Q. What is the difference between Time Last Known Well and time of symptom onset?

A. The time last known well is the time the patient was last known to be without the signs and symptoms of the current stroke or at his or her prior baseline.  When the onset of symptoms is clearly witnessed, then the time last known well is identical to the time of symptom onset.  Under other circumstances, these terms are not synonymous.  For more information, refer to the data element definition for Time Last Known Well in the Stroke Performance Measurement Implementation Guide, 2nd Edition, available at: http://www.jointcommission.org/CertificationPrograms/PrimaryStrokeCenters/stroke_pm_edition_2.htm.