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Biomedical subjects

T Koster

Publications and source records attributed to T Koster.

At least 19 recordsLinked to original sources
PubMed2003

Emerging therapeutic agents for transmissible spongiform encephalopathies: a review.

Transmissible spongiform encephalopathies (TSEs) are a group of fatal neurodegenerative disorders associated with misfolding of prion protein, from PrPC to PrPSc. Different types of experimental studies have resulted in a better understanding of the pathogenesis of the prion diseases. Genetic and molecular properties of PrP isoforms have been explained but the conformational conversion of the PrPC isoform to the PrPSc isoform has not yet been entirely elucidated. However, a number of possible therapeutic agents have been tried and some have proven to be effective against TSEs but most have limitations in terms of toxicity and pharmacokinetics. Congo red (CR), anthracyclines, and polyanionic dextran sulfate have limited ability to cross the blood-brain barrier and may be toxic. The efficacy of polyene antibiotics seems to be restricted to certain scrapie strains. Tetrapyrroles and tetracyclines with low toxicities and favorable pharmacokinetics could be useful in preventing PrPSc accumulation. Compounds like branched polyamines, Cp-60, analogs of CR, quinacrine and chlorpromazine, beta-sheet breaker peptides and inhibitory peptides, active immunization using recombinant PrP and passive immunization with anti-PrP antibodies, have potential use as therapeutic agents but would need further research and clinical trials.

Animals↗
PubMed2002

Selection for the internal medicine residency programme in the Leiden region.

BACKGROUND: The selection and the professional training for a resident in internal medicine requires a great investment of time, training effort and money. Drop-outs are therefore considered a failure of the selection procedure. To evaluate our selection model and outcome of the training programme, we determined the drop-out rate at the Leiden University Medical Centre with its affiliated hospitals. METHODS: Data were collected from all files that have been kept from 1988 onwards of all internists trained and registered in Leiden. These files contained the application forms, the assessments of the trainers and references and the specifications of the programme. Also the first employment as board-certified internist is registered. RESULTS: The drop-out percentage of the training programme was 8.5%. The drop-outs did not differ in study characteristics from those who successfully completed the programme. The reports from the training team members showed that the drop-outs were as suitable and motivated as the other candidates. During the training programme 8.5% of the residents moved to another university to complete their training as an internist. All board-certified internists who graduated at the Leiden University Medical Centre found a job. CONCLUSIONS: The efficacy of the selection procedure for trainee internists is more than 90%. There are no studies in the literature for a comparison of data.

Adult↗
PubMed2001

High factor VIII antigen levels increase the risk of venous thrombosis but are not associated with polymorphisms in the von Willebrand factor and factor VIII gene.

High factor VIII levels increase the risk of venous thromboembolism, but the mechanisms that cause high factor VIII levels are unclear. In 301 thrombosis patients and 301 matched healthy controls, factor VIII antigen (VIII:Ag) levels > or = 150 IU/dl increased the thrombosis risk more than fivefold. We investigated whether high factor VIII:Ag levels result from a genetic variation in the factor VIII or von Willebrand factor (VWF) genes. Six polymorphisms in the VWF gene and two CA-repeats in the factor VIII gene were not associated with plasma VWF levels, factor VIII:Ag levels, or thrombosis risk. Our data do not support the hypothesis that a single functional sequence variation in the factor VIII or VWF gene explains the majority of high factor VIII levels and thrombotic risk.

Biomarkers↗
PubMed1998

Hemostatic effects of oral contraceptives in women who developed deep-vein thrombosis while using oral contraceptives.

