What is the role of lab studies in the workup of methemoglobinemia?

Updated: Dec 09, 2018
  • Author: Mary Denshaw-Burke, MD, FACP; Chief Editor: Emmanuel C Besa, MD  more...
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Investigations to rule out hemolysis (complete blood count [CBC], reticulocyte count, peripheral smear review, lactate dehydrogenase [LDH], bilirubin, haptoglobin and Heinz body preparation) and end-organ dysfunction or failure (liver function tests, electrolytes, renal function tests) should be included in the workup. Urine pregnancy tests should be performed in females of childbearing age.

Investigations to evaluate a hereditary cause for methemoglobinemia should be ordered when appropriate. Hemoglobin electrophoresis and DNA sequencing of the globin chain gene can be used to identify hemoglobin M.

Specific enzyme assays (nicotinamide adenine dinucleotide [NADH]–dependent reductase, cytochrome b5 reductase) may be determined, often in multiple cell lines (ie, platelets, granulocytes, and fibroblasts), to diagnose inherited cases. A quick and easy bedside test for determining whether dark blood is due to methemoglobinemia is to bubble 100% oxygen in a tube that contains the dark blood. Blood that remains dark likely does so because of the presence of methemoglobin.

Another simple test (and one that is less likely to splash potentially infectious blood) is to place 1-2 drops of blood on white filter paper, then evaluate for color change upon exposure to oxygen. (This test can be accelerated by gently blowing supplemental oxygen onto the filter paper.) Deoxygenated hemoglobin changes from dark red or violet to bright red, whereas methemoglobin remains brown.

Serum levels of nitrites or other offending drugs may be determined. Often, these results are not immediately available, and treatment may have to be started empirically if the index of suspicion is high.

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