[Switching from acenocoumarol to phenprocoumon: step in personalised anticoagulation?] / Overstappen van acenocoumarol naar fenprocoumon.

Acenocoumarol and phenprocoumon are vitamin K antagonists (VKA) with average half-lives of 11 hours and 160 hours, respectively. They are used to treat and prevent thrombosis in mechanical cardiac valve replacement, atrial fibrillation and venous thromboembolism. There are historical regional differences in preferred VKA in the Netherlands. Safe and effective treatment requires the international normalized ratios (INRs) to be in the therapeutic range, and stable. Theoretically, the longer-acting phenprocoumon would yield a higher time in therapeutic range (TTR) and lower INR variability. In practice, switching from acenocoumarol to phenprocoumon eventually improves INR variability and in some patients TTR as well. However, during the preceding transition period, INRs are more often volatile and supratherapeutic. Furthermore, switching to an alternative VKA could weaken integrated care, as other healthcare providers are less experienced with it. Healthcare providers must coordinate an intended switch with the anticoagulation clinic.

Convenience and satisfaction in direct oral anticoagulant-treated patients with atrial fibrillation.

BACKGROUND: Direct oral anticoagulants (DOACs) are the preferred anticoagulants for thromboprophylaxis in atrial fibrillation. We aimed to identify determinants of quality of life related to DOAC treatment to optimize DOAC treatment convenience and satisfaction. METHODS: We conducted a cross-sectional study in DOAC users. DOAC treatment-related convenience and satisfaction were measured by Perception of Anticoagulant Treatment Questionnaire. Higher scores are more favorable (range, 0-100). Patient-reported outcome measures and drug- and organization-related factors were collected. Multiple regression analyses were used to evaluate the association between these factors (ie, exposure variables) and DOAC treatment-related convenience and treatment satisfaction (ie, outcome variables). RESULTS: Of 1598 patients invited, 1035 responded, and 962 were included. The median convenience score was 98.1 (94.2-100.0), mean satisfaction score 66.5± 14.9. Twenty-four percent felt not well informed at the start of DOAC; 6.9% did not know who to turn to with questions. Multiple regression analyses showed that lacking sense of security, the predefined composite of receiving insufficient information at start of DOAC and/or not knowing who to turn to with questions was associated with lower convenience (regression coefficient, -1.29; 95% confidence interval [CI], -2.16 to -0.41). Bleeding, gastrointestinal complaints, and lower medication adherence were also associated with lower convenience. Missing sense of security (regression coefficient -6.59; 95% CI, -8.94 to -4.24) and bleeding without consultation were associated with lower treatment satisfaction. CONCLUSIONS: Accessible interventions to improve DOAC care could be providing more instruction at treatment initiation and ensuring that patients know who to contact in case of problems.

Effect of switching from acenocoumarol to phenprocoumon on time in therapeutic range and INR variability: A cohort study.

BACKGROUND: Treatment with vitamin K antagonists (VKA) requires a high proportion of time in the therapeutic range (TTR) and a low international normalised ratio (INR) variability to be maximally safe and effective. Switching from short-acting acenocoumarol to long-acting phenprocoumon could improve VKA control. AIMS: We assessed whether switching from acenocoumarol to phenprocoumon improves the time in the therapeutic range (TTR) and INR variability. METHODS AND RESULTS: In a retrospective cohort with data on 236,957 patients-years of VKA management from two first-line anticoagulation clinics in the Netherlands, we identified 124 patients in target range 2-3, 269 patients in target range 2-3.5 and 98 patients in target range 2.5-3.5 who switched from acenocoumarol to phenprocoumon. They were matched in a 1:2 ratio to non-switching controls using propensity score matching. Over the first 180 days after a switch, switchers' TTR declined 5 (95% CI 1 to 10), 10 (95% CI 7 to 13) and 5 (95% CI 0 to 11) percentage points relative to non-switchers, in target ranges 2-3, 2-3.5 and 2.5-3.5. Anticoagulation was more often supra-therapeutic in switchers, and switchers had a higher INR variability. In the following 180 days, TTR in switchers became 1 (95% CI -4 to 6), 4 (95% CI 0 to 7) and 6 (95% CI 1 to 12) percentage points better than in non-switchers. Switchers' INRs were much more stable than non-switchers'. CONCLUSION: Eventually, a switch from acenocoumarol to phenprocoumon leads to a higher TTR and a lower INR variability. However, this is preceded by a transition period with opposite effects. An improved conversion algorithm could possibly shorten the transition period. Until then, physicians and patients should decide whether switching is worth the increased risk during the transition phase.

