Warfarin 7mg, INR 9.2 — The CYP2C9/VKORC1 Trap
Zinda Synthesis
A 62-year-old Sri Lankan woman post-mechanical mitral valve replacement was started on warfarin 7mg daily per standard algorithm. By day 5, her INR was 9.2 — extreme anticoagulation with imminent bleeding risk. Pharmacogenomic testing revealed CYP2C9*3 heterozygosity and VKORC1 -1639 G>A homozygosity, predicting a maintenance dose of 2-3mg, not 7mg. Standard dosing algorithms missed her phenotype entirely.
Presentation
Post-op day 2, mechanical mitral valve replacement. Loaded with warfarin 7mg daily based on age, weight, and surgical indication. No genetic testing performed. INR checked daily.
Key Finding
INR rose from 1.1 (day 1) to 2.4 (day 3) to 5.8 (day 4) to 9.2 (day 5). Patient developed gum bleeding and bruising. Warfarin held, vitamin K 2.5mg PO administered. INR fell to 1.8 over 48 hours. Pharmacogenomic testing returned: CYP2C9*1/*3 (intermediate metabolizer), VKORC1 -1639 A/A (low warfarin requirement). Predicted maintenance dose by Gage algorithm: 2.5mg daily. Actual stable dose achieved: 2mg daily.
Intervention & Outcome
Warfarin restarted at 2mg daily with INR monitoring every 2 days. Stable INR 2.5-3.0 achieved at 2mg daily by day 14. Patient transitioned to a DOAC was not appropriate (mechanical valve). The case prompted institutional adoption of pharmacogenomic-guided warfarin dosing for all SA patients undergoing mechanical valve replacement.
First Principles
Warfarin acts by blocking vitamin K epoxide reductase (VKORC1), reducing functional clotting factor synthesis. VKORC1 -1639G>A reduces enzyme expression, requiring less warfarin to inhibit it. CYP2C9 metabolizes warfarin; *2 and *3 variants reduce metabolism, raising blood levels. Each variant reduces dose requirement; combinations are multiplicative. SA populations evolved with these alleles potentially because of dietary or selective pressures specific to the subcontinent.
The Clinical Blindspot
"South Asians carry CYP2C9*3 (Arg144Cys) and VKORC1 -1639A alleles at substantially higher frequencies than Europeans. The combined effect can reduce warfarin requirement by 50-70%. Standard dosing algorithms were validated on European cohorts and systematically overshoot in this population. Pharmacogenomic-guided dosing should be standard for SA patients on warfarin — and the same logic extends to clopidogrel (CYP2C19), tacrolimus (CYP3A5), and codeine (CYP2D6)."
Clinical Q&A
AI / LLM Access
Plain-text Markdown version of this case: /llms/cases/case-201-pharmacogenomic-warfarin-cyp2c9-vkorc1-south-asian.md
Patient Profile
- Patient
- 62F, Sri Lankan, AF, post-valve replacement
- Domain
- Pharmacogenomics
- Evidence
- Case Report
Source Data
- Journal: Pharmacogenomics Journal 2023
- Authors: Krishnamurthy A, et al.
- PMID:37234419
Conditions
Framework Links
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