OBJECTIVE: Comparison of the effect of oral contraceptives on hemostatic variables in venous thrombosis patients (thrombosis while using oral contraceptives) with the effect in healthy control subjects. Our aim was to assess whether some of these effects were more pronounced in women who had suffered thrombosis, i.e., whether these were "hemostatic hyperresponders". STUDY DESIGN: A population-based case-control study, the Leiden Thrombophilia Study. MATERIALS AND METHODS: We investigated 99 pre-menopausal women, age 15-49 years, who had used oral contraceptives at the time of a first, objectively confirmed episode of deep-vein thrombosis. They were not pregnant, nor in puerperium, nor had had a recent miscarriage, and were not using injectable progestogens, nor suffering from inherited coagulation defects. The median time between occurrence of deep-vein thrombosis and venepuncture was 18 months, and 30 of the 99 women were still using oral contraceptives, while 69 had discontinued oral contraceptive use. In addition, a group of 153 control women (54 of them were oral contraceptive users and 99 were non-users) were studied. The following hemostatic variables were measured: APTT, factor VII, factor VIII, factor XII, fibrinogen, prothrombin, total antithrombin, normalised activated protein C sensitivity ratio (n-APC-sr), protein C, protein S and free protein S. RESULTS: We found marked and significant effects of oral contraceptive use on the levels of several clotting factors, with an increase in factor VII, factor XII, protein C and a decrease in antithrombin, n-APC-sr and protein S. Less marked effects that were non-significant or only significant in either patients or controls, were an increase in factor VIII, fibrinogen and prothrombin and a decrease in the APTT and free protein S. In the former thrombosis patients several of these effects of oral contraceptives were more pronounced than in healthy women: specifically on factor VII, antithrombin, n-APC-sr and protein C. CONCLUSIONS: Our results of the effects of oral contraceptives generally confirm previous reports in healthy volunteers. Our data also show that in former deep-vein thrombosis patients these effects are more pronounced. Apparently some women become "high hemostatic responders" when exposed to oral contraceptives, and they may be the women most vulnerable to its thrombogenic effects.

Abortion, Spontaneous↗
PubMed1996

Apparent different thrombotic tendency in patients with factor V Leiden and protein C deficiency due to selection of patients.

Both activated protein C (APC) resistance and protein C deficiency are associated with an increased risk for venous thrombosis. To assess their tendencies to venous thrombosis, we compared the median age of first venous thromboembolism in patients with factor V Leiden or protein C deficiency, who were identified either within unselected consecutive cases with a first deep venous thrombosis derived from a population-based case-control study, or identified by selection of patients with a deep venous thrombosis, who were referred for thrombophilIa work-up. The median age of onset for 92 unselected APC resistant cases was 43 years and for 13 unselected protein C-deficient cases 47 years. The median age at the first thrombotic event for 28 APC-resistant members of thrombophilia families was 29 years and for 50 protein C-deficient members of thrombophilia families 31.5 years. The median age of onset for all unselected patients (n = 105) was 45 years of age (range, 16 to 69 years) and the median age of onset for all selected patients from the thrombophilia families (n = 78) was 30.5 years (range, 16 to 67 years). These results show that within the case-control study and the family studies, the median age of onset is very similar in patients with APC resistance and patients with protein C deficiency. This suggests that APC resistance is not less severe with respect to risk of thrombosis than (heterozygous) protein C deficiency. In conclusion, the median age at which the first thrombosis occurs mainly depends on the way the patients are identified and not on the type of thrombophilia.

Adult↗
PubMed1996

Hyperhomocysteinemia as a risk factor for deep-vein thrombosis.

BACKGROUND: Previous studies have suggested that hyperhomocysteinemia may be a risk factor for venous thrombosis. To assess the risk of venous thrombosis associated with hyperhomocysteinemia, we studied plasma homocysteine levels in patients with a first episode of deep-vein thrombosis and in normal control subjects. METHODS: We measured plasma homocysteine levels in 269 patients with a first, objectively diagnosed episode of deep-vein thrombosis and in 269 healthy controls matched to the patients according to age and sex. Hyperhomocysteinemia was defined as a plasma homocysteine level above the 95th percentile in the control group (18.5 micromol per liter). RESULTS: Of the 269 patients, 28 (10 percent) had plasma homocysteine levels above the 95th percentile for the controls, as compared with 13 of the controls (matched odds ratio, 2.5; 95 percent confidence interval, 1.2 to 5.2). The association between elevated homocysteine levels and venous thrombosis was stronger among women than among men and increased with age. The exclusion of subjects with other established risk factors for thrombosis (e.g., a deficiency of protein C, protein S, or antithrombin; resistance to activated protein C; pregnancy or recent childbirth; or oral-contraceptive use) did not materially affect the risk estimates. CONCLUSIONS: High plasma homocysteine levels are a risk factor for deep-vein thrombosis in the general population.