Quality of life after switching from well-controlled vitamin K antagonist to direct oral anticoagulant: Little to GAInN.

BACKGROUND: Direct oral anticoagulants (DOAC) and vitamin K antagonists (VKA) prevent thromboembolism in atrial fibrillation (AF). DOAC have a fixed dosing regimen and obviate INR monitoring. Therefore, DOAC presumably affect quality of life (QoL) less than VKA. However, some VKA users appreciate the monitoring. A high time in the therapeutic range (TTR) leads to a lower impact on QoL. We assessed the influence of switching from well-controlled VKA to a DOAC on QoL. METHODS: In the GAInN study, 241 patients with AF, a TTR ≥ 70%, and neither bleeding nor thrombosis while on VKA were randomised to switching to DOAC (n = 121) or continuing VKA (n = 120). Health-related (SF-36) and anticoagulation-related QoL (PACT-Q) was assessed at baseline and after six and twelve months of follow-up. RESULTS AND CONCLUSION: SF-36 development did not differ between groups. After one year, average PACT-Q Convenience improvement was 2.5 (0.3-4.7) higher on DOAC. DOAC users were 6percentage points (95%CI -4-16) more likely to improve >5 points on Convenience; 22 pp. (95%CI 1-43) in patients who scored <95/100 at baseline. The probability to meaningfully improve on PACT-Q Satisfaction was 12 pp. (95%CI 0-25) higher on DOAC. However, 5 (4.1%) and 4 (3.3%) DOAC users resumed VKA because of side-effects and patient preference. Switching from well-controlled VKA to DOAC for AF leads to a higher probability of improved PACT-Q convenience and satisfaction, but also to a higher risk of side-effects. Arguably only patients who are not satisfied with VKA should switch, because they have more to gain by switching.

Perioperative bridging of vitamin K antagonist treatment in patients with atrial fibrillation: only a very small group of patients benefits.

AIMS: Bridging anticoagulation in atrial fibrillation (AF) patients who need to interrupt vitamin K antagonists for procedures is a clinical dilemma. Currently, guidelines recommend clinicians to take the stroke and bleeding risk into consideration, but no clear thresholds are advised. To aid clinical decision making, we aimed to develop a model in which periprocedural bridging therapy is compared with withholding anticoagulation in AF patients, for several bleeding and stroke risk groups. METHODS AND RESULTS: A model was developed to simulate both a bridge and a non-bridge cohort, using simulated international normalized ratio (INR) values for patients on warfarin, acenocoumarol, and phenprocoumon. For both clinical strategies, stroke and bleeding risks were included and outcomes were stratified by CHA2DS2-VASc or CHADS2 and HAS-BLED groups. Quality-adjusted life expectancy was the main outcome considered. Our analyses show bridging to only be beneficial for patients with HAS-BLED scores equal or lower to 2 and with CHA2DS2-VASc scores of 6 or higher. For patients using acenocoumarol bridging may be beneficial starting at a CHA2DS2-VASc score of 7. Post-procedural time to therapeutic INR has a significant influence on the results: no significant benefit of bridging was found for patients reaching therapeutic INR values within 5 days. CONCLUSION: When deciding whether to bridge anticoagulation, clinicians should consider the patient's individual stroke and bleeding risk, while also considering the patient's post-procedural INR management. In practice, only a small subset of patients is expected to benefit from bridging anticoagulation treatment.

Individualised versus standard duration of elastic compression therapy for prevention of post-thrombotic syndrome (IDEAL DVT): a multicentre, randomised, single-blind, allocation-concealed, non-inferiority trial.

BACKGROUND: Therapy with elastic compression stockings has been the cornerstone for prevention of post-thrombotic syndrome for decades in patients after acute deep venous thrombosis. It is uncertain who benefits most from therapy, and what the optimum duration of therapy should be. We therefore aimed to assess the safety and efficacy of individualised duration of compression therapy versus the standard duration of 24 months following an initial treatment period of 6 months. METHODS: We did a multicentre, randomised, single-blind, allocation-concealed, non-inferiority trial at 12 hospitals in the Netherlands and two in Italy. We randomly assigned patients (1:1) with acute proximal deep vein thrombosis of the leg and without pre-existent venous insufficiency (Clinical Etiological Anatomical and Pathophysiological score

Risk of Bleeding and Thrombosis in Patients 70 Years or Older Using Vitamin K Antagonists.

