Calciphylaxis is a syndrome of microvascular calcification and thrombosis leading to painful purpuric skin lesions that progress to necrotic ulcers. Calciphylaxis occurs primarily in patients with ESRD, for which its prevalence has been estimated at 1% annually, and carries a high mortality rate.1,2 Increasingly, sodium thiosulfate (STS) is used off-label for treating calciphylaxis. The STS mechanism of action in this condition is unknown. One postulated mechanism is by binding to calcium phosphate salts to form soluble calcium thiosulfate, though additional antioxidant, vasodilatory, and direct inhibitory actions on vascular calcification have been proposed.