Methemoglobinemia

By Ilene B. Anderson, PharmD; and Susan Y. Kim, PharmD

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Bedside Tests

There are several bedside tests that are of some historical interest and are widely cited in various reviews of the subject or in older texts. First, the venous blood in Mhgb is classically a dark, chocolate-brown color. This brown color will remain unchanged even when oxygen is bubbled through it in a test tube. In contrast, normal venous blood, even if it appears dark, will turn bright red after this procedure. Second, a drop of blood may be placed on a piece of white filter paper alongside a normal drop of blood. The methemoglobin-containing blood will appear dark brown in color. Third, lysis of the RBCs may be carried out by adding deionized water (1:100 dilution) and then one crystal of potassium cyanide. If methemoglobin is present, cyanomethemoglobin will be formed and the blood will turn bright red.¹³ This test is particularly useful in differentiating between sulfhemoglobin and methemoglobin, since there will be no color change for sulfhemoglobin.¹⁴ In a practical sense, these bedside tests are rarely used today because of concerns relating to bloodborne pathogens and toxic chemical exposure.

 

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