BACKGROUND AND PURPOSE: The Carotid Occlusion Surgery Study (COSS) was an 
improvement over the Extracranial-Intracranial Bypass Study, which did not 
utilize physiological selection. To assess possible reasons for early closure of 
the COSS trial, we reviewed COSS methods used to identify high-risk patients and 
compared results with separate quantitative data.
METHODS: Increased oxygen extraction fraction (OEF) by positron emission 
tomography is a gold standard for ischemia, but the specific thresholds and 
equivalency of the semiquantitative OEF ratio utilized in COSS and quantitative 
OEF are at issue.
RESULTS: The semiquantitative hemispheric OEF ratio used in COSS did not 
identify the same group of patients as did quantitative OEF using a threshold of 
50%.
CONCLUSIONS: The failure of COSS is likely caused by a failure of the 
semiquantitative, hemispheric OEF ratio method rather than by the selection for 
bypass based on hemodynamic compromise.