Which of these is not a cause of rightward shifted to right in?

A decrease in the pH shifts the curve to the right, while an increase in pH shifts the curve to the left. This occurs because a higher hydrogen ion concentration causes an alteration in amino acid residues that stabilises deoxyhaemoglobin in a state (the T state) that has a lower affinity for oxygen Ref: guyton and hall textbook of medical physiology 12 edition page number:348,349,350