Challenging the dogma: Red blood cell-directed autoimmunity as risk factor for red blood cell alloimmunisation after blood transfusion

Red blood cell autoimmunity and alloimmunity are potentially linked. Quantification of this association can tailor extensively matched red blood cell transfusions in patients with autoimmunity. Using an incident new-user cohort comprising 47 285 previously non-transfused, non-alloimmunised patients, we compared transfusion-induced red blood cell alloimmunisation incidences in direct antiglobulin test (DAT)-positive and control patients. Additionally, we performed case-control analyses to handle potential confounding by clinical immunomodulators. Among (IgG and/or C3d) DAT-positive patients (N = 380), cumulative red blood cell alloimmunisation incidences after 10 units transfused reached 4.5% (95% confidence interval [CI] 2.5-8.2) versus 4.2% (CI 3.9-4.5, p = 0.88) in controls. In case-control analyses, alloimmunisation relative risks among DAT-positive patients increased to 1.7 (CI 1.1-2.8). Additional adjustments for pre-DAT transfusion exposure or the extent of Rh/K mismatching did not impact results. In conclusion, while patients with DAT positivity show an intrinsically increased alloimmune red blood cell response, their absolute risk is comparable to control patients due to counteracting co-existing immunosuppressive conditions. Consequently, isolated DAT positivity in patients lacking overt haemolysis or complicated alloantibody testing does not seem to warrant extended matching strategies.

Transfusion of ever-pregnant donor red blood cells and mortality of male patients

Previous studies found exposure to red blood cell transfusions from female donors who have been pregnant reduces survival in male patients compared to exposure to male donor products, but evidence is not consistent. We postulate the previously observed association is modified by offspring sex, with an expected increased mortality risk for male patients receiving units from female donors with sons. Here, marginal structural models were used to assess the association between exposure to units from ever-pregnant donors, ever-pregnant donors with sons and ever-pregnant donors with daughters, and mortality. Clinical data were collected on first-ever transfusion recipients in the Netherlands and donor data were supplemented with information about offspring sex and date of birth. In this analysis, 56,825 patients were included, of whom 8,288 died during follow-up. Exposure to red blood cell units from everpregnant donors with sons was not associated with increased all-cause mortality risk among male transfusion recipients (hazard ratio [HR] 0.91, 95% confidence interval 0.83-1.01). Exposure to ever-pregnant donors, irrespective of offspring sex, was associated with mortality in male patients aged between 18 and 50 years (ever-pregnant donors: HR 1.81, 95% CI 1.31-2.51) compared to male donor units, but was protective in female patients. This study suggests that the observed increased mortality risk for exposure to red blood cell units from parous female donors does not depend on offspring sex. The increased risk of mortality seen in younger adult male patients is consistent with previous observations, but the underlying biological mechanism could not be identified in this study.

Effect of conventional hemodialysis on the apixaban plasma concentration

PURPOSE

Apixaban is a factor Xa inhibitor used in patients undergoing hemodialysis treatment. The objective of this study is to investigate the effect of hemodialysis on apixaban plasma concentrations.

METHODS

This observational study is on patients treated with apixaban 2.5 mg twice daily on conventional hemodialysis with standard low-molecular-weight heparin (LMWH) anticoagulation (nadroparin 3800-7600 IU). Plasma blood samples were collected before starting dialysis (t1), 2 h after starting dialysis (t2), and directly after dialysis (t3). Apixaban concentration was measured before and after dialysis. Anti-Xa activity was measured for all three samples.

RESULTS

A significant difference was observed between the apixaban concentration before and after dialysis (mean before dialysis 141.03 ng/mL; mean after dialysis 102.71 ng/mL; p = 0.003). Nonetheless, both apixaban plasma concentrations and anti-Xa levels remained within the reference range. Anti-Xa levels had a strong correlation with the apixaban concentrations (r = 0.935, p = 0.000). Thus, anti-Xa activity might be used as a surrogate for apixaban plasma concentration.

