• Intervention
• Stroke

The term ‘last known well’ (LKW) is ingrained in the culture of stroke treatment. There are good reasons for this. Successful acute thrombolytic trials used LKW as an entry criterion to prove the efficacy of IV thrombolysis.1 ,2 Based on the results of those trials, treatment of acute stroke with IV thrombolysis is a class 1 recommendation for patients presenting with acute stroke symptoms within 3–4.5 h of LKW.3 Few vascular neurologists will recommend IV thrombolysis for patients outside the time window established by LKW, and hence this term represents a crucial time metric for thrombolytic therapy and is here to stay.

Patient entry into the successful randomized trials of mechanical thrombectomy in all cases also included a (varying) criterion for LKW.4–8 Hence, the class 1, level of evidence A recommendation for mechanical thrombectomy also includes a time of onset metric, based on a time of LKW of 6 h.9

One problem with LKW is that it assumes the worst case scenario—for example, that a patient's stroke started the minute the last person to see them left the room. Although this may be a necessary assumption, it is a potentially detrimental assumption for patients with large vessel occlusion (LVO). Consider two identical twins, over 18 years of age, who both had an onset of symptoms from a middle cerebral artery embolus at 13:00, and were both discovered 30 min later at 13:30. One was last seen normal at 7:00, and one was last seen normal at 10:00. They are both transported by the emergency medical services and both arrive at the emergency department at 14:00 (they are both just 1 h into their stroke). In many hospitals across the country, one of these patients will receive an acute evaluation and consideration for mechanical thrombectomy (the one LKW 4 h ago), and …