In 2016, International Council for Standardization in Haematology (ICSH) re-energized their efforts to provide open-access guidance documents for standardizing or harmonizing laboratory practice related to hemostasis. One such document, currently in development, is providing laboratory guidance for performing mixing studies to differentiate prolonged PT and/or APTT clotting times secondary to inhibitors or factor deficiencies. The document addresses the challenges that face most laboratories that perform mixing studies including patient selection, ideal method for testing, determination thresholds to interpret a “corrected” versus “non-corrected” result, and recommendations for testing verification or validation.