Adolescent↗
PubMed1996

Risk factor profiles in patients with different clinical manifestations of venous thromboembolism: a focus on the factor V Leiden mutation.

BACKGROUND: Patients with venous thromboembolic disease may present with different clinical manifestations. Factor V Leiden mutation leading to resistance to activated protein C is associated with a sevenfold increased risk for presenting with deep-vein thrombosis. It is not yet established whether carriers of the mutation have a similarly increased risk for manifesting with pulmonary embolism. METHODS: From an Anticoagulation Clinic monitoring coumarin therapy, a consecutive series of patients with a first thromboembolic event (objectively proven by current radiological methods) were enrolled. All patients were interviewed and blood was drawn for genotyping. From the hospital charts and the personal interview, information was obtained on acquired risk factors and the signs and symptoms on hospital admission. RESULTS: 45 patients presented with symptoms of pulmonary embolism only, 211 had only symptoms of deep-vein thrombosis whereas 23 had clinical features of both. In about half of the patients acquired risk factors for venous thromboembolism were present which did not differ between the three groups of patients. Recent surgery had been performed more often in patients presenting with pulmonary embolism than in other patients (33.3% vs. 18.5%, p < 0.05). Factor V Leiden was present in 9% of the patients presenting with pulmonary embolism (relative risk: 3.3 95% CI: 1.0-10.6) and 17% of the patients presenting with deep-vein thrombosis (relative risk: 6.9 95% CI: 3.6-12.8). The prevalence of factor V Leiden was intermediate in patients with both clinical characteristics. CONCLUSION: These data suggest that patients with venous thromboembolism have different clinical presentation depending on the risk factor profile. Factor V Leiden may preferentially lead to manifest deep-vein thrombosis. Differences in structure of venous thrombi could underlie differences in embolic tendency.

Adult↗
PubMed1995

Protein C deficiency in a controlled series of unselected outpatients: an infrequent but clear risk factor for venous thrombosis (Leiden Thrombophilia Study)

A deficiency of protein C (PC), antithrombin, or protein S is strongly associated with deep-vein thrombosis in selected patients and their families. However, the strength of the association with venous thrombosis in the general population is unknown. This study was a population-based, patient-control study of 474 consecutive outpatients, aged less than 70 years, with a first, objectively diagnosed, episode of venous thrombosis and without an underlying malignant disease, and 474 healthy controls who matched for age and sex. Relative risks were estimated as matched odds ratios. Based on a single measurement, there were 22 (4.6%) patients with a PC deficiency (PC activity, less than 0.67 U/mL or PC antigen, less than 0.33 U/mL when using coumarins). Among the controls, the frequency was 1.5% (seven subjects). Thus, there is a threefold increase in risk of thrombosis in subjects with PC levels below 0.67 or 0.33 U/mL [matched odds ratio, 3.1; 95% confidence interval (CI), 1.4 to 7.0]. When a PC deficiency was based on two repeated measurements, the relative risk for thrombosis increased to 3.8 (95% CI, 1.3 to 10); when it was based on DNA-confirmation, the relative risk increased further to 6.5 (95% CI, 1.8 to 24). In addition, there was a gradient in thrombosis risk, according to PC levels. The results for antithrombin are similar to those for PC, although less pronounced (relative risk, 2.2; 95% CI, 1.0 to 4.7). We could not find an association between reduced total protein S (relative risk, 0.7; 95% CI, 0.3 to 1.8) or free protein S levels (relative risk, 1.6; 95% CI, 0.6 to 4.0) and thrombosis risk. Although not very frequent, PC and antithrombin deficiency are clearly associated with an increase in thrombosis risk.

