 being calculated on the basis of this count.