Another bottleneck in most previous investigations of Hb-O2 affinity was that the concentration of 2,3-DPG in RBCs of patients with SARS-CoV-2 infection was not usually measured [41], and the concentrations of Hb, phosphate, age (aging red cells or patients), history of heart failure [39], or a fixed value of 2,3-DPG [186,187] were used as a sensitive index in predicting 2,3-DPG concentration. The gene discussed is GSTM1; the disease is heart failure.