Time to think outside the box? Prothrombin time, international normalised ratio, international sensitivity index, mean normal prothrombin time and measurement of uncertainty a novel approach to standardisation

Evolution of Hemostasis Testing: A Personal Reflection Covering over 40 Years of History

There is no certainty in change, other than change is certain. As Seminars in Thrombosis and Hemostasis celebrates 50 years of publication, I felt it appropriate to reflect on my own 40-year plus scientific career. My career in the thrombosis and hemostasis field did not start until 1987, but the subsequent 35 years reflected a period of significant change in associated disease diagnostics. I started in the Westmead Hospital "coagulation laboratory" when staff were still performing manual clotting tests, using stopwatches, pipettes, test tubes, and a water bath, which we transported to the hospital outpatient department to run our weekly warfarin clinic. Several hemostasis instruments have come and gone, including the Coag-A-Mate X2, the ACL-300R, the MDA-180, the BCS XP, and several StaR Evolution analyzers. Some instruments remain, including the PFA-100, PFA-200, the AggRAM, the CS-5100, an AcuStar, a Hydrasys gel system, and two ACL-TOP 750s. We still have a water bath, but this is primarily used to defrost frozen samples, and manual clotting tests are only used to teach visiting medical students. We have migrated across several methodologies in the 45-year history of the local laboratory. Laurel gel rockets, used for several assays in the 1980s, were replaced with enzyme-linked immunosorbent assay assays and most assays were eventually placed on automated instruments. Radio-isotopic assays, used in the 1980s, were replaced by an alternate safer method or else abandoned. Test numbers have increased markedly over time. The approximately 31,000 hemostasis assays performed at the Westmead-based laboratory in 1983 had become approximately 200,000 in 2022, a sixfold increase. Some 90,000 prothrombin times and activated partial thromboplastic times are now performed at this laboratory per year. Thrombophilia assays were added to the test repertoires over time, as were the tests to measure several anticoagulant drugs, most recently the direct oral anticoagulants. I hope my personal history, reflecting on the changes in hemostasis testing over my career to date in the field, is found to be of interest to the readership, and I hope they forgive any inaccuracies I have introduced in this reflection of the past.

International Council for Standardization in Haematology Guidance for New Lot Verification of Coagulation Reagents, Calibrators, and Controls

The clinical laboratory uses commercial products with limited shelf life or certain expiry dates requiring frequent lot changes. Prior to implementation for clinical use, laboratories should determine the performance of the new reagent lot to ensure that there is no significant shift in reagent performance or reporting of patient data. This guideline has been written on behalf of the International Council for Standardization in Haematology (ICSH) to provide the framework and provisional guidance for clinical laboratories for evaluating and verifying the performance of new lot reagents used for coagulation testing. These ICSH Working Party consensus recommendations are based on good laboratory practice, regulatory recommendations, evidence emerged from scientific publications, and expert opinion and are meant to supplement regional standards, regulations, or requirements.

Hemostasis and Thrombosis: An Overview Focusing on Associated Laboratory Testing to Diagnose and Help Manage Related Disorders

Hemostasis is a complex but balanced process that permit normal blood flow, without adverse events. Disruption of the balance may lead to bleeding or thrombotic events, and clinical interventions may be required. Hemostasis laboratories typically offer an array of tests, including routine coagulation and specialized hemostasis assays used to guide clinicians for diagnosing and managing patients. Routine assays may be used to screen patients for hemostasis-related disturbances but may also be used for drug monitoring, measuring efficacy of replacement or adjunctive therapy, and other indications, which may then be used to guide further patient management. Similarly, "specialized" assays are used for diagnostic purposes or may be used to monitor or measure efficacy of a given therapy. This chapter provides an overview of hemostasis and thrombosis, with a focus on laboratory testing that may be used to diagnose and help manage patients suspected of hemostasis- and thrombosis-related disorders.

Standardization of Prothrombin Time/International Normalized Ratio (PT/INR)

The prothrombin time (PT) represents the most commonly used coagulation test in clinical laboratories. The PT is mathematically converted to the international normalized ratio (INR) for use in monitoring anticoagulant therapy with vitamin K antagonists such as warfarin in order to provide test results that are adjusted for thromboplastin and instrument used. The INR is created using two major PT 'correction factors', namely the mean normal PT (MNPT) and the international sensitivity index (ISI). Manufacturers of reagents and coagulometers have made some efforts to harmonizing INRs, for example, by tailoring reagents to specific coagulometers and provide associated ISI values. Thus, two types of ISIs may be generated, with one being a 'general' or 'generic' ISI and others being reagent/coagulometer-specific ISI values. Although these play a crucial role in improving INR results between laboratories, these laboratories reported INR values are known to still differ, even when laboratories use the same thromboplastin reagent and coagulometer. Moreover, ISI values for a specific thromboplastin can vary among different models of coagulometers from a manufacturer using the same method for clot identification. All these factors can be sources of error for INR reporting, which in turn can significantly affect patient management. In this narrative review, we provide some guidance to appropriate ISI verification/validation, which may help decrease the variability in cross laboratory reporting of INRs.

Top-down and bottom-up approaches for the estimation of measurement uncertainty in coagulation assays

Background

The assessment of measurement uncertainty (MU) in clinical laboratories is essential to the reliable interpretation of results in clinical laboratories. However, despite the introduction of various methods for the expression of uncertainty in measurement, the MUs of coagulation tests have not been extensively studied. The aim of this study was to quantify the MU of various coagulation assays according to international guidelines and to report an expected confidence in the quality of coagulation assays.