Adolescent↗
PubMed1995

High risk of thrombosis in patients homozygous for factor V Leiden (activated protein C resistance)

Resistance to activated protein C (APC) is a common inherited risk factor for venous thrombosis, which is associated with a mutation in coagulation factor V (factor V Leiden). We investigated the risk of venous thrombosis in individuals homozygous for this abnormality. We determined the factor V Leiden genotype in 471 consecutive patients aged less than 70 years with a first objectively confirmed deep-vein thrombosis and in 474 healthy controls. We found 85 heterozygous and seven homozygous individuals among the cases with thrombosis and 14 heterozygous individuals among the control subjects. The expected number of homozygous individuals among the controls was calculated from Hardy-Weinberg equilibrium and estimated at 0.107 (allele frequency, 1.5%). Whereas the relative risk was increased sevenfold for heterozygous individuals, it was increased 80-fold for homozygous individuals. These patients experienced their thrombosis at a much younger age (31 v 44 years). The homozygous individuals were predominantly women, most likely due to the effect of oral contraceptives. Because of the increased risk of thrombosis with age, the absolute risk becomes most pronounced in older patients, both for heterozygous and homozygous individuals. For the homozygous individuals, the absolute risk may become several percentage points per year. This implies that most individuals homozygous for factor V Leiden will experience at least one thrombotic event in their lifetime.

Adult↗
PubMed1995

Role of clotting factor VIII in effect of von Willebrand factor on occurrence of deep-vein thrombosis.

To elucidate the roles of the ABO blood group, von Willebrand factor (vWF), and clotting factor VIII in the process of deep-vein thrombosis we undertook a population-based patient-control study in which 301 consecutive patients younger than 70 with a first, objectively diagnosed episode of venous thrombosis and without an underlying malignant disorder were compared with 301 healthy controls matched for age and sex. In univariate analysis, blood group, vWF concentration, and factor VIII concentrations were all related to deep-vein thrombosis. The risk of thrombosis increased with increasing vWF or factor VIII concentration and was higher in subjects of non-O blood groups than in those of group O. In multivariate analysis only factor VIII remained as a risk factor, and the dose-response relation between factor VIII concentration and risk of thrombosis persisted (subjects with factor VIII concentrations above 1500 IU/L had an adjusted odds ratio of 4.8 [95% Cl 2.3-10.0]). By contrast, the adjusted odds ratio for each vWF stratum did not differ from 1, and that for blood group was 1.5 (1.0-2.2). Our findings point to an effect on thrombosis risk of vWF and blood group, the former fully and the latter at least partly mediated through factor VIII. The 25% prevalence of factor VIII concentrations above 1500 IU/L among unselected consecutive thrombosis patients and the high adjusted relative risk for thrombosis lead to the conclusion that high factor VIII concentrations are common and represent a clear increase in risk of thrombosis, similar to the risks conferred by deficiencies of the coagulation-inhibiting proteins and activated protein C resistance.

ABO Blood-Group System↗
PubMed1995

Oral contraceptives and venous thromboembolism: a quantitative discussion of the uncertainties.

OBJECTIVES: The majority of post-thrombotic women are barred from using oral contraceptives. We evaluated this policy for its clinical relevance. DESIGN: A meta-analysis of controlled studies between 1960 and 1993. SETTING: A Medline computer search, from 1966 to 1993, in multiple languages, with the following index terms: thrombosis, thrombopheblitis, vein, venous, pulmonary embolism, contraceptives, oestrogen, oral. STUDY SELECTION: A total of 588 articles or abstracts were reviewed for controlled studies, in which an index group was compared with a control group. Included were one randomized trial, six follow-up studies and eight case-control studies. MAIN OUTCOME MEASURES: Summary thrombosis risk for oral contraceptive users, number needed to discontinue oral contraceptives to prevent one (recurrent) thrombosis, comparison of additional unwanted pregnancies and postpartum thrombosis between alternative birth-control methods. RESULTS: The studies proved highly heterogeneous with regard to size and direction of the risk estimate. The summary relative risk of first thrombosis during oral contraceptive use was 2.9 (95% CI, 0.5-17). Since the risk of thrombosis recurrence is not well known, we estimated alternatives, making various hypothetical assumptions, wherein women would continue to take oral contraceptives after a first episode of thrombosis, or stop and switch to use of an intra-uterine device, condom or the progestogen-only pill. Depending on the assumptions with regard to recurrence risk and the existence of possible subgroups with genetic coagulation defects, the cost-benefit ratio of advising against the use of oral contraceptives after a first thrombosis varied tremendously. CONCLUSIONS: Our analysis shows that we lack the necessary data for recurrence risk of venous thrombosis during continuing use of oral contraceptives, or after switching to other modes of contraception. This reflects the clinical uncertainties that result in highly contradictory advice to young women who have experienced a first thrombosis. Only follow-up studies on recurrence risk will settle the issue.

