A Subtherapeutic International Normalized Ratio Despite Increasing Doses of Warfarin: Could This Be Malabsorption?

Supratherapeutic International Normalized Ratio Due to Reduced Vitamin K Intake Secondary to Prolonged Vomiting in a Patient on Warfarin

OBJECTIVE:

To report a case of prolonged vomiting during warfarin therapy, leading to an elevated international normalized ratio (INR).

CASE SUMMARY:

A 32-year-old female with a history of cyclic vomiting syndrome since early childhood and bilateral pulmonary emboli diagnosed 4 months prior to this acute event presented to the clinic for routine monitoring of warfarin therapy. The warfarin dose had been maintained at 35 mg/wk for 3½ months (INR 2-3.5), but it was increased to 37.5 mg/wk because the INR had trended down to the low goal range over the preceding month. At presentation, the patient reported a 15-day history of vomiting with minimal oral intake that required intravenous fluids to prevent dehydration. The INR was 4.7; warfarin was withheld, and oral vitamin K 5 mg as well as subcutaneous vitamin K 10 mg was administered the following day. The INR then decreased to 1.3. Therapy was transitioned to subcutaneous enoxaparin 1 mg/kg every 12 hours for the duration of the patient's anticoagulation therapy.

DISCUSSION:

Multiple reports have demonstrated malabsorption of warfarin and decreased INR response in the presence of underlying gastrointestinal disease. Despite a prolonged episode of cyclic vomiting syndrome, our patient had an elevated INR. In normal circumstances, warfarin is rapidly absorbed from the gastrointestinal tract and reaches maximum serum concentrations in approximately 90 minutes. Studies have shown that although the presence of food does not affect overall absorption of the medication, it can decrease the rate of absorption. Our patient was vomiting 20-30 minutes after oral dose administration. Because the patient was not consuming food, absorption of warfarin was potentially prompt, thus contributing to the elevated INR. The variability of vitamin K in the diet also can have significant impact on the response to warfarin. Our patient's INR had been stable while she consumed 3 servings each week of foods rich in vitamin K. This consumption was abruptly discontinued with the onset of the cyclic vomiting syndrome. We believe that decreased intake and retention of oral vitamin K–containing foods from the diet due to the prolonged vomiting coupled with the rapid onset of absorption resulted in a notably increased INR and subsequent bruising in our patient.

CONCLUSIONS:

In the presence of prolonged vomiting, warfarin therapy requires more frequent monitoring than usual to detect fluctuations in INR that may increase the risk of adverse events.

Malabsorption-associated warfarin resistance

PURPOSE

A case of malabsorption- associated warfarin resistance is reported.

SUMMARY

A 42-year-old, 111-kg, Caucasian man arrived at the emergency department with atypical pleuritic chest pain. The chest pain was associated with shortness of breath, diaphoresis, nausea, vomiting, and tachycardia. The patient's medical history was significant for multiple episodes of deep venous thrombosis (DVT) in the left upper extremity and both lower extremities, a right above-the-knee amputation due to complications of a previous DVT, insertion of a vena cava filter, pulmonary embolism (PE), asthma, hypertension, and multiple myocardial infarctions. During admission, he was diagnosed presumptively with PE. All potential causes of interference with warfarin absorption were investigated and ruled out. I.V. warfarin therapy at a conventional initial dosage of 5 mg once daily was started on hospital day 2. The International Normalized Ratio (INR) reached the therapeutic range after increasing the i.v. warfarin dosage to 7.5 mg once daily on hospital day 6. The ability to obtain a therapeutic INR on a relatively low dosage of i.v. warfarin but not high dosages of oral warfarin strongly suggests an inherent warfarin malabsorption defect in this patient.

CONCLUSION

A 42-year-old man with a history of recurrent thromboembolisms demonstrated resistance to oral warfarin therapy due to warfarin malabsorption.

Pharmacogenetics of oral anticoagulants

The use of oral anticoagulants (OA) is problematic due to its association with hemorrhagic complications. OA metabolism relies on the CYP2C9 complex. Genetic variations compromising metabolic competence of this complex may explain the risk of excessive and hazardous anticoagulation. A pharmacogenetics-based approach to this issue could be beneficial for choosing adequate dose and duration of treatment, in addition to having a better understanding of pharmacological interactions to which OA are susceptible. However, evidence from several basic and clinical studies indicates that both a complicated system of regulation of expression of multiple genes and the influence of a wide variety of epigenetic factors could be responsible for adverse drug reactions associated with the use of OA. Emphasis on understanding the gene-environment interactions could attain new paths to facilitate the use of these important drugs in the quotidian clinical practice.