Publication date: Dec 01, 2024

Background: Several studies have examined INR fluctuations using pharmacokinetic (PK) models or post-hoc INR values after completing nirmatrelvir/ritonavir, but further study of the effects of the drug interaction with warfarin during treatment is necessary. Case Summary: Nirmatrelvir/ritonavir is largely utilized in the outpatient setting so data regarding INR trends in hospitalized patients on warfarin is limited. However, many who receive nirmatrelvir/ritonavir outpatient experience difficulty with presenting to clinic for INR checks due to feeling acutely ill along with isolation precautions. We present the case of a patient receiving warfarin and utilizing home INR testing for monitoring. After diagnosis of coronavirus disease of 2019 (COVID-19), she was started on nirmatrelvir/ritonavir on day five after testing positive. Most recent INR prior to the start of therapy was 2. 7 and had been stable on the same dose for months prior to infection. On day two of nirmatrelvir/ritonavir, her INR rose to 4. 0 on home point of care INR testing. Despite reducing her dose of warfarin by 15%, her INR remained supratherapeutic the day after completing nirmatrelvir/ritonavir (4. 0) and for several checks after. One month after completion of therapy, her INR returned to therapeutic levels. Practice Implications: While PK models and case series have hypothesized both potential increases or decreases in INR with the nirmatrelvir/ritonavir and warfarin interaction, COVID-19 infection itself can cause several pharmacodynamic changes which can increase INR, including decreased appetite and, in severe cases, organ dysfunction. This case provides real-world insight into the drug interaction between nirmatrelvir/ritonavir and the drug-disease state interaction between warfarin and COVID-19.

Semantics

Original Article