Spontaneous agglutination of red cells with a positive direct antiglobulin test in various media

Red cells from 475 individuals (donors and patients) with a positive direct antiglobulin test (DAT) (66% were 1/2+ to 1+, 19% were 1 1/2+ to 2+, and 15% were 2 1/2+ or greater) were examined for spontaneous agglutination following incubation in various media. Twenty-three percent of the samples showed spontaneous agglutination in commercial Rh control solutions, 6 percent in 30 percent albumin, 3 percent in 10 percent albumin, 2 percent in 6 percent albumin, and 1 percent in saline. Cells suspended in serum were more prone to spontaneous agglutination. The strength of spontaneous agglutination varied; more than 50 percent of the samples reacted only 1+ or less, approximately 10 percent reacted 2+ or stronger. There was not a complete correlation with spontaneous agglutination and quantity of IgG on the cells, as determined by the antiglobulin test. Of those samples showing spontaneous agglutination, 50 percent were associated with a DAT strength of more than 2+, 27 percent with a DAT of 1 1/2+ to 2+, and 23 percent with a DAT of 1/2+ to 1+. Spontaneous agglutination occurred with the same frequency whether the cells were sensitized with both IgG and C3 or with IgG alone. Surprisingly, 7 percent of the samples sensitized with C3 but no detectable IgG also demonstrated spontaneous agglutination. Marked differences in reaction strength were seen with Rh control solutions from different manufacturers, and the degree of spontaneous agglutination was inconsistent with individual products.