Variability of INR in patients on stable long-term treatment with phenprocoumon and acenocoumarol and implications for analytical quality requirements.

Within each patient treated with vitamin K antagonist (VKA), variation of the international normalised ratio (INR) occurs over the treatment period. The purpose of the present study was to assess INR variation in selected patients on long-term treatment in whom the dose of VKA was not changed. This type of variation is considered as "biological variation" which is caused by many factors but not VKA dose changes or other medication. Four groups of long-term patients were examined: each group with a different VKA (acenocoumarol or phenprocoumon) or a different target intensity (INR 2.0-3.5 or 2.5-4.0). All patients were monitored with the same PT system (Hepato Quick, STA-R Evolution coagulation instrument) by one laboratory. The variation of the INR within each patient was expressed as coefficient of variation (CV, in %). The CV was corrected for the average imprecision of the INR measurement (CV, 2.4%). The mean corrected CV values for the four groups were: 10.9% (acenocoumarol, target INR 2.0-3.5); 10.5% (acenocoumarol, target INR 2.5-4.0); 10.4% (phenprocoumon, target INR 2.0-3.5); 9.1% (phenprocoumon, target INR 2.5-4.0). The analytical performance goal for the INR measurement (imprecision) can be derived from the within-subject biological variation. Desirable INR imprecision goals are <4.9% and <5.3% CV for monitoring of phenprocoumon and acenocoumarol, respectively. These goals were achieved using the aforesaid PT system.

Improvement in the regulation of the vitamin K antagonist acenocoumarol after a standard initial dose regimen: prospective validation of a prescription model.

BACKGROUND: In a retrospective study we have developed a model which determines the dose of acenocoumarol based on the age of the patient and on the first INR obtained after a standard initial loading dose. The group of patients of this study was used as the control group of the present study. AIM: The aim of this study was to prospectively validate the model and to assess whether the use of this model improves the quality of the treatment in the 0-2 months study period. PATIENTS AND METHODS: In 197 patients the model was evaluated by (1) in the initial phase: comparison of INRs with the control group, after assessing the dose according to the model, and (2) in the 0-2 months period: calculation of the percentage of time spent in the therapeutic target range compared to the control group. Furthermore, the eventual dose was compared to the dose of the model when the INRs were within the therapeutic target range for the first time and on two successive occasions. RESULTS: (1) When dosed according to the model, 50% of INRs in the total group were within the therapeutic target range compared to 45% in the control group, and (2) the percentage time spent within this range was 68 in the total group compared to 63 in the control group (P = 0.0013). When the INRs were within the range for the first time and successively twice, the eventual doses were similar to the model in 59 and 50%, respectively. About 20% of the patients did not achieve two successive INRs within the range. CONCLUSIONS: Using the model the quality of treatment improved. We advice to use a standardized individualized dose regimen at the initiation of vitamin K antagonist treatment.

The influence on INRs and coagulation factors of the time span between blood sample collection and intake of phenprocoumon or acenocoumarol: consequences for the assessment of the dose.

Managing treatment with vitamin K antagonists, the prothrombin time (PT), expressed as international normalized ratio (INR), may not represent the INR during the entire 24 hour (h) period, and this variation may be different between long-acting phenprocoumon and short-acting acenocoumarol. For both drugs we investigated the variation in 24 h of the PT/INR, the consequencesfor the assessment of the doses and which vitamin K-dependent factor causes the daily variation. Patients on self-management took their medication at 6 p.m. and determined their INRs for eight weeks, once a week and three times daily (8.30 a.m., 6 p.m. and 11 p.m., thus 14.5 h, 24 h and 29 h after taking the medication, respectively). Acenocoumarol showed a significant variation in INRs over the 24 h period, with 22 out of 80 INRs >20% lower at 11 p.m. versus 8.30 a.m. Phenprocoumon showed only few variations. Patients managed by the anticoagulation clinic took their medication at 6 p.m. for four weeks and then at 8 a.m. for four weeks, 15 h and 25 h, respectively, before the weekly blood collection. PT/INR and coagulation factors VII, X and II were determined. With acenocoumarol, taken 25 h before blood collection, the INRs were significantly different compared to 15 h, especially attributed to plasma levels of factor VII. Those on phenprocoumon were equal. These variations of INRs during 24 h may have major effects on the prescribed dose of short-acting vitamin K antagonists, such as acenocoumarol, especially for INRs at the limits of the therapeutic ranges.

Invasive procedures in the outpatient setting: managing the short-acting acenocoumarol and the long-acting phenprocoumon.

