Introduction

Diabetes mellitus (DM) is a systemic disease characterized by vascular and neuropathic complications and is becoming an increasing problem worldwide [1]. Diabetic retinopathy (DR) is the most common complication of DM and is the primary cause of vision loss and impairment in developed countries [2, 3]. The pathogenesis of DR is complex and inflammation plays an essential role in the development and progression of DR, along with many factors such as hyperglycemia and hypertension [4,5,6,7].

Circulating monocytes migrate to tissues and turn into macrophages and dendritic cells and are involved in antimicrobial defense mechanisms. They also produce inflammatory cytokines and contribute to local and systemic inflammation. They play a role in many diseases with inflammatory mechanisms such as atherosclerosis [8].

High-density lipoprotein cholesterol (HDL-c) prevents atherosclerosis by transporting cholesterol from peripheral tissues to the liver [9]. Also, HDL-c protects endothelial functions, has an antioxidant effect, and modulates inflammation [10].

Until now, it has been shown that the monocyte/HDL-c ratio (MHR) is an important marker of inflammation and oxidative stress in many inflammation-related diseases, especially cardiovascular diseases [11,12,13]. The reason why the MHR shows inflammation strongly is due to the anti-inflammatory and antioxidant effects of HDL-c as well as the pro-inflammatory effects of monocytes. It has been reported that MHR can also be a biomarker for vascular and neuropathic complications of type 2 DM (T2DM) [14, 15]. However, as far as we know, there are not enough studies on MHR and DR.

This study aims to evaluate the relationship between DR and MHR values in T2DM patients.

Methods

Study population

This cross-sectional study was conducted between February 2020 and June 2020 in Ordu University Training and Research Hospital, Internal Medicine, and Ophthalmology Clinic. A total of 118 patients, 60 of whom had DR and diagnosis of T2DM, were included in the study. The control group consisted of 58 healthy individuals who were matched in terms of age and gender. Participants who accepted to participate in the study were questioned in terms of age, gender, hyperlipidemia, smoking, family history, chronic diseases, and medications used.

Smokers, chronic alcoholics, patients with infection, hematological disease, acute or chronic renal failure, chronic liver disease, cardiovascular disease, solid or malignant tumors, and chronic inflammatory diseases were excluded from the study. Our study was approved by the local ethics committee and was conducted in accordance with the ethical principles defined in the Declaration of Helsinki. A written informed consent form was obtained from all participants before the study.

Clinical examination and biochemical analysis

The diagnosis of T2DM was made according to the American Diabetes Association guidelines [16]. After a complete ophthalmological examination was applied to all participants, the diagnosis of retinopathy was made by fundus photographs, fluorescein angiography, and optical coherence tomography. The Diabetic Eye Care Guideline was used for DR diagnostic criteria [17].

Venous blood samples were taken after an overnight fast. All biochemical analyses were studied freshly in our hospital. ARCHITECT c8000 clinical chemistry analyzer (Abbott, IL, USA) was used to analyze creatinine, HDL-c, low-density lipoprotein cholesterol (LDL-c), total cholesterol, triglyceride, and C-reactive protein (CRP) concentrations. Complete blood count was analyzed using the CELL-DYN Ruby automated hematology analyzer (Abbott, IL, USA). Neutrophil/lymphocyte ratio (NLR), monocyte/lymphocyte ratio (MLR), platelet/lymphocyte ratio (PLR), and MHR were calculated and recorded for each participant.

Statistical analysis

In all statistical analyses, SPSS 26.0 Statistical Package Program for Windows (SPSS Inc., Chicago, IL, USA) was used. The Shapiro-Wilk test was used to evaluate the distribution of data. The Kruskal-Wallis test, one-way ANOVA, and Fisher’s exact test were used to compare the groups. The numeric variables as mean± SD and median (minimum-maximum) and the categorical variables as number and percent were expressed. Point biserial was applied to evaluate the correlation between the presence of DR and other parameters. Logistic regression analysis was used to calculate predictors of DR. MHR cutoff value was calculated by receiver operating characteristic (ROC) curve analysis to predict DR. A p-value of <0.05 was accepted as statistically significant.

Results

The study was conducted on 176 subjects, of whom 46 (26.1%) were males and 130 (73.9%) were females. Insulin use, hemoglobin, neutrophil, HbA1c, creatinine, HDL-c, and MHR were statistically different when the groups were compared in terms of age, gender, drug use, hemogram, and biochemical parameters (Table 1). In post hoc analysis, there was a significant difference between group 1 and group 2 in terms of Hba1c, HDL-c. It was found that there were differences in neutrophil, HDL-c, and MHR between group 1 and group 3, and there was a difference between group 2 and group 3 in terms of neutrophil, HbA1c, and creatinine (Table 1).