CONCLUSION

There seems to be no need for dose adjustments of apixaban; co-administration of LMWH next to apixaban might also be unnecessary.

Donor pregnancies and transfusion recipient mortality: A role for red blood cell storage?

BACKGROUND AND OBJECTIVES

Donor characteristics have been implicated in transfusion-related adverse events. Uncertainty remains about whether sex, and specifically pregnancy history of the blood donor, could affect patient outcomes. Whether storage duration of the blood product could be important for patient outcomes has also been investigated, and a small detrimental effect of fresh products remains a possibility. Here, we hypothesize that fresh red blood cell products donated by ever-pregnant donors are associated with mortality in male patients.

MATERIALS AND METHODS

We used data from a cohort study of adult patients receiving a first transfusion between 2005 and 2015 in the Netherlands. The risk of death after receiving a transfusion from one of five exposure categories (female never-pregnant stored ≤10 days, female never-pregnant stored >10 days, female ever-pregnant stored ≤10 days, female ever-pregnant stored >10 days and male stored for ≤10 days), compared to receiving a unit donated by a male donor, which was stored for >10 days (reference), was calculated using a Cox proportional hazards model.

RESULTS

The study included 42,456 patients who contributed 88,538 person-years in total, of whom 13,948 died during the follow-up of the study (33%). Fresh units (stored for ≤10 days) from ever-pregnant donors were associated with mortality in male patients, but the association was not statistically significant (hazard ratio 1.39, 95% confidence interval 0.97-1.99). Sensitivity analyses did not corroborate this finding.

CONCLUSION

These findings do not consistently support the notion that the observed association between ever-pregnant donor units and mortality is mediated by blood product storage.

Impact of COVID-19 pandemic on the quality of test output in haemostasis laboratories

Introduction

The high incidence of thrombotic events in patients with COVID‐19 affects health care worldwide and results in an increased workload in haemostasis laboratories due to more frequent testing of D‐dimer, haemostatic parameters and anti‐Xa tests. However, the impact of this increase in assay requests on the quality of performance in haemostasis laboratories remains unclear. In this study, the impact of the COVID‐19 pandemic on the quality of performance and management of haemostasis laboratories was evaluated.

Methods

The impact on the quality of performance was studied using external quality assessment data from 2019 to 2020 derived from ECAT surveys. A questionnaire was sent to Dutch haemostasis laboratories to identify challenges and management strategies. Furthermore, the number of assays performed in 2019 and 2020 was supplied by four Dutch hospitals, located in regions with different disease incidence.

Results

No differences in response rate nor the quality of the measurements were observed between the EQA surveys in 2019 and 2020. The questionnaire results showed a large increase of >25% in the number of test requests for anti‐Xa, D‐dimer and fibrinogen assays in 2020 compared to 2019. Extreme peaks in test requests were also observed in the four evaluated hospitals. Additionally, 84% of the respondents indicated that they had experienced increased work pressure, and increased sick leave was observed in 71% of the participating laboratories.

Conclusions

The enormous increase in test requests, especially for D‐dimer assays and anti‐Xa activity, did not affect the quality of performance within haemostatic laboratories during the COVID‐19 pandemic.

The effect of extended c, E and K matching in females under 45 years of age on the incidence of transfusion-induced red blood cell alloimmunisation

Maternal alloantibodies directed against fetal red blood cell (RBC) antigens may cause potentially life‐threatening haemolytic disease of the fetus and newborn (HDFN). Dutch transfusion guidelines therefore prescribe preventive cEK matching for all (pre‐)fertile females. To quantify the impact of cEK matching, we compared overall and antigen‐specific cumulative RBC alloimmunisation incidences in females and males aged <45 years. Among a multicentre cohort comprised of patients who received their first and subsequent RBC unit between 2005 and 2019, first‐formed RBC alloantibodies were detected in 47 of 2998 (1·6%) females and 49 of 2507 (2·0%) males. Comparing females and males, overall alloimmunisation incidences were comparable (3·1% [95% confidence interval (CI) 2·1–4·4] versus 3·5% (95% CI 2·4–4·9, P = 0·853) after 10 units transfused). However, cEK alloimmunisation incidences were significantly lower among females (0·6% (95% CI 0·3–1.5) versus 2·2% (95% CI 1·5–3·4, P = 0·001) after 10 units transfused). Yet, despite cEK‐matching guidelines being in effect, 6·5%, 3·6% and 0·2% of all RBC units remained mismatched for c, E or K antigens respectively. Most of these mismatches were almost always due to emergency settings. Even though cEK alloimmunisation was not prevented completely, implementation of cEK matching resulted in an alloantigen‐exposure risk reduction of up to 98%.

