Subtherapeutic oral anticoagulant therapy: Frequency and risk factors

Subtherapeutic anticoagulation levels increase both the risk and severity of thromboembolism. The aim of this study was to determine the cumulative incidence of subtherapeutic international normalised ratios (INRs) and to identify risk factors associated with a low INR. We performed a cohort study in 7,419 patients from a Dutch anticoagulation clinic. Patients who started a first treatment with oral anticoagulants between January 2000 and December 2005 and who were stably anticoagulated (4 consecutive INRs in the therapeutic range) were included. Within the cohort a nested case control study was performed to identify risk factors of subtherapeutic INRs and to determine how often a subtherapeutic INR is the result of medical interference in case of invasive procedures, hospital admissions, haemorrhage or overanticoagulation. In patients with a stable anticoagulation, the median time to a first low INR was 40 weeks. A subtherapeutic INR occurred twice as often in patients using acenocoumarol as in those using phenprocoumon (hazard ratio [HR] 2.1, 95% confidence interval [95%CI]:2.0 – 2.3) and was more common in patients with a high therapeutic range compared to a low therapeutic range (HR 1.8, 95%CI:1.5 – 2.2). Occurrence of a low INR also depended on indication for anticoagulant therapy, with the highest risk in patients who used anticoagulants as prophylaxis and the lowest risk in patients with mechanical heart valves. In 30% of cases the subtherapeutic INR was preceded by an event necessitating vitamin K or discontinuation of the anticoagulant drug.

Subtherapeutic oral anticoagulant therapy: frequency and risk factors

Subtherapeutic anticoagulation levels increase both the risk and severity of thromboembolism. The aim of this study was to determine the cumulative incidence of subtherapeutic international normalised ratios (INRs) and to identify risk factors associated with a low INR. We performed a cohort study in 7,419 patients from a Dutch anticoagulation clinic. Patients who started a first treatment with oral anticoagulants between January 2000 and December 2005 and who were stably anticoagulated (4 consecutive INRs in the therapeutic range) were included. Within the cohort a nested case control study was performed to identify risk factors of subtherapeutic INRs and to determine how often a subtherapeutic INR is the result of medical interference in case of invasive procedures, hospital admissions, haemorrhage or overanticoagulation. In patients with a stable anticoagulation, the median time to a first low INR was 40 weeks. A subtherapeutic INR occurred twice as often in patients using acenocoumarol as in those using phenprocoumon (hazard ratio [HR] 2.1, 95% confidence interval [95%CI]:2.0 - 2.3) and was more common in patients with a high therapeutic range compared to a low therapeutic range (HR 1.8, 95%CI:1.5 - 2.2). Occurrence of a low INR also depended on indication for anticoagulant therapy, with the highest risk in patients who used anticoagulants as prophylaxis and the lowest risk in patients with mechanical heart valves. In 30% of cases the subtherapeutic INR was preceded by an event necessitating vitamin K or discontinuation of the anticoagulant drug.

Daily vitamin K supplementation improves anticoagulant stability

BACKGROUND

One of the causes of unstable anticoagulant control in patients using vitamin K antagonists is a fluctuating intake of vitamin K. Research suggests that patients with a low dietary intake of vitamin K have a less stable anticoagulant control than patients with a higher intake.

OBJECTIVES

To study whether supplementation with a low daily dose of vitamin K improves anticoagulant control.

METHODS

We performed a double-blind, randomized, placebo-controlled trial. 200 patients of the Leiden anticoagulation clinic, who used the vitamin K antagonist phenprocoumon, were randomized to receive either adjusted-dose phenprocoumon and 100 mug vitamin K once daily or adjusted-dose phenprocoumon and a placebo. Treatment duration was 24 weeks. The primary outcome was the percentage of time the International Normalized Ratio was within the therapeutic range.

RESULTS

The time in the therapeutic range was 85.5% in the placebo group and 89.5% in the vitamin K group (adjusted difference 3.6%; 95% CI -0.8% to 8.0%). The time below the therapeutic range was 3.1% in the placebo group and 2.1% in the vitamin K group (adjusted difference -0.7%; 95% CI -2.5% to 1.1%) and the time above the therapeutic range was 11.4% in the placebo group and 8.5% in the vitamin K group (adjusted difference -2.9%; 95% CI -6.9% to 1.1%). The relative risk (RR) of a maximal stability in the vitamin K group compared to the placebo group was 1.8 (95%, CI 1.1-2.7).

CONCLUSION

Supplementation of vitamin K antagonists with 100 mug vitamin K improves stability of anticoagulant therapy. Because the risk of side effects is inversely related to anticoagulant stability, such an improvement is likely to reduce the number of bleeding and thrombotic events.

Improved control of oral anticoagulant dosing: a randomized controlled trial comparing two computer algorithms

BACKGROUND

Efforts to improve dosing quality in oral anticoagulant control include the use of computer algorithms. As current algorithms are simplistic and give dosage proposals in a small fraction of patients, we developed an algorithm based on principles of system and control engineering that gives proposals in nearly all patients.

OBJECTIVE

To evaluate the new algorithm in clinical practice.

PATIENTS AND METHODS

We conducted a double-blind randomized controlled trial among 712 patients with an indication for long-term anticoagulant treatment at the Leiden Anticoagulation Clinic. We compared oral anticoagulant dosing supported by the new algorithm (ICAD) with the standard algorithm (TRODIS).

RESULTS

The percentage of time spent in the therapeutic range was similar for the new and standard algorithm groups, 79.8% vs. 80.2% (difference 0.4%, 95% CI: -1.7-2.6%). The new algorithm produced a dosage proposal in 97.5% of visits, and the standard algorithm in 60.8% (difference 36.7%, 95% CI: 35.4-38.0%). Of proposals of the new algorithm, 79.3% were accepted by the physician vs. 90.9% for the standard algorithm (difference 11.6%, 95% CI: 10.2-13.0%). This implies that the new algorithm gave an acceptable proposal in 77.4% of all patient visits vs. 55.3% for the standard algorithm (difference 22.1%, 95% CI 20.4-23.8%).

CONCLUSIONS

Substantially more dosage proposals were generated and accepted with the new than with the standard algorithm, and the new algorithm will therefore improve the efficiency of anticoagulant monitoring without loss of quality.