Don't forget methylmalonic acid quantification in symptomatic exclusively breast-fed infants.

Vitamin B12 deficiency can lead to serious haematological and neurological signs in infants. The reported clinical cases of vitamin B12 deficiency were found in exclusively breast-fed infants whose asymptomatic mothers were diagnosed later with pernicious anaemia. For the infants, the diagnosis required urinary methylmalonic acid quantification (grossly elevated in these two cases) and treatment rapidly improved the clinical signs. These cases underline the serious consequences of vitamin B12 deficiency in infants and the helpful role of early methylmalonic acid quantification for diagnosis.