What happens to the microspherocytosis of hereditary spherocytosis in folate deficiency?

The case is presented of an elderly man in whom hereditary spherocytosis (HS) had been diagnosed many years previously. He also had diverticulitis and was referred because of increasing anaemia (Hb 7.7 g/dl). He was suffering from diarrhoea and anorexia, and was taking a very poor diet. Serum investigations showed low folate and vitamin B12 levels. His blood film, despite the history of HS, showed very few spherocytes, and the red cell osmotic fragility curve was virtually normal. Instead, the blood film showed normocytes and oval macrocytes, mostly with normal central pallor. He was treated with vitamin B12 injections and folic acid tablets. On this treatment, the Hb rapidly rose, the macrocytosis disappeared, the film appearances reverted to the classical picture of HS with large numbers of microspherocytes, and the osmotic fragility curve to the picture of markedly increased fragility typical of HS.