IMPORTANCE: Previous studies have shown that, despite the higher risk of bleeding, the elderly still benefit from taking anticoagulants if they have a stringent indication. However, owing to the relatively low number of patients older than 90 years in these studies, it is unknown whether this benefit is also seen with the eldest patients. OBJECTIVE: To determine how the risk of bleeding and thrombosis is associated with age in patients older than 70 years who were treated with a vitamin K antagonist (VKA). DESIGN, SETTING, AND PARTICIPANTS: A matched cohort study was conducted of patients at a thrombosis service who were treated with a VKA between January 21, 2009, and June 30, 2012. All 1109 patients 90 years or older who were treated with a VKA were randomly matched 1:1:1 with 1100 patients aged 80 to 89 years and 1104 patients aged 70 to 79 years based on duration of VKA treatment. Data analysis was conducted from April 2015 to April 2016. MAIN OUTCOMES AND MEASURES: The primary outcome was a composite of clinically relevant nonmajor and major bleeding. Secondary outcomes included thromboses and quality of VKA control. RESULTS: During 6419 observation-years, 713 of the 3313 patients (1394 men and 1919 women) had 1050 bleeding events. The risk of bleeding was not significantly increased in patients aged 80 to 89 years (event rate per 100 patient-years [ER], 16.7; hazard ratio [HR], 1.07; 95% CI, 0.89-1.27) and mildly increased in patients 90 years or older (ER, 18.1; HR, 1.26; 95% CI, 1.05-1.50) compared with patients aged 70 to 79 years (ER, 14.8). The point estimates for major bleeding (including fatal) were comparable for patients aged 80 to 89 years (ER, 1.0; HR, 1.09; 95% CI, 0.60-1.98) and those 90 years or older (ER, 1.1; HR, 1.20; 95% CI, 0.65-2.22) compared with those aged 70 to 79 years (ER, 0.9). The increase in bleeding risk was sharper in men than in women. Eighty-five patients (2.6%) developed a thrombotic event. Risk of thrombosis was higher for patients in their 90s (HR, 2.14; 95% CI, 1.22-3.75) and 80s (HR, 1.75; 95% CI, 1.002-3.05) than for patients in their 70s. Vitamin K antagonist control became significantly poorer with rising age, which partly explained the increased bleeding risk in patients 90 years or older, but most of the increased risk of thrombosis was not mediated by VKA control. CONCLUSIONS AND RELEVANCE: These clinical practice data of patients considered eligible for anticoagulation show that the bleeding risk with a VKA only mildly increases after the age of 80 years, while there is a sharp increase in the risk of thrombosis in the same age group.

Impact of Vitamin K Antagonists on Quality of Life in a Prospective Cohort of 807 Atrial Fibrillation Patients.

BACKGROUND: Vitamin K antagonists (VKA) use is challenging because of frequent blood monitoring and complex dosing. Therefore, many patients and physicians are reluctant to start VKA. However, it is unclear whether VKA use actually lowers quality of life. We aimed to determine the impact of VKA initiation on quality of life and to analyze the correlation between patient and treatment characteristics and VKA perception in atrial fibrillation patients. METHODS AND RESULTS: In a prospective cohort of 240 new and 567 long-term VKA users, general quality of life and VKA perception (satisfaction and convenience) were measured at inclusion and at 3 months by the validated Study Short-Form 36 and Perception of Anticoagulant Treatment Questionnaire. Scores were converted to a 0 to 100 scale. Higher scores are more favorable. In the new patients, Medical Outcomes Study Short-Form 36 scores improved during the initial 3 months to a level comparable with the general population. At 3 months, the median convenience score was 95 (Q1-Q3, 88-98) and was higher in older patients (regression coefficient, 0.47 per year; 95% confidence interval, 0.25-0.69) and lower after bleeding (regression coefficient, -12; 95% confidence interval, -20 to -4.7). The median satisfaction score was 64. For the long-term patients, VKA perception scores were highly comparable with the new patients. The convenience score mildly improved in patients with increased individual time in therapeutic range (regression coefficient, 0.03; 95% confidence interval, 0.01-0.05; r(2)=0.01), and satisfaction scores decreased in patients with new comedication (regression coefficient, -7.0; 95% confidence interval, -12 to -1.9; r(2)=0.02). CONCLUSIONS: VKA were well tolerated in real-life, and the influences of patient and treatment related factors on VKA perception were very limited.

