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Back to Test Catalogue
Test ID: CYB5RED WB

Cytochrome b5 Reductase Enzyme Activity, Whole Blood

Test Overview

Clinical Utility

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

Spectrophotometry - kinetic

Method Description

Cytochrome b5 reductase (methemoglobin reductase) catalyzes the 1,4-dihydronicotinamide adenine dinucleotide (NADH)-linked reduction of several substrates, including ferricyanide. The activity at 37 degrees C is measured spectrophotometrically by measuring the oxidation of NADH to NAD(+) at 340 nm on an automated chemistry analyzer.

Result Included

Cytochrome b5 Reductase

Aliases/Synonyms

Diaphorase; Ferrihemoglobin Reductase; Methemoglobin Reductase; Methemoglobin Diaphorase

Specimen

Specimen Type

Blood

Containers

Preferred Containers

Yellow top (ACD)

Acceptable Containers

Lavender top (EDTA)

Volume

Sample Volume

6.0 mL

Minimum Volume

1.0 mL

Collection & Handling

Handling Information

Send whole blood specimen in original tube. Do not aliquot.

Stability

Refrigerated
18 days

Rejection Criteria

Criteria Specification
Hemolysis Gross

Performance & Interpretation

Turnaround Time

10 days

Results

Name
Units
Reference Range
Conversion
  1. Cytochrome b5 Reductase
    U/g Hb
    ≥12 months: 7.8 - 13.1

    Comment

    Reference values have not been established for patients who are younger than 12 months.
    Cytochrome b5 reductase (methemoglobin reductase) activity in neonates (0-6 weeks of age) 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.
    Individuals who are glucose-6-phosphate dehydrogenase deficient are not candidates for methylene blue therapy. Administration of methylene blue to such persons will cause hemolysis or methemoglobin formation.

Referral Location

Out-of-Country

Interface & Setup

HL7 Interface Codes

Order Code Result Name Result Codes Units
CYB5RED WB Cytochrome b5 Reductase, B 65501 U/g Hb

Test Version

Last Updated

2025-11-11