Association between renal failure and red blood cell alloimmunization among newly transfused patients

BACKGROUND

Renal failure and renal replacement therapy (RRT) affect the immune system and could therefore modulate red blood cell (RBC) alloimmunization after transfusion.

STUDY DESIGN AND METHODS

We performed a nationwide multicenter case-control study within a source population of newly transfused patients between 2005 and 2015. Using conditional multivariate logistic regression, we compared first-time transfusion-induced RBC alloantibody formers (N = 505) with two nonalloimmunized recipients with similar transfusion burden (N = 1010).

RESULTS

Renal failure was observed in 17% of the control and 13% of the case patients. A total of 41% of the control patients and 34% of case patients underwent acute RRT. Renal failure without RRT was associated with lower alloimmunization risks after blood transfusion (moderate renal failure: adjusted relative rate [RR], 0.82 [95% confidence interval (CI), 0.67-1.01]); severe renal failure, adjusted RR, 0.76 [95% CI, 0.55-1.05]). With severe renal failure patients mainly receiving RRT, the lowest alloimmunization risk was found in particularly these patients [adjusted RR 0.48 (95% CI 0.39-0.58)]. This was similar for patients receiving RRT for acute or chronic renal failure (adjusted RR, 0.59 [95% CI, 0.46-0.75]); and adjusted RR, 0.62 [95% CI 0.45-0.88], respectively).

CONCLUSION

These findings are indicative of a weakened humoral response in acute as well as chronic renal failure, which appeared to be most pronounced when treated with RRT. Future research should focus on how renal failure and RRT mechanistically modulate RBC alloimmunization.

Perioperative bridging of anticoagulation: towards a more reserved approach

BACKGROUND

Most invasive procedures require the interruption of oral anticoagulation. In 2015, an international randomised trial demonstrated that perioperative bridging caused more harm than benefit in most anticoagulated patients with atrial fibrillation, leading to a more restrictive Dutch national guideline in April 2016. The objective of the present study was to analyse the integration of the 2016 Dutch guideline for perioperative antithrombotic management from after publication until update of hospital protocols.

METHODS

This is a retrospective cohort study of patients on vitamin K antagonists undergoing a surgical procedure between April 2016 and June 2017.

RESULTS

The proportion of high-risk patients with venous thromboembolism or atrial fibrillation receiving bridging therapy decreased from 91% and 77%, respectively at the start of the study to 33% in both groups in the last months. In high-risk patients with a mechanical heart valve, the bridging rate remained stable at 70-80% for 12 months and increased to 100% in the last 3 months. Protocol adherence for high-risk patients decreased from 80% to below 43%. The 30-day incidence of major bleeding was 4.1% (15.2% in bridged patients and 0.7% in non-bridged patients) and 10.3% for clinically relevant non-major bleeding (23.9% in bridged patients and 6.0% in non-bridged patients). The incidence of thrombo-embolism was 0.5%.

CONCLUSION

New evidence from the Dutch national guideline on perioperative bridging was adopted by physicians before the local hospital protocol was updated. Low incidence of thromboembolism in non-bridged patients and high incidence of bleeding in bridged patients support a more restrictive bridging policy.

