- Low venous blood p50: the determination of p50 (percentage at which Hb is half saturated with oxygen) is very helpful is establishing the presence of a haemoglobin variant with high oxygen affinity or a rare 2,3-diphosphoglycerate (2,3-DPG) deficiency. In such a situation, a specific HBB, HBA1 and HBA2 mutation will be screened for using Sanger sequencing (these genes are not included in NGS analysis because of redundancy of pseudogenes).