Treatment with vitamin K antagonists (VKAs) has to be interrupted when invasive procedures are planned. We compared various methods of interruption in patients on acenocoumarol or phenprocoumon in a prospective study. In patients on acenocoumarol (n = 141), 99 stopped three days before the intervention and 42 stopped two days before. All patients on phenprocoumon (n = 111) received vitamin K two days before the intervention, and 55 of these patients discontinued phenprocoumon, whereas 56 did not stop. In a subset of 30 patients we determined International Normalized Ratios (INRs) and coagulation factors II, VII, X and protein C. The mean INR after stopping acenocoumarol for three days was significantly lower than after two days (1.1 vs. 1.3, p = <0.0001), but its clinical relevance may be trivial. In patients using phenprocoumon, the mean INR on the day of the intervention was only slightly lower after stopping the VKAs (1.5 vs. 1.6, p = 0.0407), but a similar proportion of patients had an INR

VKORC1 and CYP2C9 genotypes and acenocoumarol anticoagulation status: interaction between both genotypes affects overanticoagulation.

OBJECTIVE: Our objective was to assess the effects of VKORC1 and CYP2C9 genotypes on severe overanticoagulation and time to achieve stability and their contributions to dose requirement during the initial phase of acenocoumarol treatment. METHODS: A prospective follow-up study was conducted at 2 anticoagulation clinics in The Netherlands. We assessed the CYP2C9 genotype (CYP2C9*2 and CYP2C9*3 polymorphisms) and the VKORC1 C1173T genotype of the subjects and collected data on international normalized ratio, dose, comedication, and comorbidity. RESULTS: Of the 231 patients in the cohort, 150 (64.9%) had a VKORC1 C1173T polymorphism and 84 (36.4%) had a CYP2C9*2 or CYP2C9*3 allele. Only carriers of a combination of a CYP2C9 polymorphism and a VKORC1 polymorphism had an increased risk of severe overanticoagulation compared with subjects with no polymorphism or only 1 polymorphism (hazard ratio, 3.83 [95% confidence interval, 1.62-9.05]). The time to achieve stability was associated with the possession of the CYP2C9 genotype, not with the VKORC1 genotype (hazard ratio for CYP2C9*3 allele compared with CYP2C9 wild type, 0.59 [95% confidence interval, 0.40-0.87]). Patients with a VKORC1 polymorphism required significantly lower doses than VKORC1 CC wild-type patients. A larger part of the variability in dose requirement was explained by the VKORC1 genotype than by the CYP2C9 genotype (21.4% and 4.9%, respectively). CONCLUSION: Being a carrier of a combination of polymorphisms of VKORC1 and CYP2C9, rather than of one of these polymorphisms, is associated with severe overanticoagulation. The time to achieve stability is mainly associated with the CYP2C9 genotype.

Acenocoumarol stabilization is delayed in CYP2C93 carriers.

OBJECTIVE: Our objective was to assess whether there is an association between the presence of allelic variants of the gene for cytochrome P450 (CYP) 2C9 and anticoagulation problems during the initial 3 to 6 months of acenocoumarol treatment. METHODS: A prospective follow-up study was performed at 2 anticoagulation clinics in the Netherlands. Included subjects started with a standard dose regimen as follows: 6 mg on the first day, 4 mg on the second day, and 2 mg on the third day. CYP2C9 genotypes were assessed, and data on international normalized ratio (INR), comedication, and comorbidity were collected. RESULTS: The CYP2C9 genotype of 231 subjects was assessed. Of these, 147 (63.6%) were wild-type subjects (CYP2C9*1/*1), 38 (16.5%) were carriers of CYP2C9*2, and 46 (19.9%) were carriers of CYP2C9*3. Compared with wild-type subjects, carriers of the CYP2C9*3 allele had (1) a lower chance to achieve stability in the first 6 months of therapy (hazard ratio, 0.62; 95% confidence interval, 0.42-0.91; P <.05) and (2) an increased risk of severe overanticoagulation (INR >6.0) (hazard ratio, 3.80; 95% confidence interval, 1.54-9.39; P <.01). For both outcomes, there was no significant difference between carriers of the CYP2C9*2 allele and wild-type subjects. CONCLUSION: In carriers of the CYP2C9*3 allele more difficulties in terms of stabilization and overanticoagulation were found as compared with wild-type subjects or CYP2C9*2 carriers. CYP2C9 genotyping could be useful to identify potential candidates for more frequent INR controls to minimize problems with acenocoumarol anticoagulation status.

Coumarin anticoagulants and co-trimoxazole: avoid the combination rather than manage the interaction.

