This prospective, observational registry aims to collect real-world data regarding the safety and efficacy of ProvayBlue® (methylene blue 0.5%) used according to normal standard of care for the treatment of acquired methemoglobinemia. Methylene blue has been used for decades as a rescue medication for the treatment of methemoglobinemia, a rare and potentially life-threatening condition in which elevated levels of methemoglobin impede the delivery of oxygen from blood to body tissues. However, consistent prospective data about the safety and efficacy of this medication are sparse, simply because of the rarity of the disorder. ProvayBlue® received accelerated FDA approval for treatment of acquired methemoglobinemia in 2016. This large, prospective, multi-center observational registry has been initiated to gain more information on the use of methylene blue in the treatment of acquired methemoglobinemia.

Inclusion Criteria:

• Adult or pediatric patients diagnosed with acquired methemoglobinemia and receiving treatment with ProvayBlue® (methylene blue 0.5%) as per the treating physician's diagnosis and the acute care facility's standard of care
• Those acquired methemoglobinemia patients whose diagnosis is aided by measurement of methemoglobin and whose ongoing treatment is guided by re-measurement of methemoglobin ~1h post-treatment with ProvayBlue® in accordance with the US FDA Label prescribing information

Exclusion Criteria:

• Refusal of consent (in those subjects approached for consent where required by local institutional procedures)
• Treatment of methemoglobinemia with another methylene blue product

Patients who present in hospital / urgent care setting diagnosed with acquired methemoglobinemia.