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[Apparatus for the determination of prothrombin time].
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Effect of direct thrombin inhibitors, bivalirudin, lepirudin, and argatroban, on prothrombin time and INR values.
Direct thrombin inhibitors (DTIs) represent a new class of promising anticoagulation agents. The DTIs frequently are used to provide initial anticoagulation, with long-term therapy requiring eventual transition to coumarins. Unfortunately, DTIs not only prolong the activated partial thromboplastin time but also can affect international normalized ratio (INR) values. We approximated the DTI effect on INRs by each drug to pooled plasma at concentrations between 0.1 and 1.2 microg/mL. We then concurrently tested these samples using 14 prothrombin time (PT) reagents. By using repeated measures analysis of variance, we found significant differences (P < .05) between the median INRs for lepirudin and argatroban for all PT reagents, between lepirudin and bivalirudin for all reagents except PT-Fibrinogen HS Plus (P = .07), and between bivalirudin and argatroban for all reagents except Thromborel S (P = .05). The DTI effect on INRs was dependent on drug, drug concentration, and reagent. Argatroban had the most effect on INRs, while lepirudin had the least effect. Reagents with a lower international sensitivity index were less affected by DTI; ThromboMax HS was the least sensitive PT reagent to any DTI.
Rapid fibrinogen determination with the prothrombin time using a potentiophotometer.
The potentiophotometer (PTPH) is a spectrophotometric device that embodies the electrical, analog solution to Beer's Law. The output of the instrument gives, directly and simultaneously, the concentration of the substance being measured. The PTPH has been applied to the manual determination of fibrinogen (FBG) with the same thrombopalstin injection as the prothrombin time (PT). We have now semi-automated and computerized the PTPH to constitute the coagulation instrument named POTENS+. POTENS+ determines the FBG rapidly, within approximately three to twenty seconds after the PT. With POTENS+, precision, linearity and reference range will be given for its FBG method. The accuracy of POTENS+ will be assessed by comparing it with both automated FBG methods on the MLA Electra 1000C Coagulation Timer. Of these two methods, the automated Clauss method is used as the reference method.
Artifactual shortening of prothrombin time in a semi-automated coagulometer because of lipaemic plasma.
Widespread use of coagulometers over the past few decades has improved the accuracy and precision of coagulation tests. However, test results even from sophisticated coagulometers are prone to errors due to both analytical and pre-analytical factors. Coagulometers utilizing absorbance variation for end point detection can generate erroneous results due to turbidity in the test plasma. We present a case with artifactual shortening of prothrombin time because of lipaemic plasma.
Prothrombin time determination. The lack of need for a discard tube and 24-hour stability.
The National Committee for Clinical Laboratory Standards (NCCLS) recommends that all coagulation studies be done on a specimen from the tube drawn second or later. For patients receiving long-term anticoagulant therapy, this may require that the first tube of blood drawn be discarded for each prothrombin evaluation. In a prospective study we compared the prothrombin times (PTs) as international normalized ratios (INRs) from a series of three tubes obtained from 241 patients receiving consistent dosages of oral anticoagulant therapy to determine the need for discarding the first tube drawn, as well as the stability of PT determinations over a 24-hour period. Tube one was treated as the discard tube. Tubes one and two were analyzed within the laboratory's standard 4-hour time frame, while tube three was kept stoppered at room temperature, centrifuged a half hour before PT determination, and analyzed after a 24-hour delay. Comparisons of the INRs were made in four ranges comprising 1.2 to 2.0, 2.1 to 3.5, 3.6 to 5.9, and 6.0 or more. Most INR comparisons were less than the 10% maximum variance listed as acceptable by the NCCLS. A comparison of INR results between tube two and tube one showed a statistically significant difference only for the INR range of 6.0 or more. The comparison of the 24-hour specimen with tube one showed statistically significant differences in paired t testing for the first three INR cohorts. However, the 95% confidence intervals demonstrated that these mean differences were probably too small to be clinically significant. For the fourth cohort (INR > or = 6.0) the mean difference was not significantly different on paired t testing, but the 95% confidence interval was larger at -0.07 to 0.839. In this sample of outpatients receiving consistent dosages of oral anticoagulant therapy the use of a discard tube seemed unnecessary, and the 24-hour stability of PT determinations was documented.
Biological variation of International Normalized Ratio for prothrombin times, and consequences in monitoring oral anticoagulant therapy: computer simulation of serial measurements with goal-setting for analytical quality.
Oral anticoagulant therapy (OAT) has a well-established efficacy in prophylaxis and treatment of thromboembolic disorders. Because complications are related to intensity of OAT, optimal control of treatment is mandatory. In studies of OAT, as many as 30% of International Normalized Ratio (INR) measurements for prothrombin times fall outside the therapeutic interval. Preanalytical, analytical, and biological variation all contribute to this. Computer simulations of serial INR measurements were performed for various assumed in-treatment setpoints within the therapeutic interval INR 2.0-3.0 and for an "in-treatment within-subject variation" (CV) of 10.1%. Results are presented in difference plots with therapeutic intervals and critical differences. If the in-treatment setpoint is mid-interval (INR = 2.5), only 5% of simulated INR values fall outside the therapeutic interval. Setpoints deviating from the mid-interval and increases in the in-treatment within-subject variation considerably increase the number of observations outside the therapeutic interval and the critical differences. In conclusion, random variation, biological or analytical, and setpoints (targets) deviating from mid-interval explain a substantial number of the INR values outside therapeutic intervals observed in clinical studies. Analytical imprecision should be kept < 5% and analytical bias < +/- 0.2 INR.
Prothrombin time derived fibrinogen determination on Sysmex CA-6000.