OBJECTIVE: The objective of our study was to examine the management of the interaction between acenocoumarol or phenprocoumon and several antibiotics by anticoagulation clinics and to compare the consequences of this interaction on users of co-trimoxazole with those for users of other antibiotics. METHODS: A follow-up study was conducted at four anticoagulation clinics in The Netherlands. Data on measurements of the International Normalised Ratio (INR), application of a preventive dose reduction (PDR) of the coumarin anticoagulant, fever and time within or outside the therapeutic INR range were collected. RESULTS: The study cohort consisted of 326 subjects. A PDR was given more often to users of co-trimoxazole PDR than to users of other antibiotics. The PDR in co-trimoxazole users resulted in a significantly reduced risk of both moderate overanticoagulation (INR >4.5) and severe overanticoagulation (INR >6.0) compared with no PDR, with odds ratios (ORs) of 0.06 [95% confidence interval (CI): 0.01-0.51] and 0.09 (95% CI: 0.01-0.92), respectively. In co-trimoxazole users without PDR, the risk of overanticoagulation was significantly increased compared with users of other antibiotics. All co-trimoxazole users spent significantly more time under the therapeutic INR range during the first 6 weeks after the course than users of other antibiotics. CONCLUSION: PDR is effective in preventing overanticoagulation in co-trimoxazole users, but results in a significantly prolonged period of underanticoagulation after the course. Avoidance of concomitant use of co-trimoxazole with acenocoumarol or phenprocoumon seems to be a safer approach than management of the interaction between these drugs.

Discrepancies between medication records of anticoagulation clinics and pharmacy records.

PURPOSE: To determine whether there were discrepancies between with coumarin anticoagulants interacting medications recorded in medical files of anticoagulation clinics (AC-records) and computerized records of community pharmacies (pharmacy records). METHODS: A descriptive study was conducted at two Dutch anticoagulation clinics (AC's). Records of with coumarin anticoagulants interacting drugs at the AC's were compared with the pharmacy records. A drug registered in the pharmacy records but not in the AC-records, was recorded as a discrepancy, while a drug registered in AC-records as well as in pharmacy records, was recorded as a match. RESULTS: Of the 117 identified interacting drugs registered in pharmacy records 32 (27%) were not registered in the AC-records. In four out of seven patients of whom the use of a pharmacokinetically interacting drug was not registered in the AC-records, several INR-values exceeded the upper therapeutic range. CONCLUSION: This study demonstrates that a substantial percentage of drugs of which an interaction with coumarin anticoagulants can be expected, is not registered in the medical files of anticoagulation clinics.

Preanalytical variables and off-site blood collection: influences on the results of the prothrombin time/international normalized ratio test and implications for monitoring of oral anticoagulant therapy.

BACKGROUND: The quality of oral anticoagulant therapy management with coumarin derivatives requires reliable results for the prothrombin time/International Normalized Ratio (PT/INR). We assessed the effect on PT/INR of preanalytical variables, including ones related to off-site blood collection and transportation to a laboratory. METHODS: Four laboratories with different combinations of blood collection systems, thromboplastin reagents, and coagulation meters participated. The simulated preanalytical variables included time between blood collection and PT/INR determinations on samples stored at room temperature, at 4-6 degrees C, and at 37 degrees C; mechanical agitation at room temperature, at 4-6 degrees C, and at 37 degrees C; time between centrifugation and PT/INR determination; and times and temperatures of centrifugation. For variables that affected results, the effect of the variable was classified as moderate when <25% of samples showed a change >10% or as large if >25% of samples showed such a change. RESULTS: During the first 6 h after blood collection, INR changed by >10% in <25% of samples (moderate effect) when blood samples were stored at room temperature, 4-6 degrees C, or 37 degrees C with or without mechanical agitation and independent of the time of centrifugation after blood collection. With one combination of materials and preanalytical conditions, a 24-h delay at room temperature or 4-6 degrees C had a large effect, i.e., changes >10% in >25% of samples. In all laboratories, a 24-h delay at 37 degrees C or with mechanical agitation had a large effect. We observed no clinically or statistically relevant INR differences among studied centrifugation conditions (centrifugation temperature, 20 degrees C or no temperature control; centrifugation time, 5 or 10 min). CONCLUSIONS: We recommend a maximum of 6 h between blood collection and PT/INR determination. The impact of a 24-h delay should be investigated for each combination of materials and conditions.

Age and first INR after initiation of oral anticoagulant therapy with acenocoumarol predict the maintenance dosage.

BACKGROUND: This retrospective study was performed to develop a model to predict the maintenance dosage of the vitamin K antagonist acenocoumarol, based upon the first INR after a standard initial dosage regimen. MATERIAL AND METHODS: Outpatients with atrial fibrillation ( n = 284) and initial regimens of 6-4 or 6-4-2 mg acenocoumarol on day 1, 2 and 3, respectively, were included. The maintenance dosage of the period 3-6 months after the installment was related to the first INR after those standard initial dosage regimens, because in that period the INR was 76% of the time within the therapeutic range and therefore considered suitable to perform the analysis. RESULTS: A clear relation was found between the first INR, the maintenance dosage and the age. A model that predicts the maintenance dosage immediately after the standard initial dosage and based on the first INR and adjusted for age, has been developed, according to the formula: required dosage = 5.03-1.65 * ln (first INR) - 0.01 * age. CONCLUSION: We have developed a formula to predict the maintenance dosage of acenocoumarol. With this formula it is possible to install this maintenance dosage and thus achieve oral anticoagulant therapy within the therapeutic range at an earlier stage. This will have to be shown in a prospective study.