Dilutions or suspensions in serological tests are commonly made with a 0.85 percent (0.15M) solution of sodium chloride, usually referred to as physiological or isotonic saline, Thus, in blood grouping tests, erythrocytes of the donor (or recipient) are first suspended in a small amount of physiological saline and the suspension is then mixed with a serum known to agglutinate blood of one type or another. After centrifugation, a small button of sedimented cells appears at the bottom of the tube. Gentle shaking of the tube then reveals whether agglutination has or has not occurred. If the cells disperse and resuspend uniformly, the results are negative; if instead the button dislodges from the wall of the tube but remains in the form of one or more relatively large clumps which are not broken up by shaking, then agglutination has occurred and the results are positive.
While there are many variations of the above procedure, depending on the particular tests being conducted, the step of agitating an aggregation of cells to observe their tendency to resuspend in saline is common to most of them, whether such step takes place in a centrifuge tube or on a microscope slide. A problem frequently arises when the mass of cells neither disperses nor releases as an aggregate but instead adheres firmly to the glass surface. If cells remain in place despite such agitation, then the serologist cannot be certain whether the resistance to dispersion arises because of agglutination or simply because of the tendency of the cells to stick to the glass. On the other hand, if a probe or excessive agitation should be used to dislodge the button from the glass, and should the cells then resuspend, there may be uncertainty as to whether such resuspension occurred because of a true negative reaction or because the mechanical force applied to the button caused the agglutinated cells of a positive test to separate. Therefore, the attraction between blood cells and glass surfaces, a well known phenomenon in clinical labroatories, has created problems which have tended to complicate blood testing procedures and to effect their accuracy and reliability. Despite the long-standing nature of such problems, no effective solution has heretofore been developed.