Methods

We selected activated partial thromboplastin time, international normalized ratio (INR), protein C/S, antithrombin, fibrinogen, and Factor V/VIII/X to quantify the MUs of two coagulation testing systems: ACL TOP 750 CTS (Instrumentation Laboratory, Bedford, MA, USA) and STA Compact (Diagnostica Stago, Asnières-sur-Seine, France). We used international standards and interlaboratory comparison results in accordance with international guidelines in a top-down approach to the assessment of MU. For INR, MU was estimated in a bottom-up approach using reference thromboplastin and certified plasmas.

Results

Top-down approaches resulted in MUs between 3.3% and 21.3% for each measurand. In the bottom-up approach, MUs of INR values ranged from 10.9% to 26.4% and showed an upward trend as INR increased.

Conclusions

In this study, we were successful in quantifying MU of coagulation assays using practical methods. Our results demonstrated that top-down and bottom-up approaches were adequate for coagulation assays. However, some assays showed significant biases against international standards; therefore, standardization would be necessary to ensure more reliable patient results.

Optimizing the Verification of Mean Normal Prothrombin Time (MNPT) and International Sensitivity Index (ISI) for Accurate Conversion of Prothrombin Time (PT) to International Normalized Ratio (INR)

The Prothrombin Time (PT) assay is the most common test performed in hemostasis laboratories, most commonly as converted to an international normalized ratio (INR) to monitor anticoagulant therapy using vitamin K antagonists (VKAs) such as warfarin. Although the INR is meant to standardize PT values by taking into consideration reagent and instrument variability, substantial inter-laboratory variation in INRs still exists and suggests that this can be further improved. This paper describes the PT test, its conversion to an INR value, and methods to improve the accuracy of INRs by improving the determination of critical components to the INR, namely the mean normal prothrombin time (MNPT) and the international sensitive index (ISI).

Harmonizing the International Normalized Ratio (INR) : Standardization of Methods and Use of Novel Strategies to Reduce Interlaboratory Variation and Bias

OBJECTIVES

To reduce interlaboratory variation and bias in international normalized ratio (INR) results, as used to monitor patients receiving vitamin K antagonist therapy, including warfarin, in a large pathology network (n = 27 laboratories) by procedural standardization and harmonization.

METHODS

Network consensus to standardize to common instrument and reagent platforms was established, following development of hemostasis test specifications. Subsequent installations and implementation occurred after conclusion of a government tender process. Network-wide application of simple novel process of verification harmonization of local international sensitive index and mean normal prothrombin time initiated for each new lot of INR reagent that does not require ongoing use of reference thromboplastin or calibration/certified plasma sets.

RESULTS

We achieved reduction of different instrument manufacturers (from four to one), instrument types (10 to three), reagent types (four to one), and instrument/reagent combinations (12 to three), plus substantial reduction in INR variability and bias.

CONCLUSIONS

Results infer significant improvement in local patient management, with positive implications for other laboratories. For the United States in particular, lack of US Food and Drug Administration-cleared certified plasmas may compromise INR accuracy, and our novel approach may provide a workable alternative for laboratories and networks.

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.

Laboratory reporting of hemostasis assays: the final post-analytical opportunity to reduce errors of clinical diagnosis in hemostasis?

The advent of modern instrumentation, with associated improvements in test performance and reliability, together with appropriate internal quality control (IQC) and external quality assurance (EQA) measures, has led to substantial reductions in analytical errors within hemostasis laboratories. Unfortunately, the reporting of incorrect or inappropriate test results still occurs, perhaps even as frequently as in the past. Many of these cases arise due to a variety of events largely outside the control of the laboratories performing the tests. These events are primarily preanalytical, related to sample collection and processing, but can also include post-analytical events related to the reporting and interpretation of test results. The current report provides an overview of these events, as well as guidance for prevention or minimization. In particular, we propose several strategies for the post-analytical reporting of hemostasis assays, and how this may provide the final opportunity to prevent serious clinical errors in diagnosis. This report should be of interest to both the laboratory scientists working in hemostasis and clinicians that request and attempt to interpret the test results. Laboratory scientists are ultimately responsible for these test results, and there is a duty to provide both accurate and precise results to enable clinicians to manage patients appropriately and to avoid the need to recollect and retest. Also, clinicians will not be in a position to best diagnose and manage their patient unless they gain an appreciation of these issues.

Thromboplastin standards

The Prothrombin Time (PT) test is used for monitoring of treatment with Vitamin K-antagonists (VKA). The result of the PT test should be expressed as the International Normalized Ratio (INR). Calculation of INR is based on the availability of International Standards (IS) for thromboplastin and a calibration model. Calibration of a new PT test system is performed with the appropriate IS and fresh plasma samples of healthy (normal) volunteers and patients treated with VKA. The calibration model is based on the assumption of a linear relationship between the log(PT)'s obtained with the new PT system and the reference IS for both normal and patients' samples. Patients' samples for calibration should be selected by rejecting samples beyond the 1.5-4.5 INR range. Outliers should be rejected defined as points with a perpendicular distance greater than three residual standard deviations from the line of relationship. Selection of patients' samples and rejection of outliers result in a reduction of the between-laboratory variation of calibration. In addition to monitoring of VKA, the PT is used for management of patients with chronic liver disease. Likewise, INR(liver) should be based on calibration with an IS using samples from patients with chronic liver disease.

Prothrombin time and partial thromboplastin time assay considerations

Prothrombin times and activated partial thromboplastin times have long been used as tests of overall ("global") clotting function. Laboratory coagulation testing issues should be at the forefront of the reader's consciousness whenever critically evaluating and extrapolating published study conclusions reliant on the results of these tests. Thus, this article reviews laboratory issues and known variables influencing prothrombin time and partial thromboplastin time results and international normalized ratio determinations.