BACKGROUND: Five fulfilled hemophagocytic lymphohistiocytosis (HLH)-2004 criteria, and the HScore are widely used and recommended by international expert consensus to diagnose secondary HLH. Both diagnostic scores have never been validated in heterogeneous patient cohorts of secondary HLH patients. We aimed to systematically optimize and validate diagnostic criteria of secondary HLH using a multicenter approach. METHODS: We developed optimized criteria in our cohort of critically ill patients as a first step. We next validated these new criteria together with the original and modified HLH-2004 criteria as well as the HScore using original data of 13 published cohorts, which were identified by a systematic literature search. RESULTS: The best performing HLH diagnostic criteria sets over all 13 validation cohorts were the original HLH-2004 criteria with a decreased cut-off (cut-off 4, mean sensitivity 86.5%, mean specificity 86.1%), followed by the revised HLH-2004 criteria (natural killer cell activity removed
cut-off 4, mean sensitivity 83.8%, mean specificity 87.8%) and the HScore (cut-off 169, mean sensitivity 82.4%, mean specificity 87.6%). Our newly developed HLH diagnostic criteria showed inferior performance. Ferritin ≥500 µg/L had 94.0% mean sensitivity over all cohorts. CONCLUSIONS: In this first multicenter validation study, four fulfilled HLH-2004 criteria and an HScore of 169 were suitable to diagnose secondary HLH, which will lead to rapid diagnosis and improved patient outcomes. Ferritin proved as a reliable HLH screening marker. Our results should be taken into account in clinical recommendations and in designing new studies.