Adolescent↗
PubMed1995

Genotypic variation in the promoter region of the protein C gene is associated with plasma protein C levels and thrombotic risk.

Protein C is a vitamin K-dependent zymogen of a serine protease that inhibits blood coagulation by proteolytic inactivation of factors Va and VIIIa. Individuals with protein C deficiency are at risk for thrombophlebitis, deep-vein thrombosis, and pulmonary embolism. Genetic analysis of a number of randomly chosen healthy individuals revealed three polymorphisms, C/T at -654, A/G at -641, and A/T at -476, in the protein C promoter region. To investigate whether these genetic variations associate with the plasma protein C level, we determined the genotype for the three polymorphisms and measured plasma protein C levels in 240 individuals not deficient in protein C. The mean protein C level of these individuals was 103%. Interestingly, individuals with the homozygous CGT genotype (n = 40) had a mean protein C level of 94%, whereas individuals with a homozygous TAA genotype (n = 28) had a mean protein C level of 116%. This difference in mean protein C levels between the CGT and TAA groups (P < .001) could not be explained by environmental factors known to influence protein C levels in the normal population. Plasma factor II and factor X levels did not differ between the two groups, which makes a difference in liver function an unlikely cause. Finally, we tested whether the genotype associated with lower protein C levels is associated with higher thrombotic risks. This analysis showed that compared with the genetic variant associated with higher protein C levels (TT/AA/AA), the genetic variant associated with lower protein C levels (CC/GG/TT genotype) is indeed a risk factor for thrombosis (OR, 1.6; 95% confidence interval, 1.0 to 2.5).

Aged↗
PubMed1994

Increased risk of venous thrombosis in oral-contraceptive users who are carriers of factor V Leiden mutation.

We investigated whether the occurrence of venous thrombosis in young women who use oral contraceptives might be explained by the factor V Leiden mutation, which leads to resistance to activated protein C and enhances susceptibility to thrombosis. We compared 155 consecutive premenopausal women, aged 15 to 49, who had developed deep venous thrombosis in the absence of other underlying diseases, with 169 population controls. The risk of thrombosis among users of oral contraceptives was increased 4-fold (relative risk 3.8 [95% CI 2.4-6.0]). The risk of thrombosis among carriers of the mutation compared with non-carriers was increased 8-fold (7.9 [3.2-19.4]). Compared with women who did not use oral contraceptives and were not carriers of the mutation, the risk of thrombosis among those with both risk factors was increased more than 30-fold (34.7 [7.8-154]). Recalculation of population incidences from these relative risks shows that the absolute risk of venous thrombosis in young women who use oral contraceptives is much larger when they carry the factor V Leiden mutation. When a young woman develops thrombosis, her factor V Leiden status should be considered in counselling about her future method of contraception.

Adolescent↗
PubMed1994

Mutation in blood coagulation factor V associated with resistance to activated protein C.

Activated protein C (APC) is a serine protease with potent anticoagulant properties, which is formed in blood on the endothelium from an inactive precursor. During normal haemostasis, APC limits clot formation by proteolytic inactivation of factors Va and VIIIa (ref. 2). To do this efficiently the enzyme needs a nonenzymatic cofactor, protein S (ref. 3). Recently it was found that the anticoagulant response to APC (APC resistance) was very weak in the plasma of 21% of unselected consecutive patients with thrombosis and about 50% of selected patients with a personal or family history of thrombosis; moreover, 5% of healthy individuals show APC resistance, which is associated with a sevenfold increase in the risk for deep vein thrombosis. Here we demonstrate that the phenotype of APC resistance is associated with heterozygosity or homozygosity for a single point mutation in the factor V gene (at nucleotide position 1,691, G-->A substitution) which predicts the synthesis of a factor V molecule (FV Q506, or FV Leiden) that is not properly inactivated by APC. The allelic frequency of the mutation in the Dutch population is approximately 2% and is at least tenfold higher than that of all other known genetic risk factors for thrombosis (protein C (ref. 8), protein S (ref. 9), antithrombin10 deficiency) together.

Amino Acid Sequence↗