Lin XY, Zheng YX, Liu MM, Liang Q, Li M, Sui J, Qiang W, Guo H, Shi BY, He MQ. Indirect bilirubin is inversely associated with diabetic retinopathy risk and is a potential predictive biomarker. World J Diabetes 2025; 16(9): 110590 [DOI: 10.4239/wjd.v16.i9.110590]
Corresponding Author of This Article
Ming-Qian He, MD, Department of Endocrinology, The First Affiliated Hospital of Xi’an Jiaotong University, No. 277 Yanta West Road, Xi’an 710061, Shaanxi Province, China. mingqian_he@xjtufh.edu.cn
Research Domain of This Article
Endocrinology & Metabolism
Article-Type of This Article
Retrospective Study
Open-Access Policy of This Article
This article is an open-access article which was selected by an in-house editor and fully peer-reviewed by external reviewers. It is distributed in accordance with the Creative Commons Attribution Non Commercial (CC BY-NC 4.0) license, which permits others to distribute, remix, adapt, build upon this work non-commercially, and license their derivative works on different terms, provided the original work is properly cited and the use is non-commercial. See: http://creativecommons.org/licenses/by-nc/4.0/
Xiao-Ying Lin, Yi-Xuan Zheng, Qian Liang, Meng Li, Wei Qiang, Hui Guo, Bing-Yin Shi, Ming-Qian He, Department of Endocrinology, The First Affiliated Hospital of Xi’an Jiaotong University, Xi’an 710061, Shaanxi Province, China
Meng-Meng Liu, Department of Endocrinology, Xi’an Gaoxin Hospital, Xi’an 710075, Shaanxi Province, China
Jing Sui, Department of Endocrinology and International Medical Center, The First Affiliated Hospital of Xi’an Jiaotong University, Xi’an 710061, Shaanxi Province, China
Ming-Qian He, Med-X Institute, Center for Immunological and Metabolic Diseases, The First Affiliated Hospital of Xi’an Jiaotong University, Xi’an 710061, Shaanxi Province, China
Co-corresponding authors: Bing-Yin Shi and Ming-Qian He.
Author contributions: Lin XY contributed to the conceptualization, methodology, software, validation, visualization, writing—original draft preparation, and writing—review and editing; Zheng YX and Liu MM contributed to the formal analysis and data curation; Liang Q contributed to the investigation; Li M, Sui J, and Qiang W contributed to resources and funding acquisition; Guo H helped supervise the study; Shi BY helped supervise the study and contributed to project administration; He MQ contributed to conceptualization, methodology, validation, resources, writing—original draft preparation, writing—review and editing, supervision, project administration, and funding acquisition. As co-corresponding authors, Shi BY and He MQ played important roles in the preparation and administration of this study. The collaboration between Shi BY and He MQ was essential for the publication of this manuscript and therefore qualifies them as co-corresponding authors of the paper.
Supported by Natural Science Foundation of Shaanxi Province, No. 2024JC-YBQN-0828 and No. 2024JC-YBMS-765; Clinical Research Award of the First Affiliated Hospital of Xi'an Jiaotong University, No. XJTU1AF-CRF-2022-036; and National Innovation Center for Advanced Medical Devices, No. NMED2023AGP-013.
Institutional review board statement: The study was reviewed and approved by the Ethic Committees of the First Affiliated Hospital of Xi’an Jiaotong University (Approval No. XJTU1AF2018 LSK- 055).
Informed consent statement: All study participants, or their legal guardian, provided informed written consent prior to study enrollment.
Conflict-of-interest statement: There is no conflict of interest associated with any of the senior author or other coauthors contributed their efforts in this manuscript.
Data sharing statement: Technical appendix, statistical code, and dataset available from the corresponding author at mingqian_he@xjtufh.edu.cn. Consent was not obtained but the presented data are anonymized and risk of identification is low.
