In this study, Pearson’s correlation analysis, GLM and RCS were constructed to reveal the associations between SII and NAFLD related biomarkers values. There was negative significant association between SII and hepatic fibrosis biomarkers value (FIB-4, NFS and HFS). In contrast, there was positive significant association between SII and hepatic steatosis biomarkers value (FLI, LFS and FSI).
SII, as a novel biomarker, can be easily obtained from routine complete blood count tests (platelet counts, neutrophil, and lymphocyte) and reflect the overall status of the immune response and systemic inflammation of the subjects. In the study, SII was constructed to predict the recurrence and survival for hepatocellular carcinoma patients[7]. In the recent years, more and more studies use SII to explore the association of systemic inflammation with the subjects’ healthy status (lupus nephritis[22], psoriasis[23], arthritis[24] and NAFLD[20, 25, 26]). One study, based on NHANES, revealed that ln-transformed SII was an independent risk factor for NAFLD risk (defined by a US Fatty Liver Index (USFLI) score exceeding 30), and the corresponding OR and 95%CI was 1.46 (1.27, 1.69)[25]. Another study also found that there was a significant positive association between SII and hepatic steatosis index (HSI) and OR (95%CI) was 1.30 (1.10, 1.52)[20]. Although above studies used the different biomarkers to detect the hepatic steatosis, the result also indicted that there was a positive association between SII and hepatic steatosis, which were similar with our study.
Besides the association between SII and hepatic steatosis, two studies also explored the relationship between SII and hepatic fibrosis[21, 27]. The first study implied that there was no link between SII and liver fibrosis (based on liver stiffness measurement), and the corresponding β (95%CI) and P value were 0.000 (-0.000, 0.001) and 0.263[21]. While the second research suggested that there was a negative association between SII and advanced liver fibrosis (according to FibroScan model) in the individuals with diabetes mellitus, and the OR (95%CI) was 0.33 (0.15, 0.70)[27]. In this study, there was negative significant association between SII and hepatic fibrosis (FIB-4, NFS and HFS). The inconsistencies in the results of these studies may stem from differences in the included population and the biological markers of liver fibrosis.
The underling mechanism of relationship between SII and hepatic steatosis was not yet completed understood. Evidences suggested that immune-inflammation response played an important role in hepatic steatosis. Cell death caused by inflammasome pathways played a key role in the pathogenesis of NAFLD, and interleukin (IL)-1 and IL-18 released after cell death can cause a strong inflammatory process and induce the recruitment of inflammatory cells[28, 29]. Another theory was that nutrient overload was the primary cause of hepatic steatosis. The excess visceral fat could cause macrophage infiltration and result in a pro-inflammatory state, which induced insulin resistance. Inappropriate lipolysis resulted in aberrant fatty acid transport to the hepatic cell in the state of insulin resistance, which could induce a decrease in metabolic capacity. Lipotoxic lipids were formed as a result of lipid metabolic imbalances that lead to cellular stress and inflammasome activation[5, 30].
The mechanism underlying the relationship between SII and hepatic fibrosis remained unclear. Based on previous describe, we postulated that SII may play a pivotal role in both the initiation and progression of hepatic steatosis. However, as the disease progresses toward hepatic fibrosis, there was a discernible decline in complete blood count, predominantly lymphocytes. This could be attributed to a series of pathophysiological evolutions, such as hypersplenism, bone marrow haematopoietic inhibition, and endocrine shifts within the organism. Taken together, these observations suggested that lymphocytes, as a component of the SII formula, have different developmental trends in hepatic steatosis and hepatic fibrosis stages, and may explain the phenomenon of SII different trends in hepatic steatosis and hepatic fibrosis[27].
It was important to emphasize some limitations of this study. Firstly, the NHANES study was a cross-sectional design, which limits its ability to make established causal conclusions. Secondly, even with consideration of several influencing factors, the potential impact of hidden variables was a concern. Thirdly, due to limited information in the data set, the exclusion of subjects with incomplete data may introduce bias into the analysis. Fourthly, the current findings may not be persuasive for other racial groups, as non-Hispanic whites predominate among NHANES. In addition, utilizing the NHANES database inevitably introduces the possibility of imprecise data capture and recall bias. Although these limitations existed in this study, to some extent, the relatively large epidemiological study also could reflect the associations between SII and hepatic fibrosis or hepatic steatosis.