AIM: To evaluate PT derived fibrinogen determinations with reference to the Clauss fibrinogen assay using a Sysmex CA-6000 random access coagulation analyser. METHODS: Samples were analysed from normal subjects (n = 20), patients with renal or liver dysfunction (n = 25), critically ill patients (n = 25), patients receiving oral anticoagulant treatment (n = 50), and patients with a haemoglobinopathy (n = 127). Prothrombin times were performed using two thromboplastins: one derived from rabbit brain (Dade: Thromboplastin IS) and the other from recombinant human tissue factor (Dade: Innovin). Fibrinogen was assayed by the Clauss method using a commercial kit (Dade: Fibrinogen). RESULTS: The relation between Clauss fibrinogen and PT derived fibrinogen was found to be dependent on the patient's clinical group and source of the thromboplastin used. When the data from the above sample groups were pooled there was still a significant difference (p < 0.001) between Clauss fibrinogen and PT derived fibrinogen, irrespective of thromboplastin used. CONCLUSIONS: It is unsafe to use the PT derived fibrinogen for patient monitoring owing to non-uniform variability in response to clinical status and reagent employed; however, it may prove to be a useful screening test in a research environment for estimating fibrinogen levels among defined patient groups.
Field study of lyophilised plasmas for local prothrombin time calibration in The Netherlands.
AIM: To assess the effect of a lyophilised calibrant plasma procedure on the international normalised ratio (INR) and its interlaboratory variation. METHODS: INR equivalent values were assigned to five lyophilised plasmas (one from normal donors and four from coumarin treated patients) by a reference laboratory using three calibrated thromboplastin reagents. The calibrant plasmas and five artificial control blood specimens were mailed to 44 Dutch laboratories for prothrombin time (PT) determination. The assigned INR values were used to calculate calibration lines for each participant laboratory. The calibration lines were then used to translate the PT of the control specimens to INR. RESULTS: For all lyophilised plasmas and control blood samples, there were significant differences between INR values determined with the three thromboplastin reagents. These differences could not be explained by inaccuracy of the international sensitivity index or mean normal PT of the reagents and must, therefore, have been induced by the preparation procedures for the lyophilised plasmas and control blood samples. The interlaboratory variation of the INR obtained with the calibrant plasma procedure had a coefficient of variation (CV) ranging between 2.1% and 7.3% and tended to be lower than the interlaboratory variation found with the usual methods (3.0-12.2% CV). There was a good agreement between the mean INRs obtained with the calibrant procedure and those obtained using the normal methods. CONCLUSIONS: The present study highlights the limitations of some lyophilised plasmas and control blood samples. It is not possible to assign a single INR value to each of these lyophilised plasmas and control specimens that is valid for all thromboplastin reagents. Nevertheless, by using reagent specific INR equivalent values for the calibrant plasma procedure, the interlaboratory variation could be reduced.
[Prothrombin time in electric injuries].
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[Thrombodyn and prothrombin time].
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Influence of 3-aminophthalhydrazide on the prothrombin time.
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[Practical usefulness of suspensions of thrombokinase G 23787 in the determination of prothrombin time].
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Metabolism of vitamin K and influence on prothrombin time in milk-fed preruminant calves.
The metabolism of vitamin K was studied in 66 preruminant veal calves that were fed supplemental menadione sodium bisulfite complex or phylloquinone. Menadione sodium bisulfite complex was converted by intestinal microorganisms to menaquinone-4 and absorbed and stored in the liver as menaquinone-4. Phylloquinone was absorbed unchanged. Production of menaquinones 6, 7, 8, and 10 by intestinal microorganisms also was observed, but was not dependent upon dietary vitamin K. No difference was noted in prothrombin time among the groups. Intestinal microorganisms provide sufficient vitamin K to meet the physiological needs of calves fed milk replacers. Menaquinone-4 was the form of vitamin K used to meet the calf's requirement.
[Prothrombin time during insulin coma].
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Quality control of the prothrombin time and international normalized ratios. National and international schemes.
The international normalized ratios (INR) system allows valid comparisons in results and quality of performance to be made between users of different thromboplastin reagents. In the international quality control surveys currently over 80% of the 53 countries participating report INR. Stated local international sensitivity index (ISI) values show fair agreement with values calculated from quality control returns obtained with local and reference reagents. The coefficient of variations (CV) of the INR in these surveys are between 11-22% depending upon INR values. In comparison, CV of the UK national scheme are currently 7-13%. However, analysis of UK results has shown high CV with high ISI reagents. This is due to the ISI effect as CV of INR is CV of prothrombin ratio (PR) multiplied by the ISI. Ideal thromboplastins should show good precision of PR and a low ISI to prevent this apparent deterioration when PR results are transformed into the INR scale. Instrumentation has a further effect on the INR result. Unfortunately, the effect is not uniform even within instrument type and model or even between normal and therapeutic results. Local instrument adjustment or local calibration is therefore necessary. Thus, quality control surveys continue to highlight problems in prothrombin time standardization.
Standardization of the prothrombin time in oral anticoagulant control.
The establishment of International Reference Preparations for thromboplastin together with a recommended methodology for use has permitted the definition of a universal scale for intensity of oral anticoagulation. This scale is called International Normalized Ratio (INR). INRs can be calculated by using calibrated thromboplastin/instrument systems. A calibration model has been developed together with statistical methods to test this model. The imprecision of calculated INRs has been estimated using information from international collaborative studies and proficiency testing programs. Normalized prothrombin times obtained with calibrated thromboplastin/instrument systems have been shown to provide safe patient care in clinical practice. The development of control plasmas with assigned INR equivalents will complete the standardization system. The INR scale will facilitate the process of consensus-making with respect to optimal therapeutic ranges for anticoagulant intensity. Finally, it will improve the continuity of anticoagulant control of travelling patients.