Abstract Secretory Leukocyte Protease Inhibitor (SLPI), a protein functions as an anti-inflammatory molecule that maintains the protease-antiprotease balance. Although SLPI is gaining recognition as a potential biomarker, additional research is required to connect its levels with existing indicators and the quick Sepsis-related Organ Failure Assessment (qSOFA) score to determine its therapeutic value. This study included 50 patients with sepsis and 50 healthy controls recruited from Uttaradit Hospital. Plasma SLPI concentrations were assessed utilising a quantitative ELISA, and the results were compared with standard indicators including Procalcitonin (PCT), C-reactive protein (CRP), lactate, and white blood cell (WBC) count.  The findings indicated that plasma SLPI concentrations were markedly increased in the sepsis group relative to healthy controls. ROC curve study of diagnostic accuracy revealed that PCT and CRP had the highest discriminative efficacy, while SLPI also proved to be a meaningful indicator. SLPI had a substantial moderate positive connection with the qSOFA score, in addition to CRP, PCT, and WBC count. A significant association was not observed between SLPI and lactate levels, indicating that SLPI is indicative of systemic inflammation and organ failure rather than direct metabolic distress. In conclusion, plasma SLPI could be a sepsis biomarker that corresponds with severity and inflammatory markers. The moderate association between SLPI and qSOFA scores suggests its possible application in monitoring organ dysfunction in septic patients. Traditional markers like PCT and CRP are reliable for initial diagnosis, but SLPI's unique regulatory capabilities may help diagnose and treat sepsis-related organ damage.

Keywords: Sepsis, Secretory leukocyte protease inhibitor (SLPI), Biomarker, Systemic inflammation, quick Sepsis-related Organ Failure Assessment (qSOFA) score

Funding: This research was funded by Chiang Mai University. The Chiang Mai University Presidential Scholarship 2023 for F.P., O.S., and TA/RA scholarship for Y.H.

Citation:  Deebukkham, S., Bunnag, A., Pikwong, F., Hoiraya, Y., Srisopar, O., Pankhong, P., and Kumphune, S. 2027. Plasma human secretory leukocyte protease inhibitor (SLPI) level as a novel candidate diagnostic biomarker for sepsis. Natural and Life Sciences Communications. 26(1): e2027007.

Graphical Abstract:

INTRODUCTION

Sepsis is a life-threatening condition caused by a dysregulated host response to infection, leading to organ dysfunction. In 2021, it is anticipated that there were 166 million sepsis cases (95% uncertainty band 135–201) and 21.4 million all-cause sepsis-related deaths (20.3–22.5) worldwide, accounting for 31.5% of total global mortality (GBD 2021 Global Sepsis Collaborators, 2025).  Sepsis is characterized by an uncontrolled systemic inflammatory response that can cause widespread tissue damage. In sepsis, the body releases an excessive quantity of inflammatory mediators and proteases, which might surpass the natural defensive mechanisms, leading to a perilous feedback loop of inflammation and tissue damage. The equilibrium between pro-inflammatory and anti-inflammatory factors is disturbed. Standard supportive treatment followed the Surviving Sepsis Campaign international guidelines (Evans et al., 2021), involving appropriate antibiotics, source control, and supportive therapy. Empirical broad-spectrum intravenous antibiotics were administered within the first hour of recognition and de-escalated based on culture and susceptibility results. Source control involved abscess drainage, debridement, and removing infected intravascular devices. Supportive care included initial fluid resuscitation with crystalloids, norepinephrine to maintain a mean arterial pressure higher than 65 mmHg, supplemental oxygen, and lung-protective mechanical ventilation for acute respiratory failure. Additional measures comprised renal replacement therapy, insulin for glycemic control, pharmacological venous thromboembolism and stress ulcer prophylaxis, early enteral nutrition, and correction of electrolyte and acid-base imbalances.

