Anticoagulation variability between centres: Implications for comparative prosthetic valve assessment

Abstract

One of the major determinants in the choice of a mechanical prostheticvalve is that valve's thromboembolic record but the thromboembolic (TE)rates may be substantially influenced by the levels of anticoagulationachieved. A detailed study of anticoagulation variability was undertaken in834 patients who received one or more of a particular prosthesis(Medtronic-Hall) in one centre during a 7-year period from 1979 to 1987,but who attended 27 different anticoagulant clinics spread over a widearea. In addition, a questionnaire was sent to all 89 practising cardiacsurgeons in the UK asking for their preferred range of InternationalNormalised Ratio (INR) for patients with mechanical prosthetic valves. Boththe local study (with 16,866 INR observations) and the nationalquestionnaire (with a 53% response) revealed an enormous amount ofvariability. Median INR values (semi- interquartile range) varied from 2.2to 3.9 (0.8-2.5) according to the anticoagulant clinic attended. The rangeof INR preferred by UK cardiac surgeons, but presumably not necessarilyachieved, varied from 1.8-2.2 to 3.0-4.8, with 64% of surgeons preferringan INR less than 3.0. In comparison, standard US practice is to maintainprothrombin times equivalent to INR values of 4.0-5.0. Unless anticoagulantpractice can be standardised internationally, comparison of TEcomplications between centres is meaningless, and casts doubt on thevalidity of TE rates quoted for particular prostheses, unless accompaniedby a detailed analysis of anticoagulant control.