The OHI index recognizes patients at high mortality risk not identified by current diagnostic indices. The patients were classified based on the HLH-2004 (≥5 diagnostic criteria), optimized HLH-2004 (O-HLH2004, the HLH-2004 framework with sCD25 >3900 U/mL and ferritin >1000 ng/mL), the OHI index (sCD25 >3900 U/mL and ferritin >1000 ng/mL), or HScore ≥ 169 based on the peak/nadir of relevant laboratory values obtained during the index hospitalization. (A) Kaplan-Meier curve for patients classified concordantly or discordantly by using the HLH-2004 criteria and the OHI index. (B) Kaplan-Meier curve for patients classified concordantly or discordantly by using the optimized HLH-2004 criteria (O-HLH2004) and the OHI index. (C) Kaplan-Meier curve for patients classified concordantly or discordantly by using the HScore and the OHI index. (D) Kaplan-Meier curves of patients classified by using the OHI index or parameters of the OHI index. The number at risk is presented for each group. Statistics were calculated with the log-rank (Mantel-Cox) test. *P < .05, ***P < .001, ****P < .0001.