In sepsis, Secretory Leukocyte Protease Inhibitor (SLPI) functions as a vital regulatory molecule. It is an 11.7 kDa protein that functions as a pleiotropic regulator, performing various unique actions to safeguard the host from the excessive inflammatory response typical of sepsis. SLPI could play several important roles. SLPI is known for its anti-inflammatory effect. The anti-inflammatory mechanism of SLPI could be explained by its intracellular action to inhibit the NF-κB pathway, consequently diminishing the synthesis of pro-inflammatory cytokines like TNF-α, IL-1β, and IL-6 (Osbourn et al., 2022). Furthermore, SLPI also provide anti-proteolytic agent, preserving the intricate protease-antiprotease equilibrium within the host. In sepsis, extensive neutrophil infiltration releases potent serine proteases, notably Neutrophil Elastase (NE) and Cathepsin G, which can lead to considerable collateral tissue damage (Averdunk et al., 2019; Mongkolpathumrat et al., 2024). Anti- serine protease activity of SLPI directly neutralises these enzymes, offering a protective barrier for the lungs, liver, and kidneys, hence reducing the risk of Multiple Organ Dysfunction Syndrome (MODS). Furthermore, SLPI also has direct antimicrobial activity against a broad range of bacteria, viruses, and fungi (Gomez et al., 2009). This function helps to control the initial infection that triggers the septic response.

In clinical and scientific environments, SLPI is progressively acknowledged as both a diagnostic biomarker and a potential therapeutic target. Plasma concentrations of SLPI increase markedly during human sepsis and frequently have a stronger correlation with the severity of organ failure and mortality than conventional indicators such as IL-6 (Averdunk et al., 2019). Experimental models have shown that administering recombinant SLPI or employing targeted delivery systems can markedly enhance survival rates and diminish systemic inflammation (Osbourn et al., 2022). In contrast, research utilising SLPI-deficient mice demonstrates a significant vulnerability to septic challenges, underscoring that endogenous SLPI is essential for host survival and the mitigation of sepsis-induced organ damage (Osbourn et al., 2022).

Studies indicates that plasma concentrations of SLPI are frequently increased in sepsis patients, and these levels may correspond with sickness severity and the extent of organ dysfunction (Grobmyer et al., 2000). Elevated SLPI concentrations have been reported in patients with more severe bloodstream infections, especially those originating from pulmonary sources (Lange et al., 2019). Although further investigation is required to comprehensively ascertain its efficacy, the fluctuating SLPI levels during the progression of sepsis indicate its potential as a significant instrument for diagnosis, prognosis, and monitoring. However, the correlation of plasma SLPI and other established markers of sepsis severity (procalcitonin [PCT], C-reactive protein [CRP], Lactate, and White Blood Cell Count), and multiorgan dysfunction (Sepsis-related Organ Failure Assessment [SOFA]) score, or the subsequently developed quick Sepsis-related Organ Failure Assessment (qSOFA) score, in septic patients, has never been investigated. Therefore, this study aimed to quantify plasma SLPI levels in septic patients and evaluate the correlation with established inflammatory markers (PCT, CRP, Lactate, WBC) and clinical severity scores (qSOFA). Additionally, we sought to evaluate the potential of SLPI as a valuable new biomarker for prognosis or management in sepsis.

MATERIALS AND METHODS

Ethical approval and patient consent

Approval has been secured from the Human Committee of Uttaradit Hospital in accordance with the Declaration of Helsinki, The Belmont Report, CIOMS Guidelines, and the International Conference on Harmonisation in Good Clinical Practice (ICH-GCP). The protocol for this study was approved by the Human Committee of Uttaradit Hospital, Uttaradit, Thailand (COA No. U2003/2569, Project No. UTTH032/2568).

Study population and inclusion criteria

This study involved the recruitment of 50 consecutive patients (23 men, 27 women) based on sepsis criteria established by the American College of Chest Physicians/Society of Critical Care Medicine (ACCP/SCCM) Consensus Conference, as modified by the Polish Working Group on Sepsis. Plasma from 50 healthy blood donors (24 males, 26 females) was obtained for use as control samples.

