Does preincubation of the red blood cells contribute to the capability of the osmotic fragility test to detect very mild forms of hereditary spherocytosis?

During incubation for 24 h at 37 degrees C, erythrocytes from patients with hereditary spherocytosis (HS) undergo a greater increase in osmotic fragility than do normal cells, and this procedure has been recommended for differentiating more clearly between patients with very mild HS and normal subjects. The greater effect of preincubation on erythrocytes from patients with HS was confirmed, but, except in cases demonstrating a markedly increased osmotic fragility before incubation, this effect was outweighed by a simultaneous loss of test precision. It therefore seems that preincubation does not significantly contribute to the capability of the osmotic fragility test to detect very mild forms of HS.