Open Access: This article is an open-access article that was selected by an in-house editor and fully peer-reviewed by external reviewers. It is distributed in accordance with the Creative Commons Attribution NonCommercial (CC BY-NC 4.0) license, which permits others to distribute, remix, adapt, build upon this work non-commercially, and license their derivative works on different terms, provided the original work is properly cited and the use is non-commercial. See: https://creativecommons.org/Licenses/by-nc/4.0/
Corresponding author: Ming-Qian He, MD, Department of Endocrinology, The First Affiliated Hospital of Xi’an Jiaotong University, No. 277 Yanta West Road, Xi’an 710061, Shaanxi Province, China. mingqian_he@xjtufh.edu.cn
Received: June 12, 2025 Revised: July 10, 2025 Accepted: August 19, 2025 Published online: September 15, 2025 Processing time: 93 Days and 21.4 Hours
Abstract
BACKGROUND
Diabetic retinopathy (DR) is a major cause of visual impairment and blindness. However, the current DR biomarkers are insufficient for accurately predicting its onset.
AIM
To identify a novel marker for predicting the risk of developing DR in patients with type 2 diabetes mellitus (T2DM).
METHODS
We conducted a cross-sectional study involving 6993 hospitalized T2DM patients between 2013 and 2020. Patients were divided into two groups: The DR group and the non-DR group. Data were analyzed using univariate, correlation, multivariate, subgroup, and receiver operating characteristic curve analyses.
RESULTS
Total bilirubin, indirect bilirubin (IBIL), and direct bilirubin were negatively correlated with the risk of developing DR (P < 0.001). Moreover, these three factors were all positively correlated with clinical indicators related to DR, including the estimated glomerular filtration rate, the albumin/creatinine ratio, and the 1,25-dihydroxyvitamin D3 level (P < 0.001). After adjusting for multiple variables, greater IBIL levels remained independently associated with a lower risk of developing DR (odds ratio = 0.500; 95% confidence interval: 0.363-0.686; P < 0.001). The optimal IBIL cutoff point for predicting the risk of DR in male patients with elevated diastolic blood pressure was 0.655 μmol/dL (area under the curve = 0.662).
CONCLUSION
These findings suggest that IBIL could be a valuable biomarker for predicting DR risk, offering a noninvasive, cost-effective, and readily available clinical tool for the early identification of high-risk patients. Future multicenter and longitudinal studies are warranted to validate these findings and further explore the biological mechanisms underlying the protective role of IBIL in DR.
Core Tip: This cross-sectional study distinguished the role of indirect bilirubin (IBIL) in diabetic retinopathy (DR) risk in patients with type 2 diabetes mellitus. While total and direct bilirubin showed some protective effects, only higher IBIL levels remained independently associated with a lower risk of developing DR after comprehensive multivariable adjustment (odds ratio = 0.500; 95% confidence interval: 0.363-0.686; P < 0.001). IBIL is thus a potential predictive biomarker for DR.
Citation: Lin XY, Zheng YX, Liu MM, Liang Q, Li M, Sui J, Qiang W, Guo H, Shi BY, He MQ. Indirect bilirubin is inversely associated with diabetic retinopathy risk and is a potential predictive biomarker. World J Diabetes 2025; 16(9): 110590
Type 2 diabetes mellitus (T2DM) is a metabolic disorder characterized by chronic hyperglycemia[1] and is predicted to affect over 1.27 billion individuals globally by 2050[2], posing a critical public health burden. Diabetic retinopathy (DR), a common and specific neurovascular complication of T2DM[3], affects approximately 15%-26% of patients[4,5]. DR is the leading cause of irreversible visual impairment and blindness worldwide[6], and thus DR significantly reduces patients’ quality of life and increases the healthcare burden. Therefore, a comprehensive understanding of DR risk factors is crucial for preventing blindness and improving patients' quality of life. Major risk factors for the development of DR include chronic hyperglycemia[7], hypertension[8], dyslipidemia[9], diabetes duration[3], and nephropathy[10]. However, reliable biomarkers for early identification and diagnosis of DR are still lacking, which hinders timely intervention and management.
Bilirubin, once regarded only as a waste product, has recently gained recognition for its protective effects against various conditions, including diabetes, metabolic syndrome, and atherosclerosis[11,12]. Total bilirubin (TBIL) includes both direct bilirubin (DBIL) and indirect bilirubin (IBIL), and of these, IBIL is the predominant form[13]. IBIL is derived from heme, which is released primarily during the breakdown of senescent red blood cells. IBIL is not water soluble, but through bilirubin glucuronidation, IBIL is converted into DBIL, which is water soluble and can be excreted into the bile[14]. Previous studies have suggested that greater TBIL levels may serve as a protective factor against chronic complications of diabetes, including diabetic microvascular complications[15,16]. However, the exact mechanism remains unclear, as these studies did not clarify whether IBIL or DBIL played the predominant role, despite their substantial differences in quantity and lipid solubility.
