Failure in labelling of red blood cells with 99mTc: interaction between intravenous cannulae and stannous pyrophosphate.

The effect of different methods of stannous pyrophosphate (SnPYP) administration on a modified in vivo technique for labelling red blood cells (RBCs) with 99mTc has been studied. Retention of 99mTc in the bloodstream was measured and parallel in vitro RBC labelling experiments were performed. After administration of SnPYP or a mixture of SnPYP and heparin via the metal needles of intravenous cannulae, the mean 30-min retentions of 99mTc were 96.8% (SD 8.6) and 95.3% (SD 12.4) respectively and the mean in vitro labelling efficiencies were 86.9% (SD 5.0) and 81.9% (SD 18.2) respectively. When the SnPYP was administered via teflon cannulae, the mean 30-min retention of 99mTc was 40.0% (SD 16.7) and the in vitro labelling efficiency was 18.7% (SD 27.0). Reduced RBC labelling and rapid clearance of 99mTc from the bloodstream were not explained by adsorption of stannous ion or pyrophosphate onto the teflon cannulae or retention of SnPYP in the cannulae. When labelling RBCs with 99mTc it is important that SnPYP is not injected via a teflon cannula.