Methemoglobinemia diagnostic study of choice
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Aksiniya Stevasarova, M.D.
Overview
The diagnostic study of choice for methemoglobinemia is direct measurement of methemoglobin by a multiple wavelength co-oximeter. Also clinical cyanosis in the presence of normal arterial oxygen tensions is highly suggestive of methemoglobinemia.
Diagnostic Study of Choice
Study of choice
Direct measurement of methemoglobin by a multiple wavelength co-oximeter is the gold standard test for the diagnosis of methemoglobinemia.
Co-oximetry
The co-oximeter is the most accurate device to measure methemoglobin. The newer generation devices can actually differentiate between methemoglobin, carboxyhemoglobin, oxyhemoglobin, deoxyhemoglobin, and also sulfhemoglobin.[1]
The comparison of various diagnostic studies for methemoglobinemia
| Test | Sensitivity | Specificity |
|---|---|---|
| Co-oximetry | 100% | 100% |
| Pulse-oximetry | 85% | 85% |
[Co-oximetry] is the preferred investigation based on the sensitivity and specificity
Diagnostic results
In methemoglobinemia patients, we often see the so called “saturation gap” which can help us diagnose the condition. The gap is calculated by subtracting the oxygen percentage from the ABG analysis (typically normal in methemoglobinemia patients 100%) from the percentage of oxygen saturation given by the pulse oximeter (always ~85% in methemoglobinemia patients). Saturation gap more than 5% is significant.
The following finding on performing saturation gap is confirmatory for methemoglobinemia:
- Saturation gap more than 5%.
Sequence of Diagnostic Studies
The various investigations must be performed in the following order:
- Direct measurement with co-oximetry
- Calculation of the saturation gap
- The pulse oximetry in methemoglobinemia patients is inaccurate and unreliable, and will always show a value around 85%, regardless of the level of MetHb in the blood. This is very important to know as this value can be misleading especially in patients with very high MetHb levels. Fortunately new multiwavelength pulse oximeters have been developed recently and they can detect the levels of MetHb more accurately. [2]
Name of Diagnostic Criteria
There are no established criteria for the diagnosis of methemoglobinemia.
References
- ↑ Feiner JR, Bickler PE, Mannheimer PD (2010). "Accuracy of methemoglobin detection by pulse CO-oximetry during hypoxia". Anesth Analg. 111 (1): 143–8. doi:10.1213/ANE.0b013e3181c91bb6. PMID 20007731.
- ↑ Van Leeuwen SR, Baranoski GVG, Kimmel BW (2017). "Three-wavelength method for the optical differentiation of methemoglobin and sulfhemoglobin in oxygenated blood". Conf Proc IEEE Eng Med Biol Soc. 2017: 4570–4573. doi:10.1109/EMBC.2017.8037873. PMID 29060914.