There are five classes of normal white blood cells or leukocytes: neutrophils, lymphocytes, monocytes, eosinophils and basophils. It is a known medical diagnostic procedure to examine a dried, stained smear of blood on a microscope slide to determine the relative proportions of these five normal types of white blood cells, as well as the concentration of any abnormal cells. Such procedure is referred to as a differential white blood cell count and is described in Miale, J. B., "Laboratory Medicine - Hematology", pp. 822-830, 1126, 1127 and 1130, C. V. Mosby Company, St. Louis, Mo. (1967).
Recently, automated processes and automated flow system apparatus therefor have been developed to ease the burden of differential white blood cell counting, such as described in U.S. Pat. Nos. 3,741,875 and 4,099,917. These are cytochemical procedures to specifically identify and label individual cell types.
The procedure for preparing a cell suspension for use in such systems comprises treating an uncoagulated blood sample with a surfactant for about 1.5 minutes to precondition the red blood cells for lysis; thereafter adding a fixative to the cells for about 1 minute while maintaining a neutral pH; and incubating the mixture at 58.degree. C. to 60.degree. C. for about 2 minutes to lyse the red blood cells and fix the white blood cells, as described in U.S. Pat. No. 4,099,917. It is imperative in such processes that all of the red blood cells be lysed because the red blood cells outnumber the white blood cells by about 700 to 1. Because of this, even if one percent of the red blood cells remain unlysed, the differential white blood cell count cannot be accurately arrived at.
A major drawback to the prior art methods is the relatively extended period of time each analysis requires, e.g., as much as five miutes just to prepare the cell suspension, thereby rendering such methods undesirable for emergency sample analysis or for other situations in which rapid results are desirable. Accordingly, there is need for a method for the determination of a differential white blood cell count which is relatively rapid as compared to the prior art procedures. Such a method would have to completely lyse the red blood cells in the sample without damaging the white blood cells, causing no extra-cellular precipitation or clumping of cells, since such precipitates or cell clumps could generate ambiguities in the cell detecting and recognizing phase of the process.