Psoriasis is a chronic dermatological condition characterized by a complex pathogenesis and impacts approximately 3% of adults 20 years or older in the United States (1, 2). Research findings suggest that the prevalence of psoriasis among adults in the United States has remained relatively stable since 2003, showing no significant differences (2). Furthermore, psoriasis imposes considerable financial and social burdens, accounting for the largest global burden of disability due to skin diseases (3).

In the systemic inflammatory system of the human body, immune cells play a critical role in various diseases. Many studies have raised that integrated peripheral lymphocyte, neutrophil, and platelet counts may be more effective in predicting the inflammatory state. Importantly, SII, which possesses considerable clinical application, was calculated by counting three types of circulating immune cells: lymphocytes, platelets, and neutrophils (4, 5). Moreover, SII was initially identified as a prognosis of kidney stones, hepatic steatosis, and cancer investigations (6). Psoriasis is a chronic skin disorder with systemic involvement and inflammatory etiopathogenesis. The treatment of psoriatic skin lesions has seen notable effectiveness with interleukin-17 (IL-17), IL-23, and tumor necrosis factor-alpha (TNF-α) inhibitors (7). However, the impact of SII on psoriasis in the outpatient US population is not completely clarified, and its prognostic ability for psoriasis remains largely unknown. We hypothesized that SII could predict the risk of psoriasis, and we aimed to investigate the association between SII and psoriasis.

This cross-sectional study involved 50,938 participants from 2003–2006 and 2009–2014. First, 33,172 participants <20 and >59 years old were excluded. Then, 914 adults were excluded for missing SII data. After excluding 21 adults with missing psoriasis data, 16,831 participants were finally enrolled. As shown in Table 1 , the prevalence rate of psoriasis was 3.0% in this cross-sectional study. Of the 16,831 adults, there were 8,801 women and 8,030 men. Compared with SII lower than 479.15 × 10⁹/L, adults with SII higher than 479.15 × 10⁹/L were more likely to be non-Hispanic White, to be current smokers, and to have an educational level of high school or less.

The results of the sample-weighted logistic regression analyses are displayed in Table 2 . A positive association between a SII higher than 479.15 × 10⁹/L and a high risk of psoriasis was revealed in model 2 (OR, 1.55; 95% CI, 1.20–2.01, p = 0.001).

We found that SII was not positively associated with psoriasis among participants aged 20–39 (OR, 1.41; 95% CI, 0.88–2.26, p = 0.147) and those not married (OR, 1.50; 95% CI, 0.98–2.29, p = 0.059). In addition, we found that SII was positively associated with psoriasis in the sex, alcohol drinking status, and obesity subgroups. The results of the subgroup analyses are displayed in Figure 2 .

Many studies have found the significant contribution of chronic inflammation to the progression of psoriasis (16, 17). The immune system’s role as a prognostic determinant in the pathogenesis of many diseases has been recognized over a long period of time. SII was determined by tallying up the numbers of three different types of immune cells: lymphocytes, platelets, and neutrophils. The SII level, which reflects inflammatory responses, shows potential as a valuable diagnostic biomarker for evaluating overall inflammatory activity. A retrospective study suggests SII could function as a prognostic indicator for adults with psoriasis (18). However, it is still not clear if there is a link between SII levels and psoriasis among outpatient US adults. Our previous study, published in JAMA Dermatology, indicated that in outpatient US adults, psoriasis was positively associated with non-alcoholic fatty liver disease (NAFLD) (14). A recent study has shown a positive association between SII and hyperlipidemia among outpatient US adults (9). Hyperlipidemia is recognized as a risk factor for NAFLD. Therefore, it raises the question of whether there is a positive association between SII and psoriasis. In this cross-sectional study, our findings suggest that participants with SII higher than 479.15 × 10⁹/L had a higher risk of psoriasis after fully adjusting. Furthermore, SII characterized by a non-intrusive methodology, cost-effectiveness, and easy accessibility is a widely available method. Therefore, SII can be identified as a biomarker for psoriasis in the outpatient US adult population.

This is the first cross-sectional study focused on the risk of higher SII on psoriasis in the outpatient US population. Due to our consideration of the NHANES design for obtaining nationally representative estimates in the United States, our findings should be applicable to the outpatient population of US adults.

Our subgroup analysis showed that participants with SII higher than 479.15 × 10⁹/L and obesity had a higher risk of psoriasis. Due to the restricted sample size of participants in this cross-sectional study, it is advisable to interpret these results with caution, and more well-designed prospective studies are warranted.

As a cross-sectional study, the reliability of drawing causal conclusions is not robust, and we cannot completely rule out the potential impact of residual confounding from unmeasured variables. While NHANES is a nationally representative survey, the generalizability of our conclusions to other countries is uncertain, and further research is needed to explore the precise associations among them.

Our findings suggest a positive association between a SII higher than 479.15 × 10⁹/L and a high risk of psoriasis among outpatient US adults. SII characterized by a non-intrusive methodology, cost-effectiveness, and easy accessibility is a widely available method. Therefore, SII can be identified as a biomarker for psoriasis in the outpatient US adult population.

The original contributions presented in the study are included in the article/supplementary material. Further inquiries can be directed to the corresponding author.

The studies involving humans were approved by National Center for Health Statistics (NCHS). The studies were conducted in accordance with the local legislation and institutional requirements. The participants provided their written informed consent to participate in this study.

QD: Writing – original draft, Writing – review & editing. XL: Writing – review & editing, Writing – original draft. LL: Writing – review & editing, Writing – original draft. XX: Writing – original draft, Writing – review & editing. WJ: Writing – review & editing, Writing – original draft. XC: Writing – review & editing, Writing – original draft. JC: Writing – review & editing, Writing – original draft. TL: Writing – review & editing, Writing – original draft.