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Test ID METR Methemoglobin Reductase, Blood

Reporting Name

Methemoglobin Reductase, B

Useful For

Confirming cases of suspected methemoglobin reductase (cytochrome b5 reductase) deficiency


Functional studies in families with methemoglobin reductase (cytochrome b5 reductase) deficiency

Specimen Type

Whole Blood ACD-B

Specimen Required


Preferred: Yellow top (ACD)

Acceptable: Lavender top (EDTA)

Specimen Volume: 6 mL

Collection Instructions: Do not transfer blood to other containers.

Additional Information: Patient's age is required.

Specimen Minimum Volume

1 mL

Specimen Stability Information

Specimen Type Temperature Time Special Container
Whole Blood ACD-B Refrigerated 22 days

Reference Values

≥12 months: 6.6-13.3 U/g Hb

Reference values have not been established for patients who are <12 months of age.

Day(s) and Time(s) Performed

Tuesday, Thursday

Test Classification

This test was developed and its performance characteristics determined by Mayo Clinic in a manner consistent with CLIA requirements. This test has not been cleared or approved by the U.S. Food and Drug Administration.

CPT Code Information


LOINC Code Information

Test ID Test Order Name Order LOINC Value
METR Methemoglobin Reductase, B 32703-1


Result ID Test Result Name Result LOINC Value
9322 Methemoglobin Reductase, B 32703-1

Clinical Information

Methemoglobin reductase, also called "diaphorase," and more properly called cytochrome b5 reductase, is the enzyme within the erythrocyte that maintains hemoglobin in the reduced (non-methemoglobin) state.


Persons who are heterozygous for methemoglobin reductase mutations have no clinical or laboratory abnormalities, are not cyanotic, and have normal methemoglobin concentrations in their blood. However, they hold an increased risk for more severely symptomatic acute episodes of methemoglobinemia with exposure to inducing agents.


Persons who are homozygous for methemoglobin reductase mutations have normal arterial oxygen saturation but have varying quantities of methemoglobin in their blood, generally 15% to 20%, and are quite cyanotic. Paradoxically, homozygotes typically have normal blood counts; the condition only rarely causes polycythemia. The presence of methemoglobin shifts the hemoglobin-O2 dissociation curve to the right, so that although the transport of oxygen is diminished, the delivery of oxygen to tissues is normal. Because of the chronicity, the homozygous condition is usually compensated and therefore quite benign, but may cause concern to parents of affected children, be a cosmetic embarrassment to the children, and alarm the attending physician. The cyanosis may be treated with methylene blue.


Methemoglobin reductase (cytochrome b5 reductase) activity in neonates (0-6 weeks) is normally 60% of the normal adult value. Adult values are attained by 2 to 3 months of age.


Heterozygotes have results slightly lower than the reference range. Homozygotes demonstrate little to no methemoglobin reductase activity and increased levels of methemoglobin.

Clinical Reference

Beutler E: Methemoglobinemia and other causes of cyanosis. In Williams Hematology. Sixth edition. Edited by E Beutler, M Lichtman, WJ Williams, TJ Kipps. New York, McGraw-Hill Book Company, 2001, p 611

Analytic Time

5 days (not reported Saturday or Sunday)

Method Name

Kinetic Spectrophotometry (KS)


If not ordering electronically, complete, print, and send a Benign Hematology Test Request Form (T755) with the specimen.