All patients received both verbal and written information regarding the potential risks and benefits of participating in the study and provided informed consent before recruitment. Participants were consecutively recruited from patients admitted to the Intensive Care Unit (ICU), Department of Medicine, at Uttaradit Hospital, Uttaradit, Thailand, between February 2026 and March 2026. All patients received treatment from the same team of physicians, and care was provided in accordance with established guidelines. The standard treatment was the provision of appropriate antibiotics, management of the infection source, and supportive therapy.

A patient with an infection is described, in accordance with the previously published report (Tancharoen et al., 2022), as one exhibiting clinical manifestations of local or systemic infection, including fever and localised symptoms and signs, and who has undergone antibiotic treatment, regardless of the presence of positive cultures. Proven infection was characterised as an infection in which the causative pathogen was identified through culture, antigen or antibody detection, or polymerase chain reaction (PCR) of specimens obtained from the suspected site of infection, blood, or histopathologic specimens from the suspected site of infection.

Study design and data collection

Patients who presented to the Emergency Room (ER) and were diagnosed with sepsis were subsequently admitted to the Medical Intensive Care Unit (ICU). Blood samples were obtained for laboratory analyses, including complete blood count (CBC), urine examination, electrolyte panel, procalcitonin, lactate, haemoculture, C-reactive protein (CRP), troponin I, B-type natriuretic peptide (BNP), Creatine kinase-MB (CK-MB), and SLPI.

Sample collection and preparation

Five milliliters of the whole blood from healthy donors and patients were collected using EDTA anticoagulant tube and centrifuge at 3,000 RPM for 10 minutes. All specimen collection processes were performed within 2 hours after blood collection. The plasma samples were frozen at -80°C or lower until the experiments were performed.

Measurement of plasma biomarkers for sepsis

Whole blood samples were analysed for complete blood count (CBC) using an automated haematology analyser, model BC-760, manufactured by Shenzhen Mindray Bio-Medical Electronics Co., Ltd., Shenzhen, China. Plasma procalcitonin, C-reactive protein (CRP), troponin I, B-type natriuretic peptide (BNP), CK-MB and lactate levels were measured using the Alinity ci analyser (Abbott Laboratories, Abbott Park, IL, USA), with reagents provided by the same manufacturer.

Measurement of plasma SLPI level

The SLPI-ELISA kit is designed for quantitative measurement of SLPI protein. Before the measurement, the reagents and samples were prepared. The reagents of SLPI-ELISA kit included an antibody cocktail and 1X wash buffer, the antibody cocktail was prepared by diluting the 10X capture human SLPI antibody and 10X detector human SLPI antibody in antibody diluent. The 1X wash buffer was prepared by diluting 10X wash buffer with deionized water. For each assay performed, a minimum of two wells must be used as the zero control by using sample diluent.

Initially, 50 µL of collected plasma samples or standards was adding to pre-coated well then adding 50 µL of antibody cocktail to each well and incubate at room temperature for 1 hour on a plate shaker 400 rpm. Next, washed each well 3 times with 350 µL 1X wash buffer by aspirating or decanting the solution from wells then adding 350 µL 1X wash buffer into each well. After washing step, added 100 µL of TMB solution to each well and incubated for 10 minutes in the dark on a plate shaker 400 rpm. Subsequently, 100 µL of stop solution was added to each well. The absorbance was measured at a wavelength of λ450 nm. Before calculating the SLPI protein concentration, the absorbance of each sample was adjusted by subtracting with absorbance of zero control. In the following step, the SLPI protein concentrations were determined by comparing them to the standard curve of the kit. The data that was found to be below zero was considered as having a concentration of zero. According to the manufacturer’s instructions, the limit of detection for the SLPI ELISA was 5.59 pg/mL. The intra-assay and inter-assay coefficients of variation (CV) were 6.10% and 3.60%, respectively.

