Osmotic fragility is:

Ans: a (Negative in iron deficiency anaemia)Ref: Lewis Dacie and Lewis Practical Haematology, 10th ed.Osmotic fragility is measured by lysis in serial dilutions of hypotonic buffered saline (NaCl, NaH,PO4, Na2 HPO4).Heparinized venous blood or defibrinated blood used; oxalated or citrated blood is not suitable because of the additional salts added to it.The sigmoid shape of the normal osmotic fragility curve indicates that normal red cells vary in their resistance to hypotonic solutions. The youngest cells are the most resistant and the oldest cells are the most fragile.ConditionOsmotic fragilityHereditaryIncreasedspherocytosis (HS) HereditaryIncreasedelliptocytosis (HE) HereditaryIncreasedstomatocytosis AutoimmunehaemolyticanaemiaTail of fragile cells roughly proportional to number of spherocytes; rest of curve normal (oreven left-shifted on account of high reticulocytosis)High reticulocytosis decreases osmotic fragilityThalassaemiaDecreased in all forms of thalassaemia, except in some a-thalassaemia heterozygotesEnzymeabnormalitiesOF usually normal (anaemia originally referred to as hereditary nonspherocytic), but tail of highly resistant cells maybe seen on account of high reticulocyte court.Iron deficiencyCurve shifted to left (ie, OF decreased), wholly or partly, depending on proportion of hypochromic red cells