Guideline References

One-Stage Prothrombin Time (PT) Test and Activated Partial Thromboplastin Time (APTT) Test

The primary standard governing PT/INR system calibration. H47-A2 specifies that the Normal Patient Mean must be established by testing at least 20 fresh citrated plasma samples from healthy subjects on the local analyzer, and that the geometric mean — not the arithmetic mean — must be used as the NPM.

Standard: Establishment and Verification of Method Performance Specifications

Requires that laboratories verify or establish reference intervals as part of method validation for non-waived tests. PT and INR reference interval verification must be documented and retained.