DFP32, used to label erythrocytes in vitro, combines with cell constituents in two stages, the first almost immediate and involving tributyrinase inactivation, the second slower (more than 40 minutes) involving cholinesterase inactivation. Raising the DFP concentration increases the amount irreversibly bound, but increases even more the immediate post-transfusion elution, and DFP is unsuited for investigating erythrocyte viability of stored samples. In vivo tagging by intramuscular injection is satisfactory and normal survival curves are linear since the sample tagged has normal age distribution of cells in absence of random destruction. Here DFP32 curves are easier to interpret than Cr51 curves. In sheep, chromium elution occurs at two different rates producing a rapid initial drop followed by a slower one of about 3 per cent daily.

Random destruction alters cell age distribution. New equations are derived for cases in which this is constant both with and without chromium elution; they were applied satisfactorily to dog and sheep blood. Analysis of such curves is difficult; approximate values for random destruction rates can be obtained though not potential life spans. Chromium curves can be analyzed only with the help of DFP32 or similar curves, and yield little additional information. DFP32 and chromium can be used simultaneously to provide controls.

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