Given the urgent need for effective markers to predict DR, the aim of this study was to investigate novel markers for predicting the risk of developing DR in patients with T2DM. We hypothesized that serum bilirubin levels are inversely associated with the risk of developing DR and sought to determine which bilirubin subtype—total, direct, or indirect—plays the predominant role in this risk.
MATERIALS AND METHODS
Study design and population
This study was conducted at the Department of Endocrinology of the First Affiliated Hospital of Xi’an Jiaotong University. Patients with T2DM who were hospitalized in the Endocrinology Department of The First Affiliated Hospital of Xi’an Jiaotong University from 2013 to 2020 were initially enrolled. The exclusion criteria were: (1) Patients with nondiabetic retinopathy; (2) Patients with severe liver or renal dysfunction or cardiac disease; (3) Patients with endocrine diseases other than T2DM, such as thyroid disease, parathyroid disease, adrenal diseases, and pituitary diseases; and (4) Patients with malignant tumors. The study was conducted in accordance with the Declaration of Helsinki and was approved by the Ethics Committee of the First Affiliated Hospital of Xi’an Jiao Tong University (protocol code XJTU1AF2018 LSK-055; date: 31 May 2018). Informed consent was obtained from all subjects involved in the study.
Clinical and laboratory data
The data were extracted from medical records and included demographic information such as sex and age, alongside key indicators such as duration of diabetes, presence of DR, blood pressure, and body mass index (BMI). Additionally, biochemical parameters, including fasting plasma glucose (FPG), glycosylated hemoglobin (HbA1c), lipid profiles [total cholesterol, triglycerides, high-density lipoprotein (HDL) and low-density lipoprotein (LDL)], the albumin/creatinine ratio (ACR), the estimated glomerular filtration rate (eGFR), liver function tests [aspartate aminotransferase (AST), alanine aminotransferase (ALT), gamma-glutamyl transferase (GGT), TBIL, IBIL and DBIL], high C-reactive protein (CRP) status (defined as elevated when > 10 mg/L), and levels of 1,25(OH)2D3, were recorded. All biochemical assays were performed on venous blood samples obtained after an 8-hour fasting period. Furthermore, the collected data also included diabetes treatment regimens [insulin therapy, biguanides, sulfonylureas, glinides, α-glucosidase inhibitors, thiazolidinediones, dipeptidyl peptidase-4 (DPP-4) inhibitors, glucagon-like peptide-1 (GLP-1) analogs, and sodium-glucose cotransporter-2 (SGLT-2) inhibitors] and the use of lipid-lowering drugs.
Diagnosis of DR
The assessment of DR included a thorough history review, covering the duration of diabetes, past glycemic control, medication use, medical history, and ocular history. Comprehensive eye examinations were performed according to DR Preferred Practice Pattern[17], including assessments of visual acuity, slit-lamp biomicroscopy, intraocular pressure, gonioscopy, and thorough fundus examination to evaluate the presence of retinal hemorrhages/microaneurysms, venous beading, intraretinal microvascular abnormalities, optic nerve head neovascularization and/or neovascularization elsewhere, vitreous or preretinal hemorrhage, and macular edema. The diagnosis of DR was confirmed by experienced ophthalmologists from the Department of Ophthalmology at The First Affiliated Hospital of Xi’an Jiaotong University.
Statistical analysis
Statistical analysis was conducted via SPSS version 26.0 software (SPSS, Inc., Chicago, IL, United States). The normality of all variables was evaluated. Normally distributed continuous variables are expressed as the mean ± SD, whereas nonnormally distributed continuous variables are presented as medians with interquartile ranges (25th-75th percentiles). Group differences in these variables were assessed via either Student’s t-test or the Mann-Whitney U test on the basis of data distribution. Categorical variables are presented as frequencies or percentages, with group comparisons performed via the χ2 test. Univariate logistic regression models were applied to explore the relationships between T2DM factors and the risk of developing DR. Spearman’s correlation analysis was used to examine the relationships between bilirubin levels and other clinical features in T2DM patients. Multiple logistic regression analysis was conducted to identify independent risk factors for the development of DR. A receiver operating characteristic (ROC) curve analysis was performed to determine the IBIL cutoff point for predicting the risk of developing DR. A P value < 0.05 was considered to indicate statistical significance.
