Platelet distribution width as an useful indicator of influenza severity in children

Influenza is an acute viral respiratory disease caused by influenza A and B viruses. Annual seasonal influenza epidemics of variable severity lead to morbidity and mortality worldwide [1‐3]. Influenza is one of the commonest causes of acute respiratory illness and loss of school days in children. Household-based and community-based studies in different countries have shown that influenza virus infections and illness rates are highest in children [2]. The infection is usually self-limiting, although some individuals particularly young children have a higher risk of complications, such as pneumonia, septic shock and multiple organ dysfunction, resulting in hospital admissions, or in hospital deaths [4]. It is worth noting that most individuals with influenza disease, both children and adults, that required intensive care, had no underlying conditions [5, 6]. Therefore, biomarkers that help with early diagnosis are essential to detect severe influenza infection in children.

Platelets (PLTs), fragments of megakaryocyte cytoplasm, play a central role in pathological thrombosis [7]. In addition, platelets significantly contribute to the inflammatory reactions and immune response in various types of infections including influenza [8, 9]. Previous studies indicate that platelet parameters are associated with the inflammatory response and severity of many infectious diseases [10‐13]. Platelet parameters consist of PLT count, platelet distribution width (PDW), mean platelet volume (MPV), and plateletcrit (PCT), obtained by automated cell counters. Yu et al. showed that PLT count was an independent predictor of severe hemorrhagic fever with renal syndrome (HFRS) caused by Hanta virus [11], Barrett et al. showed that MPV associate with increased critical illness and all-cause mortality among hospitalized COVID-19 patients [12]. Additionally, PDW has been reported to be a widely applied and reliable marker of platelet activation and have recently been shown to be an independent risk factor of postoperative pneumonia in patients with type A acute aortic dissection (AAAD) [10], as well as an useful indicator for patients with severe burns [13]. However, the role of these parameters in influenza has been rarely investigated. There are very few reports about their clinical values to distinguish non-severe from severe influenza in children. The aim of the current study, therefore, was to investigate whether platelet parameters would be useful in distinction and evaluating the severity of influenza virus infection.

Influenza primarily affects the respiratory system and represents one of the most frequent causes of acute upper respiratory tract infections during the winter season. During 4 non-pandemic influenza seasons (2017–2020), about 424 influenza A and 148 influenza B cases were identified in this study. The prevalence rate of influenza A was much higher than that of influenza B. This is consistent with our previous findings and other research that influenza A had more ability to cause pandemics than influenza B [17, 18]. Additionally, being a little different from report of other research group [19], this study showed that influenza B positive children were more prone to be infected in the lower respiratory tract (bronchitis and pneumonia), the possible reasons for this difference are uncertain and require further study. On the other hand, influenza A was more likely to cause other system symptoms in addition to respiratory symptoms. It may be because influenza A virus is the most virulent influenza virus type and leads to the most severe disease outcomes [20]. Furthermore, we found that young age, as already reported, is an important risk factor for the severe influenza infection [21].

It is very difficult to diagnose influenza in young children based on its clinical symptoms alone, as no existing specific signs [22]. Therefore, laboratory indicators play an essential role in the early accurate diagnosis and severity assessment. In view of the opposite change trend of lymphocyte and neutrophil with age, and platelet parameters may be partly age-dependent [23, 24], the subjects were divided into the < 4 years group and ≥ 4 years group. We analyzed the results of laboratory indicators and found that lymphocyte count, neutrophil count, NLR, Platelet count, MPV, PCT, and PDW all showed a significant difference for children(≥ 4 years) with influenza A and B when compared to healthy individuals, while different situations were present among children under 4 years old. The different performance between these two groups may be due to the fact that children under 4 years old are an immunologically naïve population, indicating less immune response and inflammatory reaction [25, 26]. This pattern seemed to reflect the development of acquired and adaptive immune system in children [27]. These results indicated that lymphocyte count, neutrophil count, NLR, Platelet count, MPV, PCT, and PDW could be used as indicators for distinguishing children(≥ 4 years) with influenza from health, while neutrophil count and PDW could be used as biomarkers to differentiate influenza infection in children younger than 4 years old. Lymphocyte count and Neutrophil count are convenient and quick index of inflammation detections in laboratory examination, they are used in the diagnosis, treatment and prognosis evaluation of many disease [28, 29]. Platelet parameters, as newer, easily accessible, and inexpensive biomarkers had been reported to increase the accuracy of the diagnosis of many other infections [30‐32]. We found that the joint detection of platelet parameters and other inflammatory indicators showed better performance than detection of single indicator.