Statistical analysis

Data were analysed using GraphPad Prism version 5.00 (GraphPad Software, San Diego, CA, USA). Before selecting the statistical tests, the distribution of each continuous variable (plasma SLPI, PCT, CRP, lactate, WBC count, age, and BMI) in each group was assessed for normality using the Shapiro–Wilk test. Plasma SLPI, PCT, CRP, lactate, and WBC count did not follow a normal distribution in the sepsis group (Shapiro–Wilk P < 0.05), therefore, differences in these variables between the sepsis and healthy control groups were analysed using the non-parametric two-tailed Mann–Whitney U test. Receiver operating characteristic (ROC) curve analysis was performed for plasma SLPI, PCT, CRP, lactate, and WBC count to discriminate sepsis patients from healthy controls; the area under the curve (AUC) with its 95% confidence interval was calculated. Because plasma SLPI levels were not normally distributed and the qSOFA score is an ordinal variable, the associations between plasma SLPI and the qSOFA score, PCT, CRP, lactate, WBC count, and age in sepsis patients were analysed using Spearman's rank correlation coefficient (rs). A two-tailed P-value < 0.05 was considered statistically significant. 

RESULTS

Characteristics of the study population

Characteristic of the participants was shown in Table 1. The study evaluated 100 volunteers, consisting of 50 patients and 50 healthy controls. The average age of the sick group was 63.89 ± 19.21 years, significantly exceeding the healthy control group's average of 43.20 ± 8.48 years. The gender distribution was similar in both cohorts; the sick group had 23 males (46.00%) and 27 females (54.00%), whereas the control group included 24 males (48.00%) and 26 females (52.00%). The Body Mass Index (BMI) physical characteristics were comparable between the two groups, with the sick group exhibiting a mean of 23.47 ± 7.17 and the healthy controls showing a mean of 23.68 ± 3.00.

Clinical assessment of the patient cohort revealed multiple microbial aetiologies. Haemoculture was positive in 16 of 50 patients (32%), whereas 34 patients (68%) showed no growth after a five-day incubation period. Among the culture-positive cases, Escherichia coli was the most frequently isolated organism (n = 7; 14% of the cohort and 43.8% of positive haemocultures), followed by Klebsiella pneumoniae (n = 4; 8% of the cohort and 25.0% of positive haemocultures). Single isolates (n = 1 each; 2% of the cohort and 6.3% of positive haemocultures) of Vibrio vulnificus, Streptococcus gallolyticus subsp. gallolyticus, Streptococcus agalactiae, Staphylococcus aureus complex, and Staphylococcus epidermidis were also identified. Overall, Gram-negative bacteria accounted for 12 of the 16 positive haemocultures (75.0%) and Gram-positive bacteria for 4 of 16 (25.0%). Every positive haemoculture yielded a single organism; no polymicrobial (mixed) bacteraemia was detected in any patient in this cohort.

Regarding pharmacological intervention, the vast majority of patients (n= 48; 97.96%) were administered Ceftriaxone, while a single patient (n = 1, 2.04%) received Tazocin.

The predominant comorbidities identified in the patient cohort were hypertension (n = 28; 56%), diabetes (n = 21; 42%), and dyslipidaemia (n = 14; 28%). Additional underlying conditions comprised chronic kidney disease (n = 3; 6%) or end-stage renal disease (n = 1; 2%), respiratory diseases including asthma or COPD (n = 3; 6%), and thalassaemia (n = 2; 4%), along with single cases (n = 1 each; 2%) of cancer, thyroid disorder, and neurodegenerative disease. Thirty-two patients had more than one comorbidity, whereas 18 patients (36.73%) had no documented comorbidity. No fatalities were documented within the study cohort during the reporting interval.

Table 1. Baseline characteristics of participants.

Note: Data are presented as number (percentage) unless otherwise stated. Percentages for comorbidities were calculated from the total sepsis cohort (n = 50); because several patients had more than one comorbidity, the percentages do not sum to 100%.

