Hemodynamic changes and systemic inflammation are always observed in patients with acute TBI. The release of inflammatory mediators following injury could induce tissue damage, red blood cell destruction and structural changes, as well as platelet aggregation and accelerated consumption, which finally result in the dysfunction of neurovascular unit and increased risk of poor prognosis . Routinely conducted during hospitalization, the CBC analyses contain several parameters that reflect the above pathological changes. Of these parameters, RDW and Plt count which have potential to indicate the mortality risk of TBI was selected to be investigated in this research. Using the MeDICS method, we found that their ratio – RPR, a novel easily accessible index, is a reliable predictor for the outcome of acute TBI.
RDW represents the heterogeneity in size of erythrocytes, of which the higher values indicate greater variation. Recently, it has gained substantial attention as an indicator of inflammation , and a prognostic marker for various diseases independent of hemoglobin values . A research on the association between aging of hematopoietic stem cells and oxidative stress molecules, such as reactive oxygen species, super-oxide dismutase and glutathione peroxides, revealed that abnormally increased RDW can well indicated the above pathological changes . It has also been suggested that elevated RDW was related to suboptimal health status that involves chronic inflammatory response and impairment of red cell generation. Specifically, pro-inflammatory cytokines can affect the survival of erythrocytes in circulation, suppress maturation, and accelerate the entry of newer, larger reticulocytes into the peripheral circulation, thereby leading to the increase of RDW . In the present study, RDW was observed to be increased in patients with TBI at 3–5 days post-admission. However, although the RDW value were lower in survivors than in non-survivors, further statistical analysis found that it had no effect on predicting the prognosis of acute TBI.
Platelets, along their well-known roles in hemostasis, are an active participant in regulating inflammation. Specifically, while adhering to coagulation factors, platelets also carry a large number of inflammatory factors such as TNF-α, interleukins and serotonin, which are involved in tissue damage and repair. Decreased Plt count is a common pathological phenomenon in patients with acute and critical illness. Its underlying mechanism may involve: 1) Reduced platelet production due to infectious and inflammatory damage to megakaryocytes, and bacterial endotoxins that inhibit the function of bone marrow megakaryocytes. 2) Increased platelet destruction and consumption caused by severe infection-induced diffuse intravascular coagulation. 3) Destruction of platelet production led by complement activation through immune pathways . Studies on sepsis observed a reduction of Plt count in patients, which was correlated with the severity of the disease, and a risk factor for poor prognosis . Besides, platelets played a pivotal role in the inflammatory response in hepatic injury and burn injury [29, 30]. In the present study, a substantial decline in Plt count was observed at 3–5 days post-admission, and the decrease of Plt count in non-survivors were lower than that of survivors. Even so, similar to RDW, Plt count could not independently infer prognostic information of acute TBI.
Recently, RPR has been considered as a novel index that reflects inflammation severity by combining the prognostic advantages of RDW and Plt count. As a routinely available marker, RPR was recognized as a strong predictor for hepatic fibrosis and hepatitis [31, 32], inflammation in acute pancreatitis , ascending thoracic aortic aneurysm , and myocardial infarction . In addition, high RPR on days 3 and 7 could be observed in patients with severe burn injury, which indicated poor prognosis of the disease . Parallel to these studies, the increased RPR was also reported to be correlated with the severity (disease scores) of inflammatory factors in systemic lupus erythematosus . From this, RPR would be a powerful indicator of inflammation.
Although the exact mechanism underlying the poor prognosis of TBI patients with elevated RPR remains unclear, it may be partially attributed to the development inflammation following injury. In-vivo experiments were thus designed to explore related mechanisms. As an upstream switch of inflammatory response, NF-κB signaling exerts important effects on regulating the development of neuroinflammation in acute TBI  and chronic traumatic encephalopathy . In addition, our previous researches found that pyroptosis in injured brain after TBI can trigger the inflammatory cascade, and lead to the dysfunction of neurovascular unit [16, 40]. In the present study, we used specific inhibitors that block NF-κB and pyroptosis-activated receptor NLRP3 to observe the level changes on RDW, Plt count, and RPR under the condition of inflammatory suppression. RDW decline and Plt count increase with marginal statistical differences were observed after anti-inflammatory treatments at 3DPI. Meanwhile, RPR was also decreased notably after the treatments, suggesting that it is a powerful indicator of inflammation in acute TBI. In addition, the results of neuro-functional tests indicated that RPR value was closely related to the neurological outcome of TBI mice receiving anti-inflammatory treatments. These findings confirmed the association between RPR, post-traumatic inflammation and neuro-outcome after TBI, and further emphasized the importance of controlling inflammatory response in clinical treatment.
Our research simultaneously explored the association between RPR and the mortality risk of acute TBI at two important time points post-injury. We found that 4-DAA RPR turned out to be strongly associated with the endpoint of TBI, but first-day RPR had no prognostic value. Besides, similar results were also observed in in-vivo experiments. We believe that these findings could be explained by the development of inflammation. Specifically, the inflammation peak after TBI mostly occurs at 3–5 days post-injury. This time point is also the peak period of brain edema with massive activation of microglia and astrocytes, release of inflammatory mediators into the CNS and peripheral circulation, and homing of activated neutrophils and immune cells (e.g., T cells and NK cells) to the injured brain tissue . However, in the hyper-acute phase of TBI (< 24 h post-injury), the CBC parameters may be interfered by shock and acute stress reaction. In addition, the glial cells in the brain are not widely activated at this time, and the peripheral inflammatory and immune cells have not completed their homing, leading to the limitation of focal brain inflammation to the injury site . Consequently, the prognostic value of 4-DAA RPR for TBI was much higher than that of first-day RPR. Moreover, with the use of our nomogram that consisted of RPR, GCS and other risk factors, the predicting value of RPR can be further improved. The strength of this nomogram is that it was built from physical examination and easily diagnosed medical history or comorbidities. Therefore, it can be used in resource-limited settings, where clinicians are still likely to have all the data required to use it effectively. The nomogram is able to be applied to all levels of medical system, thus we propose it as a prognosis evaluation aid for all patients with TBI.
Several limitations of this study should be taken into account when interpreting the results. First, the retrospective nature of the design limited the research. Patients’ information of Abbreviated Injury Scale - Injury Severity Score at admission and Glasgow Outcome Scale – Extended on discharge were not designed to be collected in the databases. Second, this research failed to include other risk factors, such as chronic liver disease, chronic renal disease, diabetes mellitus, sepsis, and malignancy, as well as other endpoints including ICU mortality and hospital/ICU length of stay. Third, the continuous fluctuations in RDW, Plt count, and RPR in acute TBI were not determined. These fluctuations may be necessary for the formulation of a comprehensive conclusion on their prognostic value. To address the questions, a multi-center prospective study is being planned to further confirm the findings of this research and promote its clinical application in the future.