Direct Oral Anticoagulants in the Cardiorenal Patient: Pharmacology, Kidney Function Estimation and Dose Selection Across the CKD Spectrum.

Atrial fibrillation (AF) and chronic kidney disease (CKD) are not two coincident diseases but two expressions of the cardiorenal syndrome, the disorder in which dysfunction of the heart or the kidney induces dysfunction of the other. Their coexistence spans the chronic renocardiac and cardiorenal subtypes of that syndrome, and fits neither cleanly: the relationship is genuinely bidirectional, and each organ accelerates the deterioration of the other. Anticoagulation sits precisely at the point where this bidirectional relationship becomes a therapeutic dilemma: the same patient is simultaneously hypercoagulable and haemorrhagically fragile, and the drug intended to protect the brain is cleared by an organ whose function is both impaired and unstable. Direct oral anticoagulants (DOACs) have redefined therapeutic standards in AF and venous thromboembolism (VTE), offering predictable pharmacokinetics, fewer food and drug interactions and reduced monitoring requirements compared with vitamin K antagonists (VKAs). In the cardiorenal patient, however, these advantages are conditional. Renal elimination varies more than three-fold across the class-from approximately 27% for apixaban to approximately 80% for dabigatran-so that the choice of molecule, and not merely the decision to anticoagulate, becomes a kidney-related decision. The cardiorenal argument in favour of DOACs extends beyond bleeding: VKAs promote vascular calcification through inhibition of matrix Gla protein, are associated with calciphylaxis, and can cause anticoagulant-related nephropathy, an under-recognised cause of accelerated loss of glomerular filtration rate (GFR). Randomised and observational data indicate that GFR declines faster on VKAs than on DOACs, making the kidney an outcome of anticoagulation and not only a route of elimination. A second, largely unresolved problem is metrological rather than pharmacological. Dose thresholds for DOACs were derived from creatinine clearance (CrCl) estimated with the Cockcroft-Gault (CG) equation, whereas contemporary nephrology stages kidney disease with estimated GFR (eGFR) from the CKD-EPI or EKFC equations. The two quantities are not interchangeable, and the discordance between them translates directly into discordant doses in 4-34% of patients depending on the agent, with the largest discrepancies in the oldest patients and in those with the lowest measured function. This review reframes DOAC therapy as a cardiorenal problem. We examine the shared pathophysiological substrate of AF and CKD, the pharmacology of each agent through the lens of renal handling, the evidence across CKD stages G1-G5D including dialysis, the kidney as an outcome rather than only a covariate of anticoagulation, and the clinical consequences of the equation used to estimate kidney function at the point of prescription.