Filterability of blood cells: methods and clinical applications

It could be claimed that expansion in clinical aspects of haemorheology has largely been fueled by the development of a simple test of blood flow properties, i.e., analysis of filterability. With time, the level of sophistication in equipment, theory and sample preparation has increased. Theories for the development of flow in the filter have been described and these enable cellular parameters, such as transit times, to be calculated from experimental data. These theories can be quite general, and applied to filtration of red or white cells. Ideally, experimental design requires an understanding of cell behaviour at the filter and of the effects of factors such as the sample concentration and volume, and the presence of different types of cells or subpopulations. Otherwise, results are susceptible to misinterpretation, particularly if impurities or mixed populations of cells are present. It is thus very important to know accurately the constituents of the test suspension. In clinical applications the trend has been to move away from whole blood filtration, toward use of relatively pure suspension of separated red cells and white cells. In the area of red cell filtration this has led to reappraisal of some previously reported abnormalities. The relatively new study of white cell filtration should benefit from previous experience, but there is the added problem of the reactivity of the cells under test, and their mixed nature. In any case, critical evaluation of the meaning and clinical relevance of results is necessary.