The lack of subcutaneous partial pressure of oxygen (PO2) rise in response to high fraction of inspired oxygen (FiO2), called the "oxygen challenge test (OCT)", was associated with higher morbidity and mortality in human subjects. Patients had negative O2 challenge test results, indicating that flow-dependent O2 consumption might have been present. Recent reports using the noninvasive transcutaneous PO2 (PtO2) and transcutaneous partial pressure of carbon dioxide probes have observed a relationship between low oxygen challenge test values to mortality and organ failure. The OCT values provides an accessible noninvasive method of detecting early shock.