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Congenital Methemoglobinemia via the CYB5R3 Gene

Summary and Pricing

Test Method

Exome Sequencing with CNV Detection
Test Code Test Copy GenesTest CPT Code Gene CPT Codes Copy CPT Codes Base Price
CYB5R3 81479 81479,81479 $990
Test Code Test Copy Genes Test CPT Code Gene CPT Codes Copy CPT Code Base Price
9947CYB5R381479 81479,81479 $990 Order Options and Pricing

Pricing Comments

Our favored testing approach is exome based NextGen sequencing with CNV analysis. This will allow cost effective reflexing to PGxome or other exome based tests. However, if full gene Sanger sequencing is desired for STAT turnaround time, insurance, or other reasons, please see link below for Test Code, pricing, and turnaround time information. If the Sanger option is selected, CNV detection may be ordered through Test #600.

An additional 25% charge will be applied to STAT orders. STAT orders are prioritized throughout the testing process.

Click here for costs to reflex to whole PGxome (if original test is on PGxome Sequencing platform).

Click here for costs to reflex to whole PGnome (if original test is on PGnome Sequencing platform).

The Sanger Sequencing method for this test is NY State approved.

For Sanger Sequencing click here.

Turnaround Time

3 weeks on average for standard orders or 2 weeks on average for STAT orders.

Please note: Once the testing process begins, an Estimated Report Date (ERD) range will be displayed in the portal. This is the most accurate prediction of when your report will be complete and may differ from the average TAT published on our website. About 85% of our tests will be reported within or before the ERD range. We will notify you of significant delays or holds which will impact the ERD. Learn more about turnaround times here.

Targeted Testing

For ordering sequencing of targeted known variants, go to our Targeted Variants page.


Genetic Counselors


  • Luke Drury, PhD

Clinical Features and Genetics

Clinical Features

In normal individuals, hemoglobin can become oxidized to form methemoglobin which is unable to bind oxygen. Congenital methemoglobinemia is a disorder characterized by chronically elevated methemoglobin levels due to impaired reduction of methemoglobin back to hemoglobin. There are two forms of the disease. Type I disease individuals present with cyanosis, slate-blue skin color, due to decreased hemoglobin oxygen saturation. However, most people are asymptomatic with benign methemoglobinemia. Type II disease symptom onset occurs in infants and is characterized by cyanosis, severe developmental abnormalities including mental retardations and failure to thrive. Other symptoms may include neurologic problems such as microcephaly, opisthotonus, seizures, athetoid movements, and spastic quadriparesis resulting from abnormal lipid elongation and desaturation and damage to the central nervous system. About 10-15% of cases of congenital methemoglobinemia are type II (Ewenczyk et al. 2008; Percy and Lappin 2008).

Both type I and II methemoglobinemia are due to cytochrome B5 deficiency with type I deficiency being restricted to erythrocytes compared to type II which affects all cells. Type I mutations are associated with decreased stability of the cytochrome b5 reductase protein. In non-erythroid cells, both soluble and membrane bound forms are synthesized replacing the unstable form. Erythrocytes are unable to compensate via protein synthesis as they are anuclear (Percy and Lappin 2008; Warang et al. 2015). Methemoglobinemia may also be caused by Hemoglobin M disease through mutations in either the HBA1 or HBB genes. Acquired methemoglobinemia can result from drug and topical anesthetics including dapsone and benzocaine (Singh et al .2014; Vallurupalli and Manchanda 2011).


Type I and II methemoglobinemia are inherited in an autosomal recessive manner through mutations in the CYB5R3 gene. The CYB5R3 gene encodes cytochrome b5 reductase and is involved in transfer of electrons from NADH to cytochrome b5. There are several isoforms generated through use of alterative promoters of cytochrome b5 resulting in both soluble and membrane bound forms of the protein. However, these isoforms only differ at exon 1 where there have been no reports of causative variants for methemoglobinemia. 

In type I methemoglobinemia, cytochrome b5 reductase deficiency is limited to erythrocytes which are incapable of replenishing easily degraded, heat-liable mutated proteins. This results in an impaired ability to convert methemoglobin back to its oxygen binding form, hemoglobin. Type I mutations are predominantly missense and non-overlapping with type II methemoglobinemia (Ewenczyk et al. 2008; Warang et al. 2015).

Type II methemoglobinemia, cytochrome b5 reductase deficiency is throughout the body and occurs in the membrane bound isoform present on the endoplasmic reticulum and outer mitochondrial membrane. The membrane bound form of cytochrome b5 reductase plays important roles in fatty acid desaturation and drug metabolism. Type II mutations primarily include deletions, nonsense, and splice site alterations leading to loss of protein. In rarer cases, missense mutations have been reported in patients with type II disease (Kugler et al. 2001; Percy and Lappin 2008).