Level of plasma biomarker for sepsis

To characterize the systemic response and inflammatory profile of the study population, several plasma biomarkers and haematological indices were compared between the sepsis group and healthy controls. Plasma levels of SLPI were significantly elevated in the sepsis cohort (n = 50) compared to the healthy group (n = 50) (99.38 ± 61.58 ng/ml vs 32.49 ± 13.45 ng/ml, P < 0.001). This trend of high statistical significance was also observed in other key inflammatory markers; specifically, plasma C-reactive protein (CRP), and procalcitonin (PCT) (84.29 ± 58.91 ng/ml vs 1.43 ± 0.85 ng/ml, P < 0.001) were both markedly higher in sepsis patients than in the healthy controls (Figure 1).

Figure 1. Profile of inflammatory and metabolic markers in study participants. Box-and-whisker plots illustrate the distribution of various clinical parameters measured in healthy controls and patients with sepsis. Box-and-whisker plots show the median (horizontal line), interquartile range (box), and minimum to maximum values (whiskers). Because the data were not normally distributed, differences between groups were compared with the two-tailed Mann–Whitney U test. Plasma SLPI (A), Plasma Lactate (B), Plasma Procalcitonin (C), Plasma C-reactive protein (CRP) (D), and White Blood Cell (WBC) Count (E) (* P < 0.05, **** P < 0.0001).

In addition to biochemical markers, haematological and metabolic indicators further differentiated the two groups. The white blood cell (WBC) count showed a significant increase in the sepsis group relative to the healthy participants (1.51 ± 0.92 x 10⁶ /mL vs 6.27 ± 1.51 x 10⁶ /mL, P < 0.001). Furthermore, metabolic distress was evidenced by significantly higher plasma lactate levels in the sepsis patients compared to the control group (5.93 ± 3.90 nmol/L vs 4.56 ± 2.43 nmol/L, P = 0.0422). While the difference in lactate levels was statistically significant, it demonstrated a lower degree of significance than the inflammatory and haematological markers.

The diagnostic value of plasma SLPI and other biomarkers for sepsis

The diagnostic accuracy of various plasma biomarkers and haematological indicators for detecting sepsis was assessed by Receiver Operating Characteristic (ROC) curve analysis. The Area Under the Curve (AUC) was computed for each parameter to assess its discriminative efficacy.

The diagnostic potential of plasma SLPI was assessed. The results demonstrated high diagnostic accuracy with an AUC of 0.9269 (95.00% CI, 0.8797 to 0.9741, P <0.0001), with a cut-off value of 43.62, sensitivity of 82.00%, and specificity of 79.19%.

Among the existing inflammatory biomarkers, plasma procalcitonin (PCT) exhibited excellent diagnostic efficacy with an AUC of 1.000, and plasma C-reactive protein (CRP) also showed remarkable diagnostic accuracy, attaining an AUC of 0.9810 (95.00% CI, 0.9457 to 1.000, P <0.0001) (Figure 2).

Figure 2. Receiver Operating Characteristic (ROC) curve analysis of plasma biomarkers and haematological indices. The plots illustrate the trade-off between sensitivity and specificity for various markers used to distinguish sepsis patients from healthy controls. The area under the curve (AUC) represents the overall diagnostic accuracy for each parameter. Plasma SLPI (A), Plasma Lactate (B), Plasma Procalcitonin (C), Plasma C-reactive protein (CRP) (D), and White Blood Cell (WBC) Count (E).

In contrast, several clinical markers demonstrated varying levels of discriminative ability. The white blood cell (WBC) count exhibited considerable diagnostic value, as indicated by an AUC of 0.8739 (95.00% CI, 0.7932 to 0.9546, P < 0.0001). Plasma lactate demonstrated the least effective diagnostic performance among the assessed metrics, with an AUC of 0.5968 (95.00% CI, 0.4820 to 0.7116, P = 0.1022).

The findings indicate that while all evaluated markers showed differing levels of association with the septic condition, procalcitonin and CRP proved to be the most dependable indications for differentiating sepsis patients from healthy controls in this study population.

Correlation of plasma SLPI and other biomarkers for sepsis

Correlation analyses were conducted to assess the association between plasma SLPI levels and various clinical indicators or biomarkers in sepsis patients.

