A comparison of lyophilized artificially depleted plasmas and lyophilized plasmas from patients receiving warfarin in correcting for coagulometer effects on international normalized ratios

Significance of local international sensitivity index systems for monitoring warfarin and liver function

OBJECTIVES

Use of a local calibrator has been recommended for standardization of the international normalized ratio (INR) and international sensitivity index (ISI). We investigated the performance of two commercial local calibrators for warfarin monitoring and determined the significance of liver-specific INR.

METHODS

ISI values were determined using the World Health Organization (WHO) method and two commercial local calibrators. Liver-specific ISI was determined using plasma samples from patients with liver cirrhosis and normal controls.

RESULTS

In warfarin monitoring, the two local ISIs determined by the two local calibrators showed better consistency than uncorrected ISI, although they were inferior to the ISIs calibrated using the WHO method. Alternative calibration using calibration plasma from patients with liver cirrhosis instead of warfarinized plasma reduced the INR variability.

CONCLUSIONS

Local ISI determined by a commercial local calibrator improved INR standardization among thromboplastins. The alternative ISI calibration using liver-specific calibration plasma is expected to reduce INR variability for the evaluation of liver function.

Use of INR calibrator plasmas in the routine coagulation laboratory: a study of two thrombolastin reagents

INR values may be either calculated with the ISI values supplied by thromboplastin manufacturers or are directly extrapolated from certified INR calibrator plasmas. We tested the principle of local INR calibration using INR calibrator plasmas (PT-Multi Calibrator, Siemens), two thromboplastin reagents (Neoplastin Plus, rabbit brain, Stago, coagulometer-specific ISI 1.31, and Innovin, recombinant human tissue factor, Siemens) and the same coagulometer (STA-R, Stago) in 100 patients on warfarin. Using a ISI value of 0.77 with Tomenson correction for Innovin (correction factor=1.09), INR values of patients were similar with the two reagents, with a bias of 0.03 INR units and no significant regression of the difference over the average INR by method comparison analysis. With the INR calibrator plasmas, INR values with Neoplastin Plus were lower than Innovin values with an average bias of 0.39 INR units and a significant regression of the difference over the average INR (r=-0.91). Significant bias (0.16 INR units, p<0.00001) and regression (r=-0.77) was also observed by comparison of Neoplastin Plus INRs with Innovin calibrated INRs. Based on a therapeutic INR interval of 2.0 to 3.5, discordance in warfarin dosing was approximately 3 times higher with INR calibration (27% vs 11%). Because of non commutability with fresh plasma samples, local INR calibration with lyophilized calibrator plasmas may not be valid for some reagent-instrument combinations.

Simplified Method for International Normalized Ratio (INR) Derivation Based on the Prothrombin Time/INR Line: An International Study

BACKGROUND

The need to perform local International Sensitivity Index (ISI) calibrations and in particular the requirement for a manual method for prothrombin time (PT) determination, have proved to be obstacles to application of the WHO scheme for PT standardization.

METHODS

We used international normalized ratio (INR) derived with a set of only 5 European Concerted Action on Anticoagulation (ECAA) lyophilized calibrant plasmas, certified manually by expert centers with reference thromboplastins, to determine a local PT/INR Line. We compared results of an independent set of validation plasmas with INRs from conventional ISI calibrations and with manually certified INRs.

RESULTS

The mean certified INR of 5 lyophilized validation plasmas was 2.41 with human thromboplastin, 2.04 with bovine/combined, and 2.80 with rabbit. With 42 human reagents, the mean observed INR of the validation plasmas was 2.68 (11.2% deviation from certified INR). Deviation was reduced to 0.4% with both local ISI calibration and the PT/INR Line. Eight results based on bovine/combined thromboplastin gave an INR deviation of 4.9%, becoming 0.5% after ISI calibration and 2.4% with the PT/INR Line. Six results with rabbit reagents deviated from certified INR by 2.5%. After ISI calibration, deviation became 1.1%, and with the PT/INR Line, 0.7%. The PT/INR Line gave similar results with both linear and orthogonal regression analysis. The total proportion of validation plasmas giving INR within 10% deviation from certified values was 42.5% with uncorrected INR, which increased to 92.1% with local ISI calibration and 93.2% with the PT/INR Line.