Effectiveness and safety of dabigatran versus acenocoumarol in 'real-world' patients with atrial fibrillation.

AIMS: Randomized trials showed non-inferior or superior results of the non-vitamin-K-antagonist oral anticoagulants (NOACs) compared with warfarin. The aim of this study was to assess the effectiveness and safety of dabigatran (direct thrombin inhibitor) vs. acenocoumarol (vitamin K antagonist) in patients with atrial fibrillation (AF) in daily clinical practice. METHODS AND RESULTS: In this observational study, we evaluated all consecutive patients who started anticoagulation because of AF in our outpatient clinic from 2010 to 2013. Data were collected from electronic patient charts. Primary outcomes were stroke or systemic embolism and major bleeding. Propensity score matching was applied to address the non-randomized design. In total, 920 consecutive AF patients were enrolled (442 dabigatran, 478 acenocoumarol), of which 2 × 383 were available for analysis after propensity score matching. Mean follow-up duration was 1.5 ± 0.56 year. The mean calculated stroke risk according to the CHA2DS2-VASc score was 3.5%/year in dabigatran vs. 3.7%/year acenocoumarol-treated patients. The actual incidence rate of stroke or systemic embolism was 0.8%/year [95% confidence interval (CI): 0.2-2.1] vs. 1.0%/year (95% CI: 0.4-2.1), respectively. Multivariable analysis confirmed this lower but non-significant risk in dabigatran vs. acenocoumarol after adjustment for the CHA2DS2-VASc score [hazard ratio (HR)dabigatran = 0.72, 95% CI: 0.20-2.63, P = 0.61]. According to the HAS-BLED score, the mean calculated bleeding risk was 1.7%/year in both groups. Actual incidence rate of major bleeding was 2.1%/year (95% CI: 1.0-3.8) in the dabigatran vs. 4.3%/year (95% CI: 2.9-6.2) in acenocoumarol. This over 50% reduction remained significant after adjustment for the HAS-BLED score (HRdabigatran = 0.45, 95% CI: 0.22-0.93, P = 0.031). CONCLUSION: In 'real-world' patients with AF, dabigatran appears to be as effective, but significantly safer than acenocoumarol.

Feasibility and effectiveness of an asthma/COPD service for primary care: a cross-sectional baseline description and longitudinal results.

BACKGROUND: In 2007, an Asthma/chronic obstructive pulmonary disease (COPD) (AC) service was implemented in the North of the Netherlands to support General Practitioners (GPs) by providing advice from pulmonologists on a systematic basis. AIMS: To evaluate the feasibility and effectiveness of this service on patient-related outcomes. METHODS: We report baseline data on 11,401 patients and follow-up data from 2,556 patients. GPs can refer all patients with possible obstructive airway disease (OAD) to the service, which is conducted by the local laboratory. Patients are assessed in the laboratory using questionnaires and spirometry. Pulmonologists inspect the data through the internet and send the GP diagnosis and management advice. RESULTS: A total of 11,401 patients were assessed by the service, covering almost 60% of all adult patients with projected asthma or COPD in the area. In all, 46% (n = 5,268) of the patients were diagnosed with asthma, 18% (n = 2,019) with COPD and 7% (n = 788) with the overlap syndrome. A total of 740 (7%) patients were followed up after 3 months because the GP advised them to change medication. In this group, the proportion of unstable COPD patients (Clinical COPD Questionnaire (CCQ) ⩾ 1) decreased from 63% (n = 92) at baseline to 49% (n = 72). The proportion of patients with uncontrolled asthma (Asthma Control Questionnaire (ACQ) ⩾ 1.5) decreased from 41% (n = 204) to 23% (n = 115). In all, 938 (8%) patients were followed up after 12 months. From these patients, the proportion of unstable COPD patients (CCQ ⩾ 1) decreased from 47% (n = 115) to 44% (n = 107). The proportion of patients with uncontrolled asthma (ACQ⩾1.5) decreased from 16% (n = 95) to 14% (n = 85). CONCLUSION: The AC service assessed a considerable proportion of patients with OAD in the area, improved patients' outcomes, and is considered to be feasible and effective.

Clinical evaluation of eight different D-dimer tests for the exclusion of deep venous thrombosis in primary care patients.