RESULTS
Clinical and laboratory characteristics of the study participants
A total of 6993 Chinese adults diagnosed with T2DM, consisting of 4490 males (64.2%) and 2503 females (35.8%), with a mean age ± SD of 55.9 ± 14.0 years, were included in the study. Among them, 2122 participants were diagnosed with DR. To explore the clinical features associated with DR in patients with T2DM, we first compared the demographic, clinical, and biochemical characteristics between patients with and without DR (Table 1). Compared with patients without DR, those with DR were generally older and had a longer duration of diabetes; higher systolic blood pressure (SBP) and diastolic blood pressure (DBP); higher HDL levels; a higher ACR; and increased use of insulin, biguanides, and lipid-lowering drugs. Additionally, they had lower BMIs; eGFRs; AST, ALT, GGT, TBIL, IBIL, DBIL, and 1,25(OH)2D3 levels; and reduced use of sulfonylureas, DPP-4 inhibitors, GLP-1 analogs, and SGLT-2 inhibitors (P < 0.001, P < 0.01, or P < 0.05). No significant differences were observed between the two groups in terms of sex ratio; FPG, HbA1c, total cholesterol, triglyceride, or LDL levels; or the use of glinides, α-glucosidase inhibitors, or thiazolidinediones (P > 0.05).
Table 1 Characteristics of the study participants.
Univariate analysis of factors related to the risk of developing DR in T2DM patients
To identify factors potentially associated with the development of DR, univariate logistic regression analysis was performed on a range of clinical and biochemical variables. Univariate logistic regression analysis (Figure 1) revealed that higher age, duration of diabetes, SBP, DBP, and ACR were positively correlated with the risk of developing DR [odds ratios (ORs) = 1.023, 1.091, 1.017, 1.010, and 1.001, respectively; all P < 0.001]. Conversely, the levels of three liver enzymes—AST, ALT, and GGT—and the eGFR were negatively associated with the risk of developing DR (OR = 0.996, 0.992, 0.998, and 0.982, respectively; all P < 0.01). Additionally, increased levels of the three bilirubin indices—TBIL, IBIL, and DBIL—were protective factors against the development of DR (OR = 0.542, 0.392, and 0.472, respectively; all P < 0.001). Notably, the ORs associated with bilirubin were lower than those associated with the other protective factors against the development of DR.
Figure 1 Univariate analysis of the associations between factors associated with type 2 diabetes mellitus and the risk of developing diabetic retinopathy.
Univariate logistic regression models were constructed to explore the correlations between type 2 diabetes mellitus factors and the risk of developing diabetic retinopathy. aP < 0.01; bP < 0.001. Statistical significance was determined at a P value < 0.05. OR: Odds ratio; 95%CI: 95% confidence interval; SBP: Systolic blood pressure; DBP: Diastolic blood pressure; ACR: Albumin/creatinine ratio; eGFR: Estimated glomerular filtration rate; AST: Aspartate transaminase; ALT: Alanine aminotransferase; GGT: Gamma-glutamyl transferase; TBIL: Total bilirubin; IBIL: Indirect bilirubin; DBIL: Direct bilirubin.
Relationships between bilirubin levels and other clinical/laboratory features
To further elucidate the role of bilirubin in DR risk, we performed Spearman’s correlation analysis to examine the correlations between bilirubin levels and key clinical parameters known to be related to DR pathogenesis, including age, duration of diabetes, SBP, eGFR, ACR, and 1,25(OH)2D3. IBIL, DBIL, and TBIL levels were positively correlated with eGFR (r = 0.42, 0.37, and 0.41, respectively), ACR (r = 0.40, 0.37, and 0.39, respectively), and 1,25(OH)2D3 (r = 0.45, 0.46, and 0.45, respectively), with all correlations being statistically significant (P < 0.001; Figure 2).
Figure 2 Correlations between bilirubin indices and other clinical characteristics.
Spearman’s correlation analysis was used to evaluate the relationships between bilirubin indices and other clinical features in patients with type 2 diabetes mellitus. aP < 0.05; bP < 0.01, cP < 0.001. Statistical significance was determined at a P value < 0.05. IBIL: Indirect bilirubin; DBIL: Direct bilirubin; TBIL: Total bilirubin; SBP: Systolic blood pressure; eGFR: Estimated glomerular filtration rate; ACR: Albumin/creatinine ratio.