CONCLUSIONS

The PT/INR Line procedure with 5 ECAA calibrant plasmas successfully substitutes for local ISI calibrations in deriving reliable INRs.

Comparison of local International Sensitivity Index calibration and 'Direct INR' methods in correction of locally reported International Normalized Ratios: an international study

BACKGROUND

It is no longer feasible to check local International Normalized Ratios (INR) by the World Health Organization International Sensitivity Index (ISI) calibrations because the necessary manual prothrombin time technique required has generally been discarded.

OBJECTIVES

An international collaborative study at 77 centers has compared local INR correction using the two alternative methods recommended in the Scientific and Standardization Committee of the International Society on Thrombosis and Haemostasis guidelines: local ISI calibration and 'Direct INR'.

METHODS

Success of INR correction by local ISI calibration and with Direct INR was assessed with a set of 27 certified lyophilized plasmas (20 from patients on warfarin and seven from normals).

RESULTS

At 49 centers using human thromboplastins, 3.0% initial average local INR deviation from certified INR was reduced by local ISI calibration to 0.7%, and at 25 centers using rabbit reagents, from 15.9% to 7.5%. With a minority of commercial thromboplastins, mainly 'combined' rabbit reagents, INR correction was not achieved by local ISI calibration. However, when rabbit combined reagents were excluded the overall mean INR deviation after correction was reduced further to 3.9%. In contrast, with Direct INR, mean deviation using human thromboplastins increased from 3.0% to 6.6%, but there was some reduction with rabbit reagents from 15.9% to 10% (12.3% with combined reagents excluded).

CONCLUSIONS

Local ISI calibration gave INR correction for the majority of PT systems but failed at the small number using combined rabbit reagents suggesting a need for a combined reference thromboplastin. Direct INR correction was disappointing but better than local ISI calibration with combined rabbit reagents. Interlaboratory variability was improved by both procedures with human reagents only.

Enhanced standardization of the International Normalized Ratio through the use of plasma calibrants: a concise review

The INR was introduced in 1983 as a means of standardizing the prothrombin time test and improving management of patients receiving warfarin-based oral anticoagulant therapy. Despite the eventual widespread use of the INR system, unacceptable levels of inter-laboratory variation persist. The use of certified plasma calibrants has been studied as a means to reducing INR variation. This review examines the pre-analytical and analytical variables contributing to INR problems. Also, the findings of several multicenter studies in which plasma calibrants were deployed are presented. Issues such as the number of calibrants used and their composition, calibrant format, plasma citrate concentration, reference thromboplastins used, methods of calibrant certification and alternative techniques for local system calibration are examined. The development of consensus guidelines on the use of plasma calibrants for INR standardization is recommended.

An international collaborative study on the INR calibration of freeze-dried reference plasmas

A study was carried out to calibrate potential European Reference Plasmas for prothrombin time (PT) standardization. The International Normalized Ratio (INR) values of three freeze-dried candidate plasmas (one pooled normal and two pools from anticoagulated patients) were determined in 20 laboratories using six thromboplastin reagents comprising three International Reference Thromboplastins (human, rabbit and bovine), two recombinant human reagents and one placental human reagent. Interlaboratory variability of INR estimation was low with geometric coefficients of variation (gcv) <10% except in one case. Significant differences in mean INR were found between the different thromboplastins with lowest INR values found with the bovine reagent. INR values from the International rabbit and human reagents differed by <6% and were combined to give proposed assigned INR values. Significant differences in INR estimates from four thromboplastins of human origin may indicate that single assigned INR values are not applicable for use with all thromboplastin reagents. Field trials to assess the validity of single assigned INR values in clinical practice are required.

