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Test ID: METR1 Cytochrome b5 Reductase Enzyme Activity, Blood

Necessary Information

Patient's age is required.

Specimen Required

Collection Container/Tube:

Preferred: Yellow top (ACD)

Acceptable: Lavender top (EDTA)

Specimen Volume: 6 mL

Collection Instructions: Send specimen in original tube. Do not transfer blood to other containers.

Useful For

Evaluation of patients with cyanosis


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


Functional studies in families with cytochrome b5 reductase deficiency

Method Name

Kinetic Spectrophotometry (KS)

Reporting Name

Cytochrome b5 Reductase, B

Specimen Type

Whole Blood ACD-B

Specimen Minimum Volume

1 mL

Specimen Stability Information

Specimen Type Temperature Time Special Container
Whole Blood ACD-B Refrigerated 18 days

Clinical Information

Cytochrome b5 reductase (CYB5R), also called methemoglobin reductase, is the enzyme within the erythrocyte that maintains hemoglobin in the reduced (non-methemoglobin) state. A deficiency of CYB5R in erythrocytes is an autosomal recessive disorder resulting from variants in the CYB5R3 or the CYB5A genes.


Persons who are heterozygous for CYB5R genetic variants 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 CYB5R genetic variants 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 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 it 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.

Reference Values

≥12 months of age: 7.8-13.1 U/g Hb

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


Cytochrome b5 reductase (methemoglobin 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 cytochrome b5 reductase activity and increased levels of methemoglobin.

Clinical Reference

1. Agarwal AM, Prchal JT: Methemoglobinemia and other dyshemoglobinemias. In: Kaushansky K, Lichtman MA, Prchal JT, et al, eds. Williams Hematology. 9th ed. McGraw-Hill; 2016:789-800

2. Percy MJ, Barnes C, Crighton G, et al: Methemoglobin reductase deficiency: Novel mutation is associated with a disease phenotype of intermediate severity. J Pediatr Haematol Oncol. 2012;34:457-460

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.

LOINC Code Information

Test ID Test Order Name Order LOINC Value
METR1 Cytochrome b5 Reductase, B 32703-1


Result ID Test Result Name Result LOINC Value
METRB Cytochrome b5 Reductase, B 32703-1
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