Multivariate analysis of the associations of bilirubin levels and related factors with the risk of developing DR
Given the potential confounding effects of other clinical variables, multivariate logistic regression analyses were conducted to determine whether bilirubin levels were independently associated with DR after adjusting for covariates. Three logistic regression models were established: (1) Model 1 was adjusted for age; (2) Model 2 was adjusted for age and diabetes duration; and (3) Model 3 was adjusted for age, diabetes duration, SBP, the eGFR, the ACR, and 1,25(OH)2D3 (Table 2). The OR reflects the relative reduction in DR risk for each 1-unit increase in bilirubin. In both Model 1 and Model 2, IBIL, DBIL, and TBIL were significantly associated with the risk of developing DR. In Model 3, only IBIL remained an independent and significant protective factor against the development of DR [OR = 0.500; 95% confidence interval (95%CI): 0.363-0.686; P < 0.001].
Table 2 Multivariate analysis of the association between bilirubin indices and the risk of developing diabetic retinopathy.
Value of IBIL for predicting the risk of developing DR
To evaluate the predictive value of IBIL for DR risk, we performed ROC curve analyses to identify optimal cutoff values for clinical application, including subgroup analysis in high-risk populations. The results indicated that the optimal cutoff point of IBIL for predicting the risk of developing DR was 0.735 μmol/dL, with an area under the curve (AUC) of 0.612 (95%CI: 0.596-0.627; P < 0.001; Youden index = 0.175; sensitivity, 55.8%; specificity, 61.7%; Figure 3). ROC curve analysis was further performed for male patients with elevated DBP (≥ 90 mmHg; Figure 3). The optimal cutoff point of IBIL was 0.655 μmol/dL for predicting the risk of developing DR (AUC = 0.662; 95%CI: 0.621-0.704; P < 0.001; Youden index = 0.283; sensitivity, 75.5%; specificity, 52.8%).
Figure 3 Receiver operating characteristic analysis of the association between indirect bilirubin and the risk of developing diabetic retinopathy.
Receiver operating characteristic (ROC) curve analysis was carried out to determine the cutoff point of indirect bilirubin (IBIL) for predicting the risk of developing diabetic retinopathy (DR). Statistical significance was determined at a P value < 0.05. a: ROC analysis of IBIL for predicting the risk of developing DR in patients with type 2 diabetes mellitus (T2DM); b: ROC analysis of IBIL for predicting the risk of developing DR in male T2DM patients with elevated diastolic blood pressure. AUC: Area under the curve.
DISCUSSION
T2DM has become a major global health issue[2,18]. DR is a common and specific microvascular complication of T2DM, leading to vision impairment and blindness[19]. A comprehensive understanding of the risk factors for the development of DR is essential for reducing preventable blindness and improving patients' quality of life. Therefore, in the present study, we analyzed clinical data from 6993 adults diagnosed with T2DM to identify new indices that could help predict the risk of developing DR. Our analysis revealed that three bilirubin indices—TBIL, IBIL, and DBIL—were all negatively correlated with the risk of developing DR, with the OR associated with bilirubin indices being lower than the ORs associated with other protective factors. Additionally, we explored the relationships between bilirubin indices and other clinical features and found that bilirubin levels were correlated with indicators of DR. After multivariable adjustments, IBIL remained significantly associated with the risk of developing DR, whereas TBIL and DBIL did not, suggesting that IBIL was independently and negatively associated with the risk of developing DR. ROC curve analysis further supported these findings, showing that the AUC for IBIL in predicting the risk of developing DR was 0.612. In a subgroup analysis of male patients with elevated DBP, the AUC for IBIL increased to 0.662. We acknowledge that the AUC is relatively modest. Nevertheless, consistent with findings from other large-scale studies where even modest AUCs can yield clinically valuable insights[20,21], IBIL levels still provide meaningful information for detecting DR among patients with T2DM. These findings suggest that IBIL could be a valuable biomarker for predicting DR risk, offering a noninvasive, cost-effective, and readily available clinical tool for the early identification of high-risk patients.