A comparison of linear and orthogonal regression analysis for local INR determination in ECAA coagulometer studies. European Concerted Action on Anticoagulation

International sensitivity index calibrations based on the W.H.O. recommended method depend on orthogonal regression analysis. As this is not readily available in statistical packages, comparison has been made with simple linear regression analysis in a study of coagulometer effects on the International Normalized Ratio (INR) at 155 European centres. Sets of seven lyophilized normal and 20 lyophilized artificially depleted abnormal plasmas were provided with five coumarin test plasmas and two European Concerted Action on Anticoagulation reference thromboplastins (low International Sensitivity Index (ISI) human and high ISI rabbit). Local ISI based on the artificially depleted lyophilized plasmas using conventional orthogonal regression gave good correction for local coagulometer effects on the human reagent and minimal correction with the rabbit reagent INR. Results were considerably worse after attempts at correction using calibration based on linear regression analysis with both reagents. The results indicate that calibration of coagulometer prothrombin time systems using simple linear regression is not appropriate.

A comparison of artificially-depleted, lyophilized coumarin and fresh coumarin plasmas in thromboplastin calibration. European Concerted Action on Anticoagulation

Artificially-depleted lyophilized plasmas and lyophilized coumarin plasmas were prepared and compared with fresh coumarin plasmas to assess their comparative reliability in local thromboplastin calibration using the manual prothrombin time (PT) technique. Their certified PT values were inserted in turn on the vertical axis in place of the PT obtained with fresh coumarin plasmas. PT results were obtained at eight ECAA national laboratories ('test centres') and inserted on the horizontal axis. The resulting thromboplastin calibration slopes were compared with conventional fresh coumarin plasma calibration slopes at the same 'test centres'. When 60 artificially-depleted plasmas were substituted for 60 fresh plasmas, the mean calibration slopes with the human plain International Reference Preparation (IRP) were 4.2% higher. For comparison with 20 lyophilized coumarins, three sets of 20 artificially-depleted plasmas were selected in sequential order from the 60. The lyophilized coumarin plasmas gave a mean deviation of 9.6% from the fresh plasma calibration slopes compared with values of 2.0%, 6.1% and 11.7% for the three sets of 20 depleted plasmas. Although both types of lyophilized plasma calibration slopes give measurable differences from conventional fresh plasmas, these may be regarded as acceptable in clinical terms.

The importance of "like to like" ISI calibrations with freeze dried plasmas. European Concerted Action on Anticoagulation

AIM

To assess reliability of like to like and cross species calibrations using two types of certified freeze dried plasma calibrants--artificially depleted of vitamin K clotting factors, and from coumarin treated patients.

METHODS

Six ECAA national control laboratories provided certified values for the freeze dried plasmas in terms of the human plain international reference preparation (IRP) (BCT/441) with the manual prothrombin time technique. Eight other ECAA national laboratories determined international sensitivity index (ISI) values in full fresh plasma same species and cross species WHO calibrations against a low ISI human IRP (BCT/441) of the ECAA low ISI human thromboplastin and high ISI ECAA rabbit thromboplastin. Parallel calibrations were performed using the certified values.

RESULTS

Calibrations on fresh plasmas of the human ECAA reference thromboplastin (stated ISI = 0.95) gave ISI of 0.957 against the human IRP and 1.66 against the rabbit IRP. The ECAA rabbit (stated ISI = 1.67) gave an identical value on the fresh plasma calibration v the human IRP. With freeze dried depleted plasmas certified in terms of the human IRP, the ISI of the ECAA human was 1.01, but the ECAA rabbit (stated ISI = 1.67) gave a low ISI of 1.47. The freeze dried coumarin plasmas gave an ISI of 0.943 for the ECAA human but only 1.493 for the ECAA rabbit.

CONCLUSIONS

Fresh plasmas give reliable ISI when calibrating thromboplastins in same species and cross species calibrations. Freeze dried plasmas certified in terms of a single IRP, whether artificially depleted or of coumarin plasma origin, cannot be used for calibration of dissimilar thromboplastins.