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Chokri Ben Lamine: Methemoglobinemia in Hematology – Causes, Diagnosis, Antidote
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Apr 30, 2026, 12:45

Chokri Ben Lamine: Methemoglobinemia in Hematology – Causes, Diagnosis, Antidote

Chokri Ben Lamine, Assistant consultant at King Faisal Specialist Hospital and Research Center, shared a post on X:

“Methemoglobinemia in Hematology – Causes, Dx, Antidote

What is it?

Hb Fe²⁺ to Fe³⁺ leading to cannot carry O₂

Resulting in functional anemia and tissue hypoxia

Common causes in Hematology

  • Dapsone (very common in PCP prophylaxis)
  • Trimethoprim-sulfamethoxazole (rare)
  •  Benzocaine / lidocaine
  • Nitric oxide
  • Rasburicase  (esp. G6PD)
  • Primaquine
  • Nitrates / nitrites

High risk:

  • G6PD deficiency
  • Renal failure
  • High-dose or prolonged exposure

Clinical clues

  • Dyspnea, fatigue
  • Cyanosis not improving with O₂
  • Headache, confusion
  • Severe: arrhythmia, coma

Diagnosis

  • ABG: normal PaO₂
  • Pulse ox approximately 85% (fixed, not improving)
  • Co-oximetry elevated reveals metHb % (confirm)

‘Saturation gap’ is key clue

Severity guide

  • less than 10% – often asymptomatic
  • 10–20% – cyanosis
  • 20–50% – symptoms (dyspnea, fatigue)
  • more than 50% – life-threatening

Treatment

1. STOP offending drug (e.g., Dapsone)

2.Methylene blue

  • Dose: 1–2 mg/kg IV over 5 min
  • Repeat in 1 hr if needed

Requires NADPH 

  • Avoid in G6PD deficiency (risk hemolysis)

If G6PD deficiency / refractory

  • Vitamin C (ascorbic acid) high dose
  • Exchange transfusion (severe)
  • Hyperbaric O₂ (rare cases)

Monitoring

  • Serial metHb levels
  • Clinical response (O₂ sat improves)
  • Watch for rebound (esp. dapsone long half-life)

Hematology pearl

  • Dapsone – delayed and recurrent methemoglobinemia
  • May need repeated treatment

Take-home

  • Cyanosis and normal PaO₂ – think metHb
  • Dapsone is the primary culprit in hematology patients
  • Treat fast with methylene blue (unless G6PD).”

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