In this study, we found that TBIL, IBIL, and DBIL were negatively correlated with the risk of developing DR. While several studies have investigated the relationships between bilirubin indices and the risk of developing DR, the results have been inconsistent. Most previous studies have shown a negative relationship between TBIL and the risk of developing DR[22-26], which is consistent with our findings. In a 5-year follow-up study, Liu et al[27] reported that TBIL exhibited a U-shaped relationship with DR incidence. A cross-sectional study of 6713 patients with T2DM demonstrated an L-shaped association between TBIL and the risk of developing DR[28]. However, some studies have reported no association between TBIL and the risk of developing DR[29,30]. Although a few studies yielded different findings, potentially due to differences in sample size and characteristics, our results align with the majority of studies indicating that increased bilirubin levels serve as a protective factor against the development of DR.
Furthermore, IBIL, rather than TBIL or DBIL, was independently and negatively correlated with the risk of developing DR. Most studies have reported an inverse relationship between bilirubin indices and the risk of developing DR, and most have focused solely on TBIL without distinguishing between IBIL and DBIL. An exception is a cross-sectional study involving 4368 diabetic participants, in which Wan et al[31] reported that lower levels of DBIL and TBIL, but not IBIL, were associated with increased incidence and severity of DR, which is inconsistent with our findings. These discrepancies may be partly explained by differences in study populations and the covariates adjusted for in the analyses. Their study involved relatively stable community-based outpatients, whereas our study included hospitalized patients with more severe conditions, which may affect both bilirubin metabolism and DR risk. In addition, their covariate adjustments emphasized demographic and lifestyle factors, whereas our model focused more on renal and vascular parameters that are more directly related to DR. These distinctions likely contributed to the contradictory results. In contrast, another clinical study provided evidence that supported our findings. Patients with both Gilbert syndrome—a hereditary condition marked by chronically elevated IBIL—and T2DM presented a significantly lower prevalence of both DR and diabetic nephropathy compared with those without Gilbert syndrome[32]. These findings suggest that the protective role of IBIL may not be limited to DR but that it could extend to other microvascular complications, such as nephropathy.
Although the underlying mechanisms of the association between IBIL and the risk of developing DR remain unclear, there are several possible explanations. Bilirubin is a strong endogenous antioxidant[33] and a major contributor to the total antioxidant capacity of blood plasma[34]. Previous studies have suggested that bilirubin may function as an antioxidant by scavenging reactive oxygen species, inhibiting NADPH oxidase activity, and complementing other antioxidants synergistically[35-37]. Consequently, mildly elevated bilirubin levels may alleviate oxidative stress in the vascular endothelium, a key factor in the pathogenesis of DR. In addition to its antioxidant properties, bilirubin also exerts anti-inflammatory effects by modulating both innate and adaptive immune responses, including the suppression of pro-inflammatory cytokine production and the inhibition of T-cell activation[38]. This is particularly noteworthy given that chronic low-grade inflammation is increasingly recognized as a key contributor to microvascular damage in patients with DR[19]. Thus, the anti-inflammatory action of bilirubin provides a plausible mechanistic explanation for its inverse association with DR risk observed in our study. Additionally, bilirubin has recently been recognized as a hormone that primarily targets peroxisome proliferator-activated receptor alpha (PPARα) within cells to induce the expression of genes related to fatty acid β-oxidation at physiological levels[39]. Another PPARα ligand, fenofibrate, reduced the need for laser treatment for vision-threatening DR by 31% in T2DM patients in a randomized controlled trial[40]. Notably, DBIL has not been shown to interact with PPARα, which may explain why DR is more strongly associated with IBIL than with DBIL.
Our study identified male T2DM patients with elevated DBP as the optimal population in which IBIL serves as the best predictor of DR, with a cutoff point of 0.655 μmol/dL. This subgroup association may be due to sex differences in smoking prevalence. Male patients are more likely to smoke than are female patients, and smoking is a known risk factor for the development of DR[41,42]. Additionally, elevated DBP in T2DM patients can lead to endothelial dysfunction, vascular inflammation, vascular fibrosis, and arterial remodeling, all of which contribute to microvascular disease[43], potentially increasing the protective effects of IBIL. Consequently, IBIL may be an especially effective predictor of DR development within this specific subgroup.
