MCHC and p50 versus osmolarity.

(A) Measurement of intracellular C_SAT by plotting p50 versus extracellular osmolarity for AA, SS, and AS red cells. (B) Determination of MCHC at the onset of polymer formation for SS red cells by plotting MCHC versus extracellular osmolarity. MCHC and p50 for representative patients with AA, AS, and SS. As expected, patient-to-patient variation was observed (data not shown). The C_SAT for SS red cells can also be estimated from the intersection of the SS p50 line with the horizontal line because the p50 values of HbA and HbS were the same in the absence of polymer formation. Intracellular C_SAT for this patient, who had 9.7% HbF, was 18.2 g/dL. The patient with AS did not have the α-thalassemia trait and had an estimated C_SAT of 32.2 g/dL. Under fully deoxygenated conditions, the traditional method of measuring C_SAT yielded values of 15.8 g/dL for purified SS.5