D-dimer tests are an essential element in the diagnostic work-up of deep venous thrombosis (DVT). However, the poor standardization amongst assays necessitates clinical validation before implementation in daily practice. We therefore evaluated the analytical and diagnostic performance of eight D-dimer tests in a representative group of 290 prospectively identified consecutive primary care patients with suspected DVT. Seven quantitative D-dimer assays, and a qualitative test, Simplify, were evaluated. Correlation between assays was generally poor and several assays showed a significant bias in the method comparison. Nevertheless, the Vidas D-dimer, Innovance D-dimer (CA1500 and BCS), Pathfast D-dimer, and HemosIL HS500 (ACL TOP), all displayed 100% (95% CI: 85-100%) sensitivity. Tina-quant (Modular), AQT90 D-dimer, and Liatest (STA(®)) D-dimer tests showed a slightly lower sensitivity of 95% (78-100%). and the Simplify test reached a sensitivity of 91% (72-99%) that was further improved in combination with a clinical decision rule to 95% (76-100%). In concert with the low (8.2%) prevalence of proximal DVT, diagnosed by compression ultrasonography, in our study, all test reached a negative predictive value (NPV) of at least 99%. The user friendliness of the assays differed mainly by stability of reagents, calibration frequency, time required to obtain a test result and costs of a test. In conclusion, despite considerable analytical differences, in our low-risk population all tests evaluated displayed an excellent NPV. In combination with a validated clinical decision rule to identify low-risk patients, even a straightforward POC solution could safely and cost-efficiently rule out DVT.

Long-term quality of VKA treatment and clinical outcome after extreme overanticoagulation in 14,777 AF and VTE patients.

Vitamin K antagonists (VKA) are widely used in atrial fibrillation and venous thromboembolism (VTE). Their efficacy and safety depend on individual time in the therapeutic range (iTTR). Due to the variable dose-response relationship within patients, also patients with initially stable VKA treatment may develop extreme overanticoagulation (EO). EO is associated with an immediate bleeding risk, but it is unknown whether VKA treatment will subsequently restabilise. We evaluated long-term quality of VKA treatment and clinical outcome after EO. EO was defined as international normalized ratio (INR) ≥ 8.0 and/or unscheduled vitamin K supplementation. We included a consecutive cohort of initially stable atrial fibrillation and venous thromboembolism patients. In EO patients, the 90 days pre- and post-period were compared. In addition, patients with EO were compared with patients without EO using a matched 1:2 cohort. Of 14,777 initially stable patients, 800 patients developed EO. The pre-period was characterised by frequent overanticoagulation, and half of EO patients had an inadequate iTTR (< 65 %). After EO, underanticoagulation became more prevalent. Although the mean time between INR-measurements decreased from 18.6 to 13.2 days, after EO inadequate iTTR became more frequent (62 %), p-value < 0.001. A 2.3 times (95 % confidence interval [CI] 2.0-2.5) higher risk for iTTR< 65 % after EO, was accompanied by increased risk of bleeding (hazard ratio [HR] 2.1;CI 1.4-3.2), VKA-related death 17.0 (HR 17.0;CI 2.1-138) and thrombosis (HR 5.7;CI 1.5-22.2), compared to the 1600 controls. In conclusion, patients continuing VKA after EO have long-lasting inferior quality of VKA treatment despite intensified INR-monitoring, and an increased risk of bleeding, thrombosis and VKA-related death.

Minor bleeds alert for subsequent major bleeding in patients using vitamin K antagonists.

Vitamin K antagonists (VKA) are effective in primary and secondary prevention of thromboembolism, but the associated risk of bleeding is an important limitation. The majority of bleeds are clinically mild. In this study, we assessed whether these minor bleeds are associated with major bleeding, when controlling for other important risk indicators, including the achieved quality of anticoagulation. For this, 5898 patients attending a specialized anticoagulation clinic were retrospectively studied for 1 year after initiation of VKA therapy. The risk of major bleeding was estimated using a multivariate piecewise exponential model with time-varying exposure for occurring minor bleeds. In patients with a minor bleed (N=1015) subsequent major bleeding occurred more frequently than in patients without a minor bleed (N=4883), with an incidence rate of 2·3 [95% confidence interval (CI) 1·4-3·7] vs. 1·2 per 100 person-years (95% CI 0·9-1·7). The adjusted relative risk of subsequent major bleeding after a minor bleed was 2·9 (95% CI 1·1-7·2, P =0·024). The percentage of time that a patient had an International Normalized Ratio (INR) above 5 was also independently associated with major bleeding, with a 2·2-fold increased risk in patients with at least 9% of time above INR 5 (95% CI 1·3-4·0, P=0·006). Minor bleeds alert for subsequent major bleeding, independent of the quality of anticoagulation.