Analytical variation in factor VIII one-stage and chromogenic assays: Experiences from the ECAT external quality assessment programme

Background

Both one‐stage (OSA) and chromogenic substrate assays (CSA) are used to measure factor VIII (FVIII) activity. Factors explaining analytical variation in FVIII activity levels are still to be completely elucidated.

Aim

The aim of this study was to investigate and quantify the analytical variation in OSA and CSA.

Methods

Factors determining analytical variation were studied in sixteen lyophilized plasma samples (FVIII activity <0.01‐1.94 IU/mL) and distributed by the ECAT surveys. To elucidate the causes of OSA variation, we exchanged deficient plasma between three company set‐ups.

Results

On average, 206 (range 164‐230) laboratories used the OSA to measure FVIII activity and 30 (range 12‐51) used CSA. The coefficient of variation of OSA and CSA increased with lower FVIII levels (FVIII <0.05 IU/mL). This resulted in misclassification of a severe haemophilia A sample into a moderate or mild haemophilia A sample in 4/30 (13.3%) of CSA measurements, while this was 37/139 (26.6%) for OSA. OSA measurements performed with reagents and equipment from Werfen showed slightly lower FVIII activity (0.93, IQR 0.88‐0.98 IU/mL) compared to measurements with Stago (1.07, IQR 1.02‐1.14 IU/mL) and Siemens (1.03, IQR 0.97‐1.07 IU/mL). Part of this difference is explained by the value of the calibrator. For CSA, the measured FVIII levels were similar using the different kits.

Conclusions

In the lower range (<0.05 IU/mL), analytical variation of FVIII measurements is high in both OSA and CSA measurements. The variation in FVIII activity levels was partly explained by specific manufacturers. Further standardization of FVIII measurements and understanding of analytical variation is required.

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.

Age of platelet concentrates and time to the next transfusion

BACKGROUND

Storage time of platelet (PLT) concentrates has been negatively associated with clinical efficacy outcomes. The aim of this study was to quantify the association between storage time of PLT concentrates and interval to the next PLT transfusion for different types of PLT components, stored for up to 7 days and transfused to transfusion-dependent hematooncology patients with thrombocytopenia.

STUDY DESIGN AND METHODS

From a cohort of patients from 10 major Dutch hospitals, patients were selected whose transfusion patterns were compatible with PLT transfusion dependency due to hematooncologic disease. Mean time to the next transfusion and mean differences in time to the next transfusion for different storage time categories (i.e., fresh, <4 days; intermediate, 4-5 days; and old, >5 days) were estimated, per component type, using multilevel mixed-effects linear models.

RESULTS

Among a cohort of 29,761 patients who received 140,896 PLT transfusions we selected 4441 hematooncology patients who had received 12,724 PLT transfusions during periods of PLT transfusion dependency. Transfusion of fresh, compared to old, buffy coat-derived PLTs in plasma was associated with a delay to the next transfusion of 6.2 hours (95% confidence interval [CI], 4.5-8.0 hr). For buffy coat-derived PLTs in PAS-B and -C this difference was 7.7 hours (95% CI, 2.2-13.3 hr) and 3.9 hours (95% CI, -2.1 to 9.9 hr) while for apheresis PLTs in plasma it was only 1.8 hours (95% CI, -3.5 to 7.1 hr).

CONCLUSION

Our results indicate that the time to the next transfusion shortens with increasing age of transfused buffy coat-derived PLT concentrates. This association was not observed for apheresis PLTs.

Association of Blood Transfusion From Female Donors With and Without a History of Pregnancy With Mortality Among Male and Female Transfusion Recipients

IMPORTANCE

Transfusion of red blood cells from female donors has been associated with increased mortality in male recipients.

OBJECTIVE

To quantify the association between red blood cell transfusion from female donors with and without a history of pregnancy and mortality of red blood cell recipients.

