Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test
Methemoglobinemia, with or without sulfhemoglobinemia, is most commonly encountered as a result of administration of such medications as phenacetin, phenazopyridine, sulfonamides, local anesthetics, dapsone, or following ingestion of nitrites or nitrates.
Congenital methemoglobinemias are rare. They are due either to:
-A deficiency of methemoglobin reductase (also called cytochrome B5 reductase or diaphorase) in erythrocytes, an autosomal recessive disorder
-One of several intrinsic structural disorders of hemoglobin, called methemoglobin-M, all of which are inherited in the autosomal dominant mode
Sulfhemoglobinemia often accompanies methemoglobinemia. Sulfhemoglobinemia can be due to exposure to trinitrotoluene and/or zinc ethylene bisdithiocarbamate (a fungicide). The formation of sulfhemoglobin cannot be reversed and there is no therapy for sulfhemoglobinemia. Because patients with sulfhemoglobinemia also often have methemoglobinemia, therapy is directed at reversing the methemoglobinemia present.
Symptoms of both methemoglobinemia and sulfhemoglobinemia are caused by anoxia and are characterized by cyanosis.
Diagnosis of methemoglobinemia and sulfhemoglobinemia
Differentiation of methemoglobinemia and sulfhemoglobinemia from other causes of cyanosis (eg, congenital heart disease)
In congenital methemoglobinemia, the methemoglobin concentration in blood is about 15% to 20% of total hemoglobin. Such patients are mildly cyanotic and asymptomatic.
In acquired (toxic) methemoglobinemia, the concentration may be much higher. Symptoms may be severe when methemoglobin is >40% of hemoglobin. Very high concentrations may be fatal.
This is a consultative evaluation in which the history and previous laboratory values are reviewed by a hematologist who is an expert on these disorders. Appropriate tests are performed and an interpretive report is issued.
Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances
Sulfhemoglobin is exceedingly stable and does not change in stored or shipped specimens.
Methemoglobin is unstable and can degrade at a rate of about 40% per 24 hours.
A normal methemoglobin value obtained with stored or shipped specimens does not exclude prior methemoglobinemia of minimal degree. However, significant methemoglobinemia will still be demonstrable.
Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.
Definitive results and an interpretive report will be provided.
Clinical References Provides recommendations for further in-depth reading of a clinical nature
Beutler E: Methemoglobinemia and sulfhemoglobinemia. In Hematology. Fifth edition. Edited by E Beutler, MA Lichtman, BS Caller, TJ Kipps. New York, McGraw-Hill Book Company, 1995, pp 654-663