Individual time within target range in patients treated with vitamin K antagonists: main determinant of quality of anticoagulation and predictor of clinical outcome. A retrospective study of 2300 consecutive patients with venous thromboembolism.

The efficacy and safety of vitamin K antagonists (VKA) are related to the actual level of anticoagulation (given as the international normalized ratio, INR). It is often difficult to maintain an optimal INR over time. We assessed the clinical impact of the individual time spent within INR target range (ITTR) in 2304 consecutive patients with venous thromboembolism. Annual incidences of recurrent thromboembolism and major bleeding were 6.2% and 2.8% respectively. The relative risk (RR) of thromboembolism was 4.5 [95% confidence interval (CI) 3.1-6.6, P < 0.001] at INR < 2.0, for major bleeding it was 6.4 (2.5-16.1, P < 0.001) at INR > 5.0, compared with INR 2.0-3.0. Patients with ITTR < 45% were at higher risk than those with ITTR > 65% (RR 2.8, 1.9-4.3, P < 0.001), while no difference was demonstrated comparing ITTR 45-65% and ITTR > 65% (RR 1.2, 0.7-1.8, P = 0.54). Annual incidences of recurrent thromboembolism were 16.0%, 4.9% and 4.6% at ITTR < 45%, 45-60% and >65% respectively. For major bleeding these were 8.7%, 2.1% and 1.9% respectively. ITTR < 37% during the first 30 treatment days was highly predictive for the total treatment time ITTR < 45% (RR 24.2, 13.5-43.1, P < 0.001). In conclusion, ITTR can be used to identify patients on VKA at risk of recurrent thromboembolism or major bleeding. Since the 30-d ITTR is highly predictive for total treatment ITTR, these patients can be identified soon after start of treatment.

Potential interaction between acenocoumarol and diclofenac, naproxen and ibuprofen and role of CYP2C9 genotype.

NSAIDs are reported to increase the risk of bleeding in coumarin users. The mechanism underlying this risk is inhibition of platelet aggregation, however a pharmacokinetic mechanism resulting in an increased International Normalised Ratio (INR) was proposed in some case reports in warfarin treated patients. In this retrospective cohort study the influence of diclofenac, naproxen and ibuprofen on the INR of outpatients stabilised on acenocoumarol therapy was investigated. We also determined the role of cytochrome P450 2C9 (CYP2C9) polymorphism on coumarin dosage and INR in NSAID users. The study was carried out at the Groningen Outpatient Thrombosis Service. A retrospective cohort study among patients who received both acenocoumarol and one of the NSAIDs under study was performed. Patients whose INR rose above the upper level of the therapeutic range (INR above 3.5 or 4.0) after an NSAID under study was added to the acenocoumarol therapy, were compared with patients who did not show such an elevation. A two-sample t-test (average acenocoumarol dosage, age), and chi-square tests (sex, therapeutic range, type of NSAID) were used to test for differences. Genotyping was carried out by analysing blood samples for the relevant CYP2C9 alleles. The study population consisted of 112 patients on stable acenocoumarol therapy, of which 52 (46%) showed an elevation of the INR above the desired therapeutic level (INR 3.5 and 4.0 respectively) after the start of an NSAID under study. In 12 patients, the INR increased above 6. The INR of the other 60 patients (54%) remained constant after the start of one of the NSAIDs under study. There were no statistically significant differences between patients with increased INR and patients without increased INR with regard to age, sex, therapeutic range and average acenocoumarol dosage. Eighty patients, of whom 36 showed an increased INR as a result of a potential acenocoumarol-NSAID drug interaction, were included in the genotyping study. No association between CYP2C9 genotype and an increased INR as a result of the drug-drug interaction was found. In nearly half of a cohort of elderly patients, the INR increased beyond the therapeutic range (INR 3.5 or 4.0) as a result of a potential pharmacokinetic drug-drug interaction between acenocoumarol and diclofenac, naproxen and ibuprofen. The average increase in INR was between 1 and 4. Polymorphism of CYP2C9 does not seem to be a relevant risk factor for the NSAID-acenocoumarol interaction.