DESIGN, SETTING, AND PARTICIPANTS

Retrospective cohort study of first-time transfusion recipients at 6 major Dutch hospitals enrolled from May 30, 2005, to September 1, 2015; the final follow-up date was September 1, 2015. The primary analysis was the no-donor-mixture cohort (ie, either all red blood cell transfusions exclusively from male donors, or all exclusively from female donors without a history of pregnancy, or all exclusively from female donors with a history of pregnancy). The association between mortality and exposure to transfusions from ever-pregnant or never-pregnant female donors was analyzed using life tables and time-varying Cox proportional hazards models.

EXPOSURES

Red blood cell transfusions from ever-pregnant or never-pregnant female donors, compared with red blood cell transfusions from male donors.

MAIN OUTCOMES AND MEASURES

All-cause mortality during follow-up.

RESULTS

The cohort for the primary analyses consisted of 31 118 patients (median age, 65 [interquartile range, 42-77] years; 52% female) who received 59 320 red blood cell transfusions exclusively from 1 of 3 types of donors (88% male; 6% ever-pregnant female; and 6% never-pregnant female). The number of deaths in this cohort was 3969 (13% mortality). For male recipients of red blood cell transfusions, all-cause mortality rates after a red blood cell transfusion from an ever-pregnant female donor vs male donor were 101 vs 80 deaths per 1000 person-years (time-dependent "per transfusion" hazard ratio [HR] for death, 1.13 [95% CI, 1.01-1.26]). For receipt of transfusion from a never-pregnant female donor vs male donor, mortality rates were 78 vs 80 deaths per 1000 person-years (HR, 0.93 [95% CI, 0.81-1.06]). Among female recipients of red blood cell transfusions, mortality rates for an ever-pregnant female donor vs male donor were 74 vs 62 per 1000 person-years (HR, 0.99 [95% CI, 0.87 to 1.13]); for a never-pregnant female donor vs male donor, mortality rates were 74 vs 62 per 1000 person-years (HR, 1.01 [95% CI, 0.88-1.15]).

CONCLUSIONS AND RELEVANCE

Among patients who received red blood cell transfusions, receipt of a transfusion from an ever-pregnant female donor, compared with a male donor, was associated with increased all-cause mortality among male recipients but not among female recipients. Transfusions from never-pregnant female donors were not associated with increased mortality among male or female recipients. Further research is needed to replicate these findings, determine their clinical significance, and identify the underlying mechanism.

Treatments for hematologic malignancies in contrast to those for solid cancers are associated with reduced red cell alloimmunization

Red cell alloimmunization may induce severe hemolytic side effects. Identification of risk-modifying conditions will help tailor preventative strategies. This study aims to quantify the associations of hematologic malignancies and solid cancers with red cell alloimmunization in patients receiving red cell transfusions. We performed a nested multicenter case-control study in a source population of 24,063 patients receiving their first and subsequent red cell transfusions during an 8-year follow-up period. Cases (n=505), defined as patients developing a first transfusion-induced red cell alloantibody, were each compared with 2 non-alloimmunized controls (n=1010) who received a similar number of red cell units. Using multivariate logistic regression analyses, we evaluated the association of various malignancies and treatment regimens with alloimmunization during a delineated 5-week risk period. The incidence of alloimmunization among patients with acute (myeloid or lymphoid) leukemia and mature (B- or T-cell) lymphoma was significantly reduced compared to patients without these malignancies: adjusted relative risks (RR) with 95% confidence interval (CI) 0.36 (range 0.19-0.68) and 0.30 (range 0.12-0.81). Associations were primarily explained by immunosuppressive treatments [RR for (any type of) chemotherapy combined with immunotherapy 0.27 (95%CI: 0.09-0.83)]. Alloimmunization risks were similarly diminished in allogeneic or autologous stem cell transplanted patients (RR 0.34, 95%CI: 0.16-0.74), at least during the six months post transplant. Alloimmunization risks of patients with other hematologic diseases or solid cancers, and their associated treatment regimens were similar to risks in the general transfused population. Our findings suggest that, in contrast to malignancies in general, hemato-oncological patients treated with dose-intensive regimens have strongly diminished risk of red cell alloimmunization.

