Decreased hypoprothrombinemic effect of warfarin associated with furosemide

Describing the profile of patients on concurrent rifampin and warfarin therapy in western Kenya: a case series

Background

Rifampicin’s ability to induce hepatic enzymes is responsible for causing a clinically significant drug interaction with warfarin. Little data exists to guide clinicians on managing this interaction, especially in Sub-Saharan Africa where many patients are exposed to this combination due to a higher burden of tuberculosis.

Objective

The objective of the case series is to provide insight to practicing clinicians of the unique dynamics of this drug interaction in resource-constrained settings. The case series will provide details on commonly encountered scenarios and the dosage adjustments required to maintain a therapeutic INR.

Methods

A retrospective chart review was conducted of patients attending the Moi Teaching and Referral Hospital anticoagulation clinic in Eldoret, Kenya. Patients were included if they had a history of concurrent rifampicin and warfarin therapy and a minimum follow up of 2 months. Descriptive statistics were used to explain the demographic characteristics, time to therapeutic INR and average weekly warfarin dose. The inference on proportions test was conducted to compare the time in the therapeutic range (TTR) for patients on concurrent rifampicin to the rest of the patients not receiving rifampicin in the clinic.

Results

Of the 350 patient charts evaluated, 10 met the inclusion criteria. The median percentage increase of the weekly warfarin dose from baseline was 15.7 %. For the patients in this analysis, the median TTR was 47 %.

Discussion

Patients on concurrent therapy should be rigorously monitored with regular INR checks and warfarin dosage adjustments. Empiric dosage adjustments of warfarin should be avoided but patient characteristics can aid in understanding the alterations seen in INR.

Effect of oral diuretics on chronic warfarin therapy: a retrospective study

BACKGROUND

Limited clinical documentation is suggestive of a drug interaction between warfarin and diuretics.

OBJECTIVE

To evaluate the effect on international normalized ratio (INR) when a daily oral diuretic is started or increased in patients on chronic stable warfarin therapy.

METHODS

The medical records of all active patients of two hospital-based anticoagulation clinics were retrospectively reviewed to identify patients who were started on or received a dose increase of a daily oral diuretic while on stable warfarin therapy. The primary endpoint was the mean difference between an INR recorded within 30 days prior to the diuretic initiation (pre-INR) and an INR recorded within 30 days after diuretic initiation (post-INR).

RESULTS

A total of 1254 patient charts were screened and a total of 123 patients met the study criteria. The mean difference in pre-INR and post-INR was 0.09 (95% CI -0.03 to 0.21, p = 0.12). Post-INR values were outside of the patient's therapeutic range in 39 patients (32%), but no major bleeding or thromboembolic events were reported.

CONCLUSION

Based on this retrospective study, diuretics did not result in a significant change in the INR in patients on stable warfarin therapy.

Benefits and risks of furosemide in acute kidney injury

Furosemide, a potent loop diuretic, is frequently used in different stages of acute kidney injury, but its clinical roles remain uncertain. This review summarises the pharmacology of furosemide, its potential uses and side effects, and the evidence of its efficacy. Furosemide is actively secreted by the proximal tubules into the urine before reaching its site of action at the ascending limb of loop of Henle. It is the urinary concentrations of furosemide that determine its diuretic effect. The severity of acute kidney injury has a significant effect on the diuretic response to furosemide; a good 'urinary response' may be considered as a 'proxy' for having some residual renal function. The current evidence does not suggest that furosemide can reduce mortality in patients with acute kidney injury. In patients with acute lung injury without haemodynamic instability, furosemide may be useful in achieving fluid balance to facilitate mechanical ventilation according to the lung-protective ventilation strategy.

Probable Interaction Between Warfarin and Torsemide

Objective:

To report a case in which the anticoagulation effect of warfarin appeared to be potentiated by torsemide, possibly due to an interference of metabolism through competition for the CYP2C9 isoenzyme and protein-binding displacement of warfarin.

Case Summary:

A 43-year-old Hispanic female with congestive heart failure, hypothyroidism, anemia, atrial fibrillation, and a mitral mechanical valve replacement was effectively anticoagulated with a target international normalized ratio (INR) of 2.5–3.5 on a warfarin regimen of 50–52.5 mg/wk. One week following the initiation of torsemide 40 mg in the morning and 20 mg in the afternoon, a marked increase in the INR occurred (6.2), requiring a warfarin dosage reduction. Subsequent titrations over a 3-week period eventually resulted in the achievement of a therapeutic INR (from 3.3 to 2.9) with a new warfarin regimen of 47.5 mg/wk.

Discussion:

Both torsemide and warfarin are highly protein-bound to albumin and are major substrates for the CYP2C9 isoenzyme. Competition by multiple drugs for metabolism via CYP2C9 may decrease the clearance of the drugs from systemic circulation. Addition of a drug with high protein binding may result in the displacement of other drugs that circulate highly protein-bound. Therefore, it is possible that the addition of torsemide may potentiate the anticoagulant effect of warfarin by (1) competition for metabolism through CYP2C9, with a decrease in the clearance of warfarin, and (2) protein-binding displacement of warfarin from albumin, transiently potentiating anticoagulant activity. An objective causality assessment revealed that the interaction was probable. Cardiology records confirmed the absence of fluid and heart failure status changes; therefore, these were ruled out as potential etiologies. No levothyroxine dosage changes occurred over the previous 14 months; thus, this also was ruled out as a possible etiology.

Conclusions:

To our knowledge, an interaction between warfarin and torsemide has not been previously reported. While further research should be done to confirm this interaction, practitioners should be made aware of its possibility.