Legion Diagnostics

PT/INR Method Comparison

The PT/INR Method Comparison module evaluates the agreement between two coagulation analyzers or reagent systems by comparing Prothrombin Time (PT) results from the same patient samples measured on both methods. Unlike a general quantitative method comparison, this module runs the analysis in two parallel domains: raw PT values (seconds) and derived INR values. INR is computed automatically from each method's PT measurements using that method's own ISI and Normal Patient Mean, so a single study simultaneously characterizes agreement in both the PT and INR reporting spaces.

For each domain, the module applies the same analytical framework as the standard Quantitative Method Comparison module — Deming, Passing-Bablok, or OLS regression to characterize the systematic relationship between methods, and Bland-Altman difference plots (absolute and percentage) to visualize bias across the measurement range. Medical Decision Point analysis is also available, estimating predicted bias at user-selected clinically important PT and INR values.

When to Use This Module

This module is used when comparing two PT/INR measurement systems specifically:

  • New coagulation analyzer implementation — Before a new instrument or reagent system is placed into service, PT/INR results must be compared against the established method to confirm that patient results will not be meaningfully affected by the transition.
  • Reagent lot change with new ISI — When a new thromboplastin lot has a different ISI from the previous lot, the INR derivation changes. A method comparison confirms that the new lot produces equivalent PT and INR results across the clinical measurement range.
  • Multi-site or satellite lab standardization — When a laboratory system operates multiple analyzers across sites, periodic method comparison ensures that PT and INR results are consistent across instruments and locations, particularly for patients who may be tested at different sites.
  • Accreditation requirements — CAP, The Joint Commission, and CLIA require documented method comparison data for PT/INR testing as part of method validation/verification records.