Red cell alloimmunisation in patients with different types of infections

Red cell alloantigen exposure can cause alloantibody‐associated morbidity. Murine models have suggested that inflammation modulates red cell alloimmunisation. This study quantifies alloimmunisation risks during infectious episodes in humans. We performed a multicentre case–control study within a source population of patients receiving their first and subsequent red cell transfusions during an 8‐year follow‐up period. Patients developing a first transfusion‐induced red cell alloantibody (N = 505) were each compared with two similarly exposed, but non‐alloimmunised controls (N = 1010) during a 5‐week ‘alloimmunisation risk period’ using multivariate logistic regression analysis. Transfusions during ‘severe’ bacterial (tissue‐invasive) infections were associated with increased risks of alloantibody development [adjusted relative risk (RR) 1·34, 95% confidence interval (95% CI) 0·97–1·85], especially when these infections were accompanied with long‐standing fever (RR 3·06, 95% CI 1·57–5·96). Disseminated viral disorders demonstrated a trend towards increased risks (RR 2·41, 95% CI 0·89–6·53), in apparent contrast to a possible protection associated with Gram‐negative bacteraemia (RR 0·58, 95% CI 0·13–1·14). ‘Simple’ bacterial infections, Gram‐positive bacteraemia, fungal infections, maximum C‐reactive protein values and leucocytosis were not associated with red cell alloimmunisation. These findings are consistent with murine models. Confirmatory research is needed before patients likely to develop alloantibodies may be identified based on their infectious conditions at time of transfusion.

Point-of-care monitoring of vitamin K-antagonists: validation of CoaguChek XS test strips with International Standard thromboplastin

AIMS

Many patients treated with vitamin K-antagonists (VKA) use point-of-care (POC) whole blood coagulometers for self-testing. The majority of patients in the Netherlands use one type of POC coagulometer, that is, the CoaguChek XS. Each new lot of test strips for the CoaguChek XS is validated by a group of collaborating thrombosis centres. We assessed the International Normalised Ratio (INR) differences between each of 51 new lots of test strips and the International Standard for thromboplastin rTF/95 or its successor rTF/09.

METHODS

Each year, a particular lot of CoaguChek XS test strips was used as reference lot. The reference lot was validated by comparison to the International Standard, yielding a relationship between the reference lot INR and International Standard INR. Successive lots of test strips were compared to the reference lot by three centres using 19-29 capillary blood samples obtained from VKA-treated patients. Each patient provided two blood drops from the same finger prick, one for the reference lot strip and one for the new lot.

RESULTS

The mean INR differences between each lot and the International Standard varied between -8% and +4%. The mean absolute values of the relative differences varied between 2.4% and 8.1%. There were small but clinically unimportant differences in INR between the first and second drop of blood.

CONCLUSIONS

Accuracy of CoaguChek XS INR determinations can be assessed by a group of collaborating centres using a limited number of capillary blood samples. As the mean INR differences with the International Standard were smaller than 10%, the lots were approved for use by the Netherlands Thrombosis Services.

Investigating unexpected INRs: in search of the culprit--adherence, interactions, genetics, and superwarfarin

Treatment with coumarin derivatives is highly individualised due to high intra- and inter-individual variation in dose response and risks of severe bleeding or thromboembolic complications. Treatment focuses on reaching and maintaining a stable target international normalised ratio (INR). However, unexpected INRs that are not explained by noncompliance or vitamin K intake may occur. Here we describe seven cases of unexpected INRs, and provide clues that clarify the underlying mechanism.

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 <or=1.4. On the day of the intervention, in the acenocoumarol group mean plasma levels of all coagulation factors were higher than 50% and in the phenprocoumon group higher than 25%. We conclude that acenocoumarol can be stopped two days before an invasive procedure that is associated with a low or moderate bleeding risk and three days before an intervention with a higher bleeding risk. For phenprocoumon, administration of vitamin K two days before an intervention results in an acceptable INR during the intervention, regardless whether phenprocoumon is interrupted or not.