The Neutrophil to Lymphocyte Ratio as a Triage Tool in Criticality Accidents

During triage of possibly irradiated individuals after a criticality accident or nuclear weapon event, it is necessary to decide whether a patient has experienced a clinically significant dose (> 2 Gy) that would require referral for additional evaluation and medical treatment. This is a binary decision: yes or no. The neutrophil-to-lymphocyte ratio (NLR) is an appropriate decision parameter, is simple to obtain in field operations, and is recognized in clinical medicine as an independent marker of systemic inflammation. NLR is evaluated for usefulness in triage using data from the Radiation Accident Registry at the Radiation Emergency Assistance Center/Training Site (REAC/TS). A criticality accident data set has been prepared using historic complete blood counts from 12 criticality events with 33 patients. In addition, a cohort of 125 normal controls has been assembled for comparison with the radiation accident data. In the control set, NLR is found to be 2.1 ± 0.06 (mean ± SEM) and distributed consistent with a Gaussian distribution. A patient from the 1958 Y-12 criticality accident is presented as an example of the time dependence of NLR after an event. In this case, NLR is statistically elevated above controls from <4 h until ~20 d post-event, and for times >20 d post-event, NLR is less than the control value, returning to baseline > ~40 d. The latter result has been confirmed using late hematological data taken from patients at Hiroshima and Nagasaki, and this appears to be a general finding. Since triage is a binary decision, analyzing NLR with receiver operating characteristic (ROC) statistics is appropriate. Maximizing the Youden J statistic (sensitivity + specificity -1) determines an appropriate decision point. For this data set, the decision point for NLR is found to be 3.33, with area under the curve (AUC) 0.865, sensitivity 0.67, specificity 0.97, positive predictive value (PPV) 0.85, and negative predictive value (NPV) 0.92. Therefore, when a known criticality accident or nuclear weapon event has occurred and if the patient's NLR is greater than 3.33 early post-event, then that person should be referred for further health physics and medical evaluation.