Clinical Sensitivity - Sequencing with CNV PGxome

In patients with laboratory findings indicating decreased cytochrome b5 reductase activity in erythrocytes and whole blood, clinical sensitivity for detection of CYB5R3 mutations is >95% (Ewenczyk et al. 2008; Percy and Lappin 2008). Analytical sensitivity should be high because the vast majority of mutations reported are detected by the method. Only one gross deletion has been reported in a single case (Human Gene Mutation Database).

Testing Strategy

This test provides full coverage of all coding exons of the CYB5R3 gene plus 10 bases of flanking noncoding DNA in all available transcripts along with other non-coding regions in which pathogenic variants have been identified at PreventionGenetics or reported elsewhere. We define full coverage as >20X NGS reads or Sanger sequencing. PGnome panels typically provide slightly increased coverage over the PGxome equivalent. PGnome sequencing panels have the added benefit of additional analysis and reporting of deep intronic regions (where applicable).

Dependent on the sequencing backbone selected for this testing, discounted reflex testing to any other similar backbone-based test is available (i.e., PGxome panel to whole PGxome; PGnome panel to whole PGnome).

Indications for Test

Candidates for testing include cyanosis in the presence of normal arterial pO2, co-oximeter reading with peak absorbance at 631nm, and dark red, chocolate, or brownish to blue blood color. Co-oximeter readings suggesting methemoglobinemia are confirmed by Evelyn-Malloy method. It should be noted that these tests do not distinguish between inherited and acquired forms of methemoglobinemia (Percy and Lappin 2008). This test may also be considered for the reproductive partners of individuals who carry pathogenic variants in CYB5R3.


Official Gene Symbol OMIM ID
CYB5R3 613213
Inheritance Abbreviation
Autosomal Dominant AD
Autosomal Recessive AR
X-Linked XL
Mitochondrial MT


Name Inheritance OMIM ID
Methemoglobinemia AR 250800


  • Ewenczyk C, Leroux A, Roubergue A, Laugel V, Afenjar A, Saudubray JM, Beauvais P, Villemeur TB de, Vidailhet M, Roze E. 2008. Recessive hereditary methaemoglobinaemia, type II: delineation of the clinical spectrum. Brain 131: 760–761. PubMed ID: 18202104
  • Human Gene Mutation Database (Bio-base).
  • Kugler W, Pekrun A, Laspe P, Erdlenbruch B, Lakomek M. 2001. Molecular basis of recessive congenital methemoglobinemia, types I and II: Exon skipping and three novel missense mutations in the NADH-cytochrome b5 reductase (diaphorase 1) gene. Human mutation 17: 348–348. PubMed ID: 11295830
  • Percy MJ, Lappin TR. 2008. Recessive congenital methaemoglobinaemia: cytochrome b 5 reductase deficiency. British Journal of Haematology 0: 080305033838221–??? PubMed ID: 18318771
  • Singh S, Sethi N, Pandith S, Ramesh G. 2014. Dapsone-induced methemoglobinemia: “Saturation gap”-The key to diagnosis. Journal of Anaesthesiology Clinical Pharmacology 30: 86. PubMed ID: 24574600
  • Vallurupalli S, Manchanda. 2011. Risk of acquired methemoglobinemia with different topical anesthetics during endoscopic procedures. Local and Regional Anesthesia 25. PubMed ID: 22915889
  • Warang PP, Kedar PS, Shanmukaiah C, Ghosh K, Colah RB. 2015. Clinical spectrum and molecular basis of recessive congenital methemoglobinemia in India. Clinical Genetics 87: 62–67. PubMed ID: 24266649


Ordering Options

We offer several options when ordering sequencing tests. For more information on these options, see our Ordering Instructions page. To view available options, click on the Order Options button within the test description.

myPrevent - Online Ordering

  • The test can be added to your online orders in the Summary and Pricing section.
  • Once the test has been added log in to myPrevent to fill out an online requisition form.
  • PGnome sequencing panels can be ordered via the myPrevent portal only at this time.

Requisition Form

  • A completed requisition form must accompany all specimens.
  • Billing information along with specimen and shipping instructions are within the requisition form.
  • All testing must be ordered by a qualified healthcare provider.

For Requisition Forms, visit our Forms page

If ordering a Duo or Trio test, the proband and all comparator samples are required to initiate testing. If we do not receive all required samples for the test ordered within 21 days, we will convert the order to the most effective testing strategy with the samples available. Prior authorization and/or billing in place may be impacted by a change in test code.

Specimen Types

Specimen Requirements and Shipping Details

PGxome (Exome) Sequencing Panel

PGnome (Genome) Sequencing Panel

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Note: acceptable specimen types are whole blood and DNA from whole blood only.
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