TPO and severe aplastic anemia: Analysis of patients with severe aplastic anemia(SAA)treated in Institute of Hematology & Blood Diseases Hospital, Chinese Academy of Medical Sciences & Peking Union Medical College from January 2021 to December 2021 who received IST combined with TPO-RA(eltrombopag/hatrombopag)for at least 6 months, but was ineffective for at least 3 months or patients who cannot continue to use TPO-RA due to side effects, and switched from TPO-RA to avatrombopag(APAG), and treated for at least 6 months.