Given that IBIL testing is inexpensive and already included in routine liver function panels, IBIL could serve as a noninvasive, accessible screening marker to identify T2DM patients at higher risk for DR, particularly in primary care or outpatient settings. However, to translate this finding into clinical practice, future multicenter studies are necessary to validate the predictive value of IBIL across diverse populations and to develop standardized testing protocols that account for interlaboratory variability. Despite such variability, monitoring the trend of IBIL levels within the same laboratory could still provide valuable information for assessing DR risk. Moreover, IBIL would be applicable primarily to individuals without underlying liver disease, as hepatic dysfunction can significantly influence bilirubin levels. In addition to its role as a potential biomarker, IBIL may also serve as a novel therapeutic target to prevent or slow the progression of DR. Given the antioxidant and anti-inflammatory properties of bilirubin, bilirubin-based nanoparticles have been developed and shown to be effective in various mouse models of inflammatory diseases, such as ulcerative colitis and liver fibrosis[44,45]. Therefore, whether an increase in IBIL levels using nanoparticles could offer a novel strategy to reduce DR risk is worthy of further exploration. In addition, future research should consider the roles of other bilirubin metabolites, such as biliverdin, which may work in concert with IBIL to regulate oxidative stress and inflammation[46]. Moreover, to increase the accuracy of DR risk prediction on the basis of IBIL and other clinical parameters, future studies should incorporate machine learning or other advanced statistical models, which can effectively handle nonlinear relationships and high-dimensional data, thereby improving predictive performance and clinical utility. Exploring the interactions among these metabolites may provide a more comprehensive understanding of the bilirubin metabolic pathway and its contribution to diabetic microvascular complications.
Several limitations of this study should be acknowledged. First, its cross-sectional design precludes the establishment of causal relationships between IBIL levels and DR development. Prospective cohort studies that track IBIL dynamics before DR onset are necessary to confirm temporal relationships. Second, this study was conducted at a single center with a homogenous population of Chinese T2DM inpatients, which may limit the generalizability of our findings to other ethnic groups or outpatient populations. Differences in genetic background, bilirubin metabolism, dietary patterns, and healthcare access across ethnicities and regions may affect IBIL levels and DR risk. Therefore, future multicenter studies involving ethnically diverse and outpatient cohorts are warranted to validate the applicability of our results to other populations. Third, while our exclusion criteria eliminated patients with severe liver dysfunction, we acknowledge that subclinical liver conditions (e.g., non-alcoholic fatty liver disease) may also influence IBIL levels. Future studies should rigorously control for liver comorbidities to isolate the specific role of IBIL in DR pathogenesis. Finally, bilirubin was measured at a single time point, and we cannot rule out the influence of other factors on bilirubin levels. Repeated measurements over time could enhance the accuracy of future studies. Despite these limitations, the large sample size, robust statistical analyses, and consistency with biological plausibility suggest that our main conclusion—IBIL is independently and inversely associated with DR risk—is reliable and clinically meaningful.
CONCLUSION
The aim of the present study was to identify a novel biomarker for predicting the risk of developing DR in patients with T2DM. Our study revealed that IBIL was significantly associated with the risk of developing DR and acted as an independent protective factor against the development of DR in patients with T2DM. The risk of developing DR decreases when IBIL concentrations exceed 0.735 μmol/dL in T2DM patients. Thus, IBIL levels could serve as a promising predictive index and a potential therapeutic target for DR in the T2DM population. Future multicenter and longitudinal studies are essential to confirm these findings across diverse populations. In addition, mechanistic research is needed to unravel the complex biological pathways through which IBIL confers protection against DR, offering a foundation for both a deeper understanding of DR pathophysiology and improved clinical strategies.
Footnotes
Provenance and peer review: Unsolicited article; Externally peer reviewed.
Peer-review model: Single blind
Specialty type: Endocrinology and metabolism
Country of origin: China
Peer-review report’s classification
Scientific Quality: Grade A, Grade A, Grade B, Grade B, Grade B, Grade B, Grade C
Novelty: Grade A, Grade B, Grade B, Grade B, Grade B
Creativity or Innovation: Grade A, Grade B, Grade B, Grade B, Grade B
Scientific Significance: Grade A, Grade A, Grade A, Grade A, Grade A
P-Reviewer: Dabla PK, MD, Professor, India; Jiang JL, PhD, Senior Researcher, China; Kerry RG, Post Doctoral Researcher, Postdoctoral Fellow, India; Pappachan JM, MD, MRCP, Professor, Senior Researcher, United Kingdom; Zheng YY, Associate Professor, Associate Research Scientist, China S-Editor: Lin C L-Editor: A P-Editor: Zhang L
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