Figure 3 shows that plasma SLPI levels exhibited a positive correlation with several markers of inflammation and disease severity. Because plasma SLPI concentrations were not normally distributed and the qSOFA score is an ordinal variable, Spearman's rank correlation was used for all analyses. A significant moderate correlation was identified between plasma SLPI and the qSOFA score (rs = 0.3483, P = 0.0142). Moreover, plasma SLPI levels correlated significantly with the established inflammatory biomarkers C-reactive protein (CRP) (rs = 0.4028, P = 0.0045), procalcitonin (PCT) (rs = 0.3731, P = 0.0098), and white blood cell (WBC) count (rs = 0.3890, P = 0.0091). Interestingly, plasma SLPI also correlated with age (rs = 0.3556, P = 0.0004]). In contrast, no significant association was observed between plasma SLPI and lactate levels (rs = -0.01367, P = 0.9257). These data indicate that, whereas plasma SLPI levels track systemic inflammation and the clinical severity of organ dysfunction in septic patients, they do not directly reflect metabolic indicators such as lactate.

The data indicate that whereas SLPI levels correlate with systemic inflammation and organ failure in septic patients, they do not immediately correspond to metabolic indicators like lactate.

Figure 3. Relationship between plasma SLPI concentrations and clinical parameters in sepsis patients. Scatter plots represent the correlation between Secretory Leukocyte Protease Inhibitor (SLPI) levels and various indicators of organ dysfunction and inflammation. Spearman's rank correlation coefficient (rs) and the corresponding P-value are provided for each analysis. qSOFA score (A), Plasma Lactate (B), Plasma Procalcitonin (C), Plasma C-reactive protein (CRP) (D), White Blood Cell (WBC) Count (E), and Age (F).

DISCUSSION

The major finding of this study is that plasma SLPI level is significantly higher in patients with sepsis when compared to the healthy, with high performance, suggesting the potential as a diagnostic or prognostic biomarker. Moreover, the plasma SLPI level showed correlation with other biomarker for sepsis including procalcitonin (PCT), C-reactive protein (CRP), and white blood cell (WBC count). Furthermore, plasma SLPI level also correlated with the severity index of sepsis (Sequential Organ Failure Assessment or SOFA score).

Secretory Leukocyte Protease Inhibitor (SLPI) acts as a critical "alarm antiprotease" in balancing inflammatory proteases, primarily by acting as a first-line defence against excess proteolytic activity and directly modulating inflammatory signaling. SLPI regulates the protease enzymes such as neutrophil elastase, cathepsin G, trypsin, chymotrypsin, and chymase (Prompunt et al., 2018a) and provides anti-inflammation through inhibition of the transcription nuclear factor NF-ĸB activation in an LPS-induced model (Taggart et al., 2005; Zakrzewicz et al., 2019). SLPI not only play role as an anti-inflammatory protease, but it also provides therapeutic effects. The previous study demonstrated that giving recombinant human secretory leukocyte protease inhibitor (rhSLPI) by means of overexpression of the rhSLPI gene (Prompunt et al., 2018a) or treatment with recombinant protein of human SLPI provided a cytoprotective effect against ischemia/reperfusion (I/R) injury both in an in vitro (Prompunt et al., 2018a; Prompunt et al., 2018b), ex vivo (Prompunt et al., 2018b), and in vivo (Mongkolpathumrat et al., 2021; Mongkolpathumrat et al., 2022b) study model. Furthermore, the beneficial effect of SLPI could also reduce vascular endothelial cell injury and preserve the vessel graft during storage (Nernpermpisooth et al., 2017; Kongpol et al., 2019; Kongpol et al., 2020). Interestingly, although the beneficial effects of SLPI are believed to be due to its anti-protease activity, a few studies showed that those effects might be independent of its anti-protease activity (Yang et al., 2005; McGarry et al., 2015). Pervious report showed that the recombinant protein of the anti-protease-deficient mutant SLPI (L72K, M73G, and L74G) (mt-SLPI) also provided cardioprotection against I/R injury (Mongkolpathumrat et al., 2022a). Therefore, the therapeutic potential of SLPI is believed to be in part due to its direct effects. This could be a good example that SLPI not only involve in infectious and inflammatory diseases but also play important roles in non-communicable diseases (NCD). Recent published review articles were intensively reviewing the role of SLPI as a therapeutic molecules and biomarkers for several NCDs such as brain and spinal cord injuries, respiratory disorders, urology, obstetrics, and gynaecology disorders, liver disease, kidney disease, ocular disease, bone, cartilage, and dental disorders, cancer, and cardiovascular diseases (Mongkolpathumrat et al., 2024).

