Perfusion assessment is a cornerstone of the management of critical limb ischemia (CLI), and is recommended by contemporary guidelines to direct treatment. 1 However, important limitations exist when applying physiologic vascular testing suitable for the evaluation of claudication (i.e. ankle-brachial index [ABI]) to assess perfusion for the patient with CLI. Indeed, the shortcomings of ankle systolic assessment for CLI have been well established. [2][3][4] Therefore, perfusion assessment for CLI requires unique physiologic testing sensitive to regional blood flow while remaining reliable despite arterial calcification.Medial arterial calcification, common to patients with CLI-associated diabetes and chronic kidney disease, falsely elevates the ankle systolic blood pressure and represents the greatest limitation to non-invasive testing. Toe systolic pressure, transcutaneous oximetry (TCOM), and skin perfusion pressure (SPP) are the currently recommended tests to assess limb perfusion in CLI. 1 However, these tests favored for CLI are inconsistently available and preference for one test over the other is not explicitly directed by current guidelines. But which of these tests might offer the most meaningful information to save a threatened limb from CLI?Skin perfusion pressure (SPP) is a non-invasive test of regional perfusion able to assess the severity of tissue ischemia, the prognosis for ulcer healing, and as a predictor for the appropriate level of amputation. 5,6 As a measure of microperfusion at a depth 1-2 mm below the skin's surface, SPP is unaffected by medial arterial calcification, making the measure particularly suitable among patients with complex peripheral artery disease (PAD). 6,7 SPP is defined as the minimal external pressure on the skin at which capillary blood flow is present during external cuff pressure applied immediately proximal to the local zone of flow assessment. 7 During stepwise reduction in external compression, the pressure at which microcirculatory flow resumes, which is assumed to represent SPP, can then be detected by one of three techniques, including radioisotope clearance, photoplethysmography, and laser Doppler. 8 In contemporary practice, laser Doppler SPP serves as a reasonable surrogate for toe systolic pressure when the latter is not attainable, such as in the case of transmetatarsal amputation. 9 SPP values above 30 mmHg, 5,6,10 and more generally above 40 mmHg, [11][12][13] have been proposed as a mark of adequate perfusion, above which healing is more likely. The combination of SPP > 40 mmHg and toe systolic pressure > 30 mmHg is associated with particularly high rates of healing. 12 While SPP also correlates with TCOM, significant variability exists in the results of TCOM at low perfusion pressure. 7,8,12 So, is SPP the under-promoted answer to assessing limb perfusion in CLI?To help understand that question, the results of several important studies have recently been published. In this edition of Vascular Medicine, Yamamoto and colleagues explore the utility of SPP i...