According to the clinical practice guidelines for the diagnosis and management of children with influenza [15, 16], we divided children into a non-severe and severe group. The findings from this study show that the inflammatory indicators involved performed very differently between non-severe and severe group in children with influenza A and B. Indeed, there are more inflammation indicators that displayed significant differences between the non-severe and severe group in influenza A. According to the previous research, influenza A disease is more severe than influenza B disease, with a more intense inflammatory response that reflects in an increase in systemic inflammatory markers [19, 33, 34]. Our findings are consistent with these prior findings. Moreover, we found that the platelet count was significantly decreased and PDW showed a marked increase in the non-severe group, especially for children with influenza A. Many studies have confirmed that many receptors for various pathogens were expressed on the surface of the platelet, enabling platelet to directly recognize and respond to invading viruses [35, 36]. The research from Milka Koupenova.et al. displayed that influenza virus during acute influenza infection can potentially invade into blood and become engulfed by platelets, then digestion of the virus was observed in platelet [37]. Seok-Joo Kim .et al. also observed a marked increase in platelet accumulation and the formation of large platelet aggregates within the lung vasculature following influenza A virus infection [38]. Influenza virus infection lead to the activation and consumption of platelet, contributing to the obvious drop of platelet count in peripheral blood as observed in the present study. PDW had been disclosed to be a more specific indicator of platelet reactivity than MPV [39, 40], an elevated PDW reflects increasing platelet destruction and variations in the size of newly formed immature platelets [41]. Being coincident with the previous studies, our research showed that the level of PDW rather than MPV rose significantly in the non-severe group for children with influenza (A and B), compared to the healthy control.

Additionally, unlike the trend of non-severe group, the severely infected ones exhibit a significant increase in the platelet count and a marked drop in the PDW. The main reason for this difference may be the different degree of systemic inflamma-tory response between severe and non-severe group. Satoshi Nishimura.et al. found that IL-1α could induce a much rapid platelet count increase into the vessel by a new way named “megakaryocyte rupture” in response to inflammatory stimuli or acute platelet requirement for platelet recovery as a host defense. Different to the traditional thrombopoiesis occurring via proplatelet formation (PPF) in the presence of thrombopoietin (TPO),the platelets release from megakaryocyte rupture appeared to a much more number and display more spheroid just liked mature platelets while PPF-dependent platelets were longitudinal and immature [42]. There are several studies have showed that influenza patients displayed markedly increased expression of IL-1α, and the expression level positively correlate with the severity of influenza infection, as well as the elevation of platelet count [42‐45]. Notably, our earlier study has indicated that severe group of children with influenza expressed much higher levels of CRP and SAA, indicating more severe inflammatory response [18]. The present findings indirectly mirror the previous findings from a clinical point of view, this would also explain why the level of PDW decreased significantly in the severe group for children with influenza. The above results were similar to our study and confirmed that the change of platelet count and PDW may reflect the severity of influenza infection.

The limitations of this report included that, as a single-center study, the characteristics of enrolled children may not be representative, multi-center studies are needed to confirm our findings. In addition, the critically severe patients were not involved in this study, which would affect our conclusions, we would continue to collect more critically severe participants in future studies.

In conclusion, there were several differences in clinical symptoms and laboratory indicators among children with influenza A and B. The change of platelet count and PDW may reflect the severity of influenza infection. Therefore, this present study suggests that routine blood examination, being easy to perform and inexpensive, in children with influenza should be performed to provide an important laboratory basis for accuracy diagnosis and severity assessment.