| Domain | High-value finding | Quantitative threshold/data | Ontology suggestion |
|---|---|---|---|
| Normal physiology | Methemoglobin (MetHb) is continuously generated but normally reduced within erythrocytes | Usually **<1%–2% of total hemoglobin** | GO:0019825 oxygen binding; CL:0000232 erythrocyte |
| Severity: minimal | Low pulse-oximeter readings or pale, gray, or blue skin may occur; patients are generally asymptomatic | **MetHb <10%** | HP:0012418 hypoxemia; HP:0032239 abnormal blood oxygen level |
| Severity: mild | Cyanosis and dark-brown or chocolate-colored blood; asymptomatic status or confusion is possible | **MetHb 10%–30%** | HP:0000961 cyanosis; HP:0001289 confusion |
| Severity: moderate | Functional anemia causes dyspnea, dizziness, syncope, chest pain, palpitations, headache, and fatigue | **MetHb 30%–50%** | HP:0002094 dyspnea; HP:0002321 vertigo; HP:0001279 syncope; HP:0001695 palpitations; HP:0002315 headache; HP:0012378 fatigue |
| Severity: severe | Tachypnea, metabolic acidosis, dysrhythmia, seizure, delirium, or coma may develop | **MetHb 50%–70%** | HP:0002789 tachypnea; HP:0001942 metabolic acidosis; HP:0011675 arrhythmia; HP:0001250 seizure; HP:0001259 coma |
| Severity: potentially fatal | Profound tissue hypoxia and death; urgent rescue treatment is required | **MetHb >70%** | HP:0012418 hypoxemia; GO:0070482 response to oxygen levels |
| Diagnostic clue | Suspect when cyanosis or low SpO₂ fails to correct with supplemental oxygen despite a normal or high PaO₂ | SpO₂ often trends toward **~85%**; a **saturation gap >5%** is suggestive | HP:0000961 cyanosis; HP:0012418 hypoxemia |
| Confirmatory test | Multiwavelength blood co-oximetry directly quantifies MetHb and is preferred over conventional pulse oximetry | Report MetHb as **percentage of total hemoglobin**; arterial or venous blood may be used | LOINC: methemoglobin/total hemoglobin in blood; NCIT: C111159 Co-Oximetry |
| Visual finding | Blood remains chocolate-brown rather than becoming red after oxygen exposure | Commonly apparent with clinically important MetHb, often **>10%–20%** | HP:0031816 abnormal blood color |
| Key drug triggers | Dapsone, benzocaine, prilocaine, lidocaine, phenazopyridine, primaquine/chloroquine, sulfonamides, rasburicase, metoclopramide, nitroglycerin, nitroprusside, and inhaled nitric oxide | Risk is exposure- and dose-dependent; dapsone and inhaled nitric oxide predominate in monitored hospital populations | CHEBI terms for individual oxidants; NCIT: Pharmacologic Substance |
| Other triggers | Sodium/amyl/isobutyl nitrite, nitrate-contaminated food or water, aniline/nitrobenzene, chlorates, pesticides, smoke inhalation, and some recreational-drug adulterants | Intentional nitrite ingestion can produce **MetHb >30%** and fatal poisoning | CHEBI:16301 nitrite; CHEBI:17632 nitrate; NCIT: Environmental Exposure |
| Initial management | Stop or remove the oxidant, administer oxygen, obtain IV access, correct acidosis/hypoglycemia, and provide cardiopulmonary or seizure support as required | Treat the patient immediately when unstable; do not delay care while distinguishing acquired from hereditary disease | NCIT: C71943 Supportive Care; NCIT: Oxygen Therapy |
| Treatment threshold | Methylene blue is indicated for symptomatic acquired disease and for high levels even without symptoms; lower thresholds apply when oxygen delivery is already compromised | Common thresholds: **≥20% if symptomatic** or **≥30% if asymptomatic**; consider treatment near **10%** with severe anemia or cardiac/pulmonary disease | NCIT: Methylene Blue Treatment; CHEBI:6872 methylene blue |
| First-line antidote | IV methylene blue accepts electrons from NADPH; leukomethylene blue reduces ferric Fe³⁺ back to ferrous Fe²⁺ in erythrocytes | **1–2 mg/kg IV over 3–5 min** | CHEBI:6872 methylene blue; GO:0055114 oxidation-reduction process |
| Repeat and maximum dosing | Repeat only if clinical findings or MetHb fail to improve; excessive dosing can worsen MetHb and cause hemolysis | Repeat **1 mg/kg after 30–60 min**; avoid cumulative doses **>5.5–7 mg/kg** | NCIT: Repeat Dose; HP:0001878 hemolytic anemia |
| G6PD caution | Methylene blue may be ineffective and can provoke oxidative hemolysis because NADPH generation is impaired | Avoid when G6PD deficiency is known; rapidly weigh risk versus benefit in life-threatening poisoning | HP:0001878 hemolytic anemia; GO:0004345 glucose-6-phosphate dehydrogenase activity; CL:0000232 erythrocyte |
| Serotonergic-drug caution | Methylene blue inhibits monoamine oxidase A and may precipitate serotonin syndrome with serotonergic medicines | Review SSRIs, SNRIs, MAO inhibitors, and other serotonergic agents before administration when feasible | HP:0011447 serotonin syndrome; GO:0004491 monoamine oxidase activity |
| Alternative or adjunctive therapy | Ascorbic acid reduces MetHb nonenzymatically but acts more slowly; dosing is not standardized | Often requires **≥24 h**; reported adult regimens vary from **0.5 g every 12 h** to larger IV doses | CHEBI:29073 L-ascorbic acid; GO:0016209 antioxidant activity |
| Refractory disease | Exchange transfusion, red-cell transfusion, or hyperbaric oxygen may be used when methylene blue is contraindicated or ineffective | Therapeutic whole-blood exchange has reported **81.6% survival** in methylene-blue-refractory cases | NCIT: C15283 Exchange Transfusion; NCIT: C135060 Hyperbaric Oxygen Therapy |
| Rebound monitoring | Long-acting or lipophilic oxidants can cause recurrent MetHb after an initial response; dapsone is especially important because of active metabolites and enterohepatic recirculation | Rebound may occur for **up to 18 h**; dapsone cases may require repeat treatment every **6–8 h for 2–3 days** | NCIT: Therapeutic Drug Monitoring; HP:0031796 recurrent symptom |
| Clinical course | Most recognized and promptly treated episodes resolve completely after withdrawal of the trigger and restoration of functional hemoglobin | MetHb should decline substantially within **30–60 min** after effective methylene blue | MONDO: acquired methemoglobinemia; GO:0036293 response to decreased oxygen levels |


*Table: This table consolidates severity strata, diagnostic clues, common oxidant triggers, antidotal treatment, contraindications, rescue options, and rebound monitoring for drug-induced methemoglobinemia. It is based on expert recommendations, recent hospital data, and clinical reviews (pqac-00000010, pqac-00000011, pqac-00000035, pqac-00000037, pqac-00000039, pqac-00000041).*