All of the following factors influence hemoglobin dissociation curve, except

Hb-O2 dissociation curve shift to left indicates more affinity of Hb to O2 and thus less delivery of O2 to tissues. Shift to left seen in -        Increased pH -        Decreased 2,3 DPG, temperature, PCO2 -        HbF (Fetal) Shift to right seen in -        Decreased pH -        Increased 2,3DPG, temperature, PCO2 -        HbS (Sickle cell)