Aims A case report to highlight the importance of macrocytosis.

Methods A 7½ year old boy was referred with macrocytosis (MCV 108). He was fit and thriving child (weight, height 50^th centile). History and examination was unremarkable. Bloods done 5 years ago also showed high MCV (95). Routine tests for evaluation of macrocytosis were normal. Later FBC showed thrombocytopenia, leucopenia and neutropenia. He was finally diagnosed to have Fanconi anaemia. He has been put on register for bone marrow transplant, and whole family is being tested for gene mutations.

Fanconi anaemia is an autosomal recessive or X-linked form of inherited aplastic anaemia. Sixteen mutations (FA-A to FA-Q) have been identified. Hematologic findings evolve over months to years, with 70% of children developing bone marrow failure by age 10. They are at high risk for developing myelodysplastic syndrome, acute myeloid leukaemia, and squamous cell carcinoma. Characteristic congenital malformations are present in up to 70% of affected children (short stature, café-au-lait spots, hypopigmented spots, thumb abnormality, microcephaly, hydrocephaly, hypogonadism, developmental delay). Diagnosis is made by presence of increased chromosomal breakage in lymphocytes cultured in the presence of DNA cross-linking agents (mitomycin C or diepoxybutane). Bone marrow transplant is the only available curative treatment. Other treatment options include androgens, hematopoietic growth factors, and blood product transfusions with supportive care.

Results Hb 122, WCC 8.6, Platelets 164, MCV 102 in 2011. Hb 118, WCC 5.4, Neutrophils 1.34, Platelets 130, MCV 108 in 2014. U&E, LFT, immunoglobulins, B12, Folate, Ferritin, and TFT were normal. Bone marrow biopsy revealed mild aplasia, and no myelodysplastic syndrome or leukaemia. HIV, Hepatitis screen and viral serology were negative. Fanconi Anaemia test came back positive.

Conclusions Children presenting with macrocytosis should be carefully evaluated and referred to a haematologist, as it could be the first manifestation of a serious underlying bone marrow problem.

Macrocytosis in age range of six months to 12 years age is MCV >90.

Causes of macrocytosis include disorders of folate/B12 metabolism, liver disease, congenital heart disease, Down’s syndrome, hypothyroidism, drugs (anticonvulsants, zidovudine, hydroxyurea, immunosuppresants), myelodysplastic syndromes, bone marrow failure, and rare genetic syndromes (eg, Fanconi anaemia, thiamine-responsive megaloblastic anaemia syndrome).