Platelet inhibitory drugs: an in vivo method of evaluation in patients

The measurement of platelet deposition in human thrombi is essential for the evaluation of platelet-inhibitory drugs and prosthetic materials for use in patients. The rate of 111Indium-labelled platelet accumulation on Dacron arterial grafts was measured in 27 patients randomised to take either aspirin and dipyridamole (ASA + DPM) or placebo. Autologous platelets were labelled and re-injected seven days following surgery and the graft thrombogenicity index calculated as the daily rise in the ratio of emissions from the graft over a reference site. The mean (+/- SD) thrombogenicity index in 12 patients undergoing femoro-popliteal bypass was 0.25 +/- 0.09 on placebo and 0.16 +/- 0.07 on ASA + DPM started pre-operatively (p less than 0.05). Post-operative ASA + DPM therapy started two days following platelet labelling in 15 patients with aorto-femoral grafts also significantly reduce thrombogenicity to 0.12 +/- 0.05 compared with 0.25 +/- 0.08 on placebo (p less than 0.01). In the latter patients the ratio of emissions from the graft over reference fell significantly on starting ASA + DPM, suggesting a net loss of platelets from the graft. These results indicate that the rate of in vivo platelet accumulation on Dacron grafts can be quantitated and that ASA + DPM reduced this rate in man.