The current study demonstrated the notable elevation in plasma SLPI levels indicates the body's effort to re-establish the protease-antiprotease balance disturbed by substantial neutrophil infiltration. In this study, the plasma SLPI exhibit high diagnostic accuracy (AUC of 0.9269) and could be used as a novel diagnostic biomarker. While the prior study indicated that PCT possesses superior diagnostic capability compared to CRP(Meisner et al., 1999), this study demonstrates that plasma PCT and CRP remain two biomarkers with remarkable diagnostic accuracy. Although PCT and CRP possess exceptionally high diagnostic accuracy, with AUC values of 1.000 and 0.9810, respectively, this complete separation presumably indicates the differing physiological characteristics of healthy donors compared to critically ill ICU patients. In a wider clinical context involving individuals with non-septic systemic inflammatory response syndrome (SIRS), these indicators, along with SLPI, may have more overlapping distributions. Furthermore, the distinctive role of SLPI as both an anti-inflammatory and antimicrobial agent indicates it may offer supplementary diagnostic insights beyond those provided by conventional markers.

This study demonstrates an important positive correlation between SLPI and the SOFA score. This indicates that an increase in organ dysfunction correlates with elevated SLPI levels. The association strengthens the concept that SLPI levels reflect the magnitude of the host's compensatory reaction to sepsis-induced organ damage. SLPI demonstrated a correlation with CRP, PCT, and WBC count, so reinforcing its status as a reliable marker of the systemic inflammatory profile.    

Although SLPI had a strong correlation with inflammatory markers, it did not demonstrate a meaningful association with lactate levels. Lactate serves predominantly as an indicator of cellular hypoxia and metabolic strain (Schmiedeknecht et al., 2022), while SLPI functions as a regulatory protein that responds to inflammatory stimuli and protease secretion. This disparity signifies that SLPI offers an alternative perspective on the patient's condition—specifically on the inflammatory and proteolytic burden—rather than their immediate metabolic or haemodynamic stability.  

The noted elevation of SLPI in the sepsis cohort, especially among those with bacterial origins such as E. coli and K. pneumoniae, S. pneumoniae, and S. aureus underscores its significance in the innate immune response to prevalent septic stimuli (Grobmyer et al., 2000; Doumas et al., 2005; Lange et al., 2019). The observed high endogenous levels of SLPI may indicate a vital yet inadequate natural defence mechanism, given that exogenous SLPI administration has demonstrated potential in enhancing survival in experimental models.   

There are several issues that could be considered as limitation of the study. Although this study provides significant insights into the role of SLPI in sepsis, several limitations must be acknowledged. First, the study was conducted with a relatively small sample size, involving a cohort of 50 patients and 50 healthy controls from a single centre. Larger, multi-centre trials are necessary to validate these findings across more diverse populations and to ensure the broader applicability of SLPI as a diagnostic tool.

While plasma and salivary SLPI concentrations has been reported to significantly varied with increasing age (Rahman et al., 2016), but generally influenced by age and inflammation, some studies on specific diseases (e.g., Sjögren's syndrome or thyroid cancer) found no direct correlation between plasma SLPI levels and age, focusing instead on disease progression, such as in autoimmune disorders or cancer (Maruyama et al., 1998; Ameshima et al., 2000; Miyazaki et al., 2023). In this study, a significant age discrepancy was observed between the patient group (63.89 ± 19.21 years) and the healthy control group (43.20 ± 8.48 years) and the results also demonstrated moderate correlation between plasma SLPI level and age. Future research should utilize age-matched cohorts to effectively control for potential age-related variations in baseline SLPI levels and to isolate the specific impact of the septic condition on this protein's expression.

