🩸 Methemoglobin (MetHb) Test
🩸 Methemoglobin (MetHb) Test — Complete Clinical Guide
Essential diagnostic test for identifying methemoglobinemia and oxygen-carrying dysfunction
Methemoglobin (MetHb) is an oxidized form of hemoglobin in which iron exists in the ferric (Fe³⁺) state instead of the normal ferrous (Fe²⁺) form. Since ferric iron cannot bind oxygen, increased MetHb levels cause a significant drop in oxygen-carrying capacity, leading to cyanosis and tissue hypoxia.
MetHb testing is essential in emergency medicine, toxicology, pediatrics, pulmonology, and critical care.
🧬 1. What Is Methemoglobin?
Under normal conditions, hemoglobin exists in the ferrous (Fe²⁺) state and binds oxygen reversibly.
When oxidized to ferric (Fe³⁺) form:
- Oxygen binding becomes impossible
- Oxygen dissociation curve shifts left, impairing oxygen release from the remaining normal Hb
- Overall tissue oxygen delivery worsens
Even moderate increases can cause cyanosis and neurologic symptoms.
🎯 2. Purpose of the MetHb Test
Major Clinical Indications
- Diagnosis of methemoglobinemia
- Evaluation of drug- or toxin-induced oxidative stress
- Benzocaine, prilocaine
- Dapsone
- Nitrates/nitrites
- Aniline dyes
- Work-up of cyanosis of unclear cause
- When SpO₂ does not match PaO₂ (“saturation gap”)
- Assessment of oxygen-carrying capacity in critically ill patients
- Evaluation of congenital methemoglobinemia
- Cytochrome b5 reductase deficiency
- Hemoglobin M disease
🧪 3. Test Method — Co-oximetry
Pulse oximetry cannot accurately measure MetHb.
The gold standard is multi-wavelength Co-oximetry performed during blood gas analysis.
Principle of Co-oximetry
- Uses 4–128 wavelengths of transmitted light
- Distinguishes HbO₂, HHb, COHb, and MetHb based on absorption spectra
- MetHb has a characteristic peak near 630 nm
- Provides direct quantitative measurement
Specimen
- Whole blood (arterial or venous)
- Immediate analysis required
- Delays cause continued oxidation → falsely elevated results
📏 4. Reference Range
| Category | MetHb (%) |
|---|---|
| Normal | 0.0–1.5% |
| Mild elevation | 1.5–10% |
| Cyanosis | 10–20% |
| Symptomatic | 20–30% (headache, dizziness) |
| Severe | 30–50% (tachypnea, confusion) |
| Life-threatening | >50–70% |
🩺 5. Clinical Significance
A) Causes of Increased MetHb
1) Drugs and Toxic Agents (most common)
- Dapsone
- Benzocaine, prilocaine (local anesthetics)
- Nitrates/nitrites
- Nitroglycerin, nitroprusside
- Aniline dyes
- Sulfonamides
2) Congenital Methemoglobinemia
- Cytochrome b5 reductase deficiency (Type I/II)
- Hemoglobin M variants
3) Acquired diseases
- Sepsis
- Increased oxidative stress
- G6PD deficiency (risk rises with oxidative medications)
⚠️ 6. Correlation Between MetHb Level and Symptoms
| MetHb (%) | Symptoms |
|---|---|
| <1.5% | Normal |
| 10–15% | Central cyanosis; low SpO₂ but normal PaO₂ |
| 20–30% | Headache, dyspnea, dizziness |
| 30–50% | Confusion, tachycardia, hypotension |
| >50% | Coma, arrhythmia, death risk |
🔍 7. Interpretation Pearls
1) SpO₂–PaO₂ mismatch (“saturation gap”)
- Pulse oximeter: ~85% plateau regardless of true oxygenation
- PaO₂: often normal
➡ Suspicious for MetHb or COHb → confirm via Co-oximetry
2) Chocolate-brown blood
Highly suggestive of elevated MetHb.
3) G6PD deficiency consideration
The antidote methylene blue requires NADPH, which G6PD-deficient patients cannot generate.
➡ Methylene blue is contraindicated → risk of hemolysis.
4) Drug history is essential
Recent anesthesia, NO donor therapy, or chemical exposure increases risk.
⚠️ 8. Precautions and Interferences
- Pulse oximetry is not diagnostic
- Delayed sample analysis → false elevation
- Mild elevation common in sepsis
- Hyperbilirubinemia or lipemia may cause optical interference
- Neonates receiving inhaled NO must be routinely monitored for MetHb
✅ 9. Summary
- Methemoglobin is a non-functional oxidized hemoglobin form (Fe³⁺)
- Measured by Co-oximetry, not pulse oximetry
- Normal level <1.5%
- Major causes: drugs, toxins, congenital enzyme defects
- “Saturation gap” is a key diagnostic clue
- Treatment is methylene blue except in G6PD deficiency
📚 References
- Wright RO, et al. Methemoglobinemia: Clinical features and diagnosis. UpToDate.
- Ash-Bernal R, et al. Acquired methemoglobinemia: A retrospective series. Medicine (Baltimore).
- American Association for Respiratory Care. Co-oximetry principles.
- Tintinalli JE. Emergency Medicine: Methemoglobinemia.
- Clinical Methods: Hemoglobin derivatives by spectrophotometry.