MEthylene Blue In Patients With Acquired Methemoglobinemia

Inclusion Criteria: * Pediatric or adult patients (males and females of all ages are included) diagnosed with acquired methemoglobinemia and receiving treatment with ProvayBlue™ as per the treating physician's diagnosis and hospital standard of care. Acquired methemoglobinemia is defined as a level of methemoglobinemia \>30% or ≤30% in case of clinical symptoms (e.g. sleepiness, cyanosis, dizziness, etc.). * Written informed consent obtained prior to any data collection (retrospective and prospective) for this study and study specific assessments. Exclusion Criteria: * Known severe hypersensitivity reactions to methylene blue or any other thiazine dye; * Known deficiency in glucose-6-phosphate dehydrogenase (G6PD) due to the risk of hemolytic anemia as well as lack of therapeutic effect; * Known deficiency in Nicotinamide Adenine Dinucleotide Phosphate Hydrogen (NADPH) reductase. * Known use of selective serotonin reuptake inhibitors (SSRIs), serotonin and norepinephrine reuptake inhibitors (SNRIs), MonoAmine Oxidase (MAO) inhibitors or drugs metabolised via CYP isoenzymes anticipated during the treatment phase of the study. * Women who refuse to stop breastfeeding for up to 8 days after receiving the last dose of ProvayBlueTM.