Building upon the findings of this research, future studies should prioritize large-scale, multicentre clinical trials to validate the diagnostic and prognostic utility of plasma SLPI across more diverse patient populations and clinical settings. To address the demographic differences noted in this cohort, implementing age-matched control groups will be essential to accurately define the baseline variation of SLPI in healthy individuals versus those with systemic infection. Additionally, longitudinal studies that perform serial measurements of SLPI at multiple time points—such as at 24, 48, and 72 hours post-admission—could provide a clearer understanding of how SLPI levels fluctuate in response to treatment or disease progression.

While a variety of pathogens were identified, such as E. coli and K. pneumoniae, a large percentage of cases (68.00%) showed no growth in haemoculture. Although this percentage is high, it is prevalent in clinical practice; culture-negative sepsis continues to pose a substantial issue, frequently occurring in roughly 30.00% to 50.00% of cases worldwide (Kim et al., 2021; Yang et al., 2021; Huang et al., 2022; Zizzo et al., 2025). In this investigation, these individuals were classified as having sepsis due to clinical signs of local or systemic infection and the requirement for antibiotic treatment, in accordance with recognised diagnostic criteria. The influence of specific microbial aetiologies on the magnitude of the SLPI response remains to be fully elucidated through advanced molecular diagnostics. Furthermore, the correlation between SLPI level and other protein Damage-Associated Molecular Pattern (DAMPs), particularly in most vulnerable organ such as the heart and brain, could provide more useful information to prevent the fatal consequences of sepsis (Kumphune et al., 2024). Finally, exploring the potential of SLPI as a therapeutic monitoring tool may help clinicians determine the efficacy of anti-inflammatory interventions in real-time.

CONCLUSION

In conclusion, plasma SLPI represents a potential biomarker for sepsis, demonstrating correlation with recognised inflammatory markers and the extent of organ dysfunction. The capacity to reflect the SOFA score indicates its potential utility in monitoring disease progression and informing clinical management in septic patients.

ACKNOWLEDGEMENTS

We would like to acknowledge Uttaradit Hospital for provide resource and encouragement on clinical samples collection and handling, Biomedical Engineering Institute-Chiang Mai University for providing research facilities and support.

AUTHOR CONTRIBUTIONS

Suwipar Deebukkham:  Conceptualization (Lead), Data Curation (Lead), Formal Analysis (Lead), Investigation (Lead), Resources (Lead), Visualization (Lead), Writing – Original Draft (Lead), Writing – Review & Editing (Lead); Adisorn Bunnag: Formal Analysis (Equal), Investigation (Equal), Validation (Equal), Writing – Original Draft (Equal), Writing – Review & Editing (Equal); Faprathan Pikwong: Formal Analysis (Equal), Investigation (Equal), Validation (Equal), Writing – Review & Editing (Equal);Yanisa Hoiraya: Formal Analysis (Equal), Investigation (Equal), Validation (Equal), Writing – Review & Editing (Equal); Onnicha Srisopar: Formal Analysis (Equal), Investigation (Equal), Validation (Equal), Writing – Review & Editing (Equal);  Panyupa Pankhong: Conceptualization (Lead), Investigation (Supporting), Validation, Writing – Original Draft (Supporting), Writing – Review & Editing (Equal);  Sarawut Kumphune: Conceptualization (Equal), Data Curation (Equal), Formal Analysis (Equal), Funding Acquisition (Lead), Investigation (Equal), Project Administration (Lead), Resources (Equal), Supervision (Lead), Validation (Equal), Visualization (Equal), Writing – Original Draft (Equal), Writing – Review & Editing (Equal).

CONFLICT OF INTEREST

The authors declare that they have no conflicts of interest.

DISCLOSURE STATEMENT ABOUT AI TOOL

The author acknowledges the use of AI to improve grammar and clarity.

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