Quick bedside tests for Methemoglobinemia

Case: 54 years old farmer is admitted to ICU with severe exacerbation of congestive heart failure (CHF). Intrinsic nitric oxide (iNO) is started for right heart support, and sodium nitroprusside (Nipride) is started as an intravenous infusion. 48 hours later, there is a concern for Methemoglobinemia (MetHb) due to bluish discoloration of the skin and low pulse oximetry level on the monitor. What bedside tests can be performed to quickly detect MetHb?

Answer: Although labs can perform methemoglobin levels, the turnaround time varies. Most hospitals establish protocols to check methemoglobin levels daily in patients who are on iNO or high-risk medications. Our patient has three such high risks, i.e., as a farmer, potential exposure to pesticides, iNO, and sodium nitroprusside infusion. A clinician can establish MetHb by performing the following two tests:

1. Bubble 100% oxygen in a tube with the patient's dark blood. No change in Color strongly predicts Methemoglobinemia. If blood turns red on exposure to oxygen, the cause is probably a hemodynamic issue. 

2. Apply 2 drops of the patient's blood on white filter paper and expose it to atmospheric oxygen. A change in color rules out Methemoglobinemia.

On a side note, though co-oximetry is an accurate method for measuring Methemoglobinemia, not all machines can (some newer versions can) differentiate it from another rare disorder called sulfhemoglobinemia.

#hematology

References:

1. Senthilkumaran S, Senthilraj MP, Jena NN, Thirumalaikolundusubramanian P. Methaemoglobinaemia: Recognition and realisation at bedside. Indian J Anaesth. 2018 Jun;62(6):475-476. doi: 10.4103/ija.IJA_138_18. PMID: 29962533; PMCID: PMC6004751.

2. Dipen B Khanapara, Ronald A Sacher - Methemoglobinemia - Updated: Dec 07, 2023 - Link: https://emedicine.medscape.com/article/204178-overview (Last accessed July 18, 2025)

3. Demedts P, Wauters A, Watelle M, Neels H. Pitfalls in discriminating sulfhemoglobin from methemoglobin. Clin Chem. 1997 Jun;43(6 Pt 1):1098-9. PMID: 9191578.

4. Wu C, Kenny MA. A case of sulfhemoglobinemia and emergency measurement of sulfhemoglobin with an OSM3 CO-oximeter. Clin Chem. 1997 Jan;43(1):162-6. PMID: 8990240.