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D51.2 – Vitamin B12 Deficiency Anaemia Due to TCII
D51.2 is the ICD-10 code for vitamin B12 deficiency anaemia due to transcobalamin II deficiency, a rare inherited disorder affecting the transport of vitamin B12 into body cells.
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D51.2 is the ICD-10 code for vitamin B12 deficiency anaemia due to transcobalamin II deficiency, a rare inherited disorder affecting the transport of vitamin B12 into body cells.
What is D51.2?
The ICD-10 code D51.2 refers to vitamin B12 deficiency anaemia caused by transcobalamin II (TCII) deficiency. This rare, inherited metabolic disorder impairs the transport of vitamin B12 (cobalamin) from the bloodstream into body cells. As a result, cells suffer from a functional vitamin B12 deficiency even when blood levels of B12 may appear normal or only mildly reduced.
Causes
Transcobalamin II is a plasma transport protein responsible for delivering absorbed vitamin B12 to tissues throughout the body. When this protein is absent or non-functional due to a genetic mutation, cells cannot take up sufficient vitamin B12. Key causes include:
- Genetic mutation: Autosomal recessive mutations in the TCN2 gene lead to reduced production or impaired function of transcobalamin II.
- Functional deficiency: Even with normal serum B12 levels, cellular uptake is severely impaired, causing a functional deficiency at the tissue level.
Symptoms
Symptoms most commonly appear in infancy or early childhood but can sometimes present later in life:
- Megaloblastic anaemia: Abnormally large, immature red blood cells resulting from impaired DNA synthesis
- Thrombocytopenia: Low platelet count, increasing the risk of bleeding
- Leukopenia: Reduced white blood cell count, increasing infection risk
- Growth retardation in infants and children
- Neurological symptoms: Developmental delay, hypotonia (low muscle tone), and in severe cases, dementia-like features
- Mucosal changes: Inflammation of the tongue (glossitis) and oral mucosa (stomatitis)
- Vomiting and diarrhoea
Diagnosis
Diagnosis of D51.2 requires specific investigations, as standard serum vitamin B12 measurements can be misleading:
- Full blood count: Reveals megaloblastic anaemia, thrombocytopenia, or leukopenia
- Serum vitamin B12: May be normal or only mildly reduced
- Methylmalonic acid and homocysteine: Elevated levels in blood or urine serve as reliable functional markers of B12 deficiency
- Transcobalamin II measurement: Direct quantification of TCII protein in the blood
- Genetic testing: Identification of mutations in the TCN2 gene confirms the diagnosis
Treatment
Treatment aims to overcome the transport defect by supplying vitamin B12 in quantities large enough to allow passive diffusion into cells:
- High-dose vitamin B12 supplementation: Regular intramuscular injections of hydroxocobalamin or cyanocobalamin in high doses to bypass the transport defect
- Regular medical monitoring: Frequent blood tests to assess treatment response and adjust dosing
- Early initiation of therapy: Critical to prevent irreversible neurological damage, especially in infants
Lifelong treatment is generally required. With early diagnosis and consistent therapy, many serious complications can be prevented and normal development can be supported.
References
- Carmel R. - Megaloblastic anemias. In: Hematology: Basic Principles and Practice, 7th ed. Elsevier, 2018.
- Gherasim C, Lofgren M, Banerjee R. - Navigating the B12 road: assimilation, delivery, and disorders of cobalamin. Journal of Biological Chemistry, 2013; 288(19): 13186–13193.
- Whitehead VM. - Acquired and inherited disorders of cobalamin and folate in children. British Journal of Haematology, 2006; 134(2): 125–136.
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