2.1 Patient selection
After obtaining approval from the Institutional Research Board/Ethics Committee (IRB/EC) of Chiang Mai University, a retrospective medical record review was conducted for infants and children under 18 years diagnosed with HLH according to the HLH-2004 criteria, the MAS-HLH criteria for rheumatologic diseases, or the H-score. This study focused on patients undergoing treatment at Chiang Mai University hospital between 2010 and 2022. The patients exhibiting HLH-like illness but not meeting the specified diagnostic criteria were excluded. Data collected included demographic and clinical characteristics, HLH subtype, HLH-2004 criteria score4, H-score11, MAS-HLH criteria5, 6, pediatric Sequential Organ Failure Assessment (pSOFA) score12, modified ISTH-DIC score13, 14, HLH severity based on the Histiocyte Society guideline15, laboratory parameters, genetic testing, bone marrow examination results, and pathology reports. Relapsed HLH was defined as the active disease occurring post-remission with standard HLH therapy, while refractory HLH was characterized by the failure to achieve a response 2 weeks after initiating standard HLH therapy. 15, 16 All diagnostic criteria and HLH severity grading were included in Supplemental Table S1. The pSOFA score and the modified ISTH-DIC score were presented separately in Supplemental Table S2.