UK researchers are urging a switch to the Cockroft-Gault equation to estimate renal function in patients prescribed the newer oral anticoagulants.
They say use of the MDRD calculator – currently used to estimate eGFR – could lead to the new drugs being overprescribed in elderly patients, putting them at risk of major bleeding.
In a study published in BMJ Open, the team looked at the records of over 4,000 patients with atrial fibrillation.
They found that 15% of patients aged 80 and over who would be ineligible for treatment with dabigatran based on the Cockroft-Gault equation – the standard for determining drug eligibility and dosing in clinical trials of the new oral anticoagulants – would become eligible for the drug based on the MDRD calculation. For rivaroxaban, 0.3% would have been incorrectly judged eligible for treatment.
Among the under-80s, 5.3% of eligible patients would be given too high a dose of dabigatran based on the MDRD, while 13.5% would get too high a dose of rivaroxaban.
The team concluded: ‘Were the MDRD-derived eGFR to be used instead of Cockcroft-Gault in prescribing these new agents, many elderly patients with atrial fibrillation would either incorrectly become eligible for them or would receive too high a dose. Safety has not been established using the MDRD equation, a concern since the risk of major bleeding would be increased in patients with unsuspected renal impairment.’
They added: ‘Given the potentially widespread use of these agents in primary care, regulatory authorities and drug companies should alert UK doctors of the need to use the Cockroft-Gault formula to calculate eligibility for and dosing of the new oral anticoagulants in elderly patients with atrial fibrillation and not rely on the MDRD-derived eGFR.’
Professor David Fitzmaurice, head of primary care clinical sciences at the University of Birmingham, said: ‘From a primary care perspective, we simply need to remember that renal function is important and that these new agents remain potent anticoagulants. Care needs to be taken at low renal function where actually warfarin remains the drug of choice.’
Dr Chris Arden, GPSI in cardiology and a GP in Hampshire, said the study ‘highlights the potential for overestimating renal function in a small, but significant number of patients – particularly the elderly’.
He said: ‘In practice the key cohort we need to focus on are patients with borderline eGFR (thresholds of eGFR 30–40 mL/min per 1.73 m2), patients over 80 years old, and those under 60 kg in weight.
‘In these groups we do need to pay special attention when prescribing predominately renally excreted drugs, including particular anticoagulants, with caution and would agree with the authors that using the Cockcroft-Gault equation – which requires the inclusion of weight – in these at-risk groups would better allow both appropriate dosing (avoiding under-dosing) and minimise the potential for over anticoagulation.’