Table 1 Baseline characteristics and laboratory parameters of the groups
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Considering whether there is a correlation between DR and age, hemogram, biochemical parameters, and MHR, positive correlation with neutrophil, HbA1c, creatinine, NLR, and MHR (r= 0.251 p= 0.005; r= 0.423 p= 0.001; r= 0.256 p= 0.005; r=0.108 p=0.005, r= 0.256 p= 0.004, respectively) and a negative correlation with HDL-c (r = −0.299 p = 0.001) were found (Table 2).

Table 2 Correlation of presence of diabetic retinopathy with other parameters
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As a result of the regression analysis, HbA1c and MHR were found to be independent predictors of DR (Table 3).

Table 3 Univariate and multivariate logistic regression analyses to identify possible predictors of diabetic retinopathy
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ROC curve analysis for MHR is shown in Figure 1. Accordingly, the area below the ROC value of MHR to distinguish DR was 0.657, and p = 0.003 significance value. The best cutoff value was 10.27, with a sensitivity of 65.2% and a specificity of 58.3%.

Fig. 1
figure 1

Receiver operating characteristic (ROC) curve analysis for monocyte to HDL-c ratio as a predictor of diabetic retinopathy

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Discussion

In the present study, there was no difference in MHR between T2DM patients without DR and healthy subjects; however, MHR was higher in patients with T2DM with DR compared to healthy control subjects and without retinopathy patients. Also, MHR was determined to be an independent predictor of DR. To the best of our knowledge, this study is the first to show the relationship between DR and MHR.

Many epidemiological studies have reported that DM is associated with chronic inflammation and endothelial dysfunction (ED) [18, 19]. Chronic inflammation and ED are two contributing causes of the development and progression of microangiopathic and macroangiopathic complications seen in DM [6]. DR is the most common microangiopathic complication of DM. Increasing evidence indicates that systemic inflammation plays an essential role in the development and progression of DR in the early and later stages of DR by inducing the formation of new blood vessels and macular edema [20], damaging the glial cross and causing neuronal loss [21]. The relationship between DR and blood inflammatory index has been previously demonstrated [22,23,24]. In addition, studies have also found that many inflammatory cytokines such as tumor necrosis factor-α and vascular endothelial growth factor are increased in the systemic circulation in patients with DR [25].

Monocytes and macrophages are cells that play a primary key role in the synthesis and release of pro-inflammatory and pro-oxidant cytokines [26]. Macrophages that differentiate from activated monocytes that secrete cytokines, growth factors, and interleukins adhere to the outer surface of the retinal capillaries and disrupt the blood-retinal barrier in DR. Therefore, the retinal pigment epithelium acts as a gateway for monocytes that directly or indirectly damage the retina [27, 28]. Despite these known contributions of monocytes in DR development, studies have shown that there is no direct relationship between the monocyte count in the blood and DR [24, 29]. Our study supports this evidence for monocyte count and DR.

HDL-c carries cholesterol from peripheral tissues to the liver, prevents the harmful effects of LDL-c, and reduces LDL oxidation [9]. Besides, HDL-c inhibits monocyte activities, prevents the differentiation of monocytes to macrophages, and limits the inflammatory response. It prevents ED by removing cholesterol from lipid-loaded macrophages in atherosclerotic lesions. Thanks to these properties, HDL-c has antiatherosclerotic, antioxidant, anti-inflammatory, and antithrombotic effects [9, 10]. Studies on whether the HDL-c level is a risk factor for the development of DR in DM patients present conflicting results. Some studies state that there is no association between DR and HDL-c [30, 31], while others argue that high HDL-c level is a risk factor for DR [32]. Lyons et al.’s study with 988 people showed that low HDL-c levels are a risk factor for DR [33]. In our study, HDL-c levels were found to be higher in the control group compared to the other two groups, while there was no significant difference between T2DM with DR and without DR groups. These findings suggest that this apparent complexity for the relationship between DR and HDL-c may be influenced by differences in other risk factors such as hypertension and hyperglycemia.

Considering the pro-inflammatory effects of the monocyte-macrophage system and the anti-inflammatory effects of HDL-c, it makes sense to combine these two parameters as an inflammatory marker in a single index (MHR). Previous studies have shown that MHR is associated with many inflammatory diseases, primarily cardiovascular diseases [12,13,14,15]. MHR has also been studied in DM and its complications, and an association of high MHR with peripheral neuropathy, a common complication, has been proven [14]. Also, Karatas et al. [15] and Onalan’s [34] studies have shown a high MHR level relationship with nephropathy, a microvascular complication in DM, and it has been suggested that MHR may be a biomarker for diabetic nephropathy. This high correlation of MHR with DM complications is due to its being a useful marker of both inflammation and vascular ED. In our study, high MHR rates were observed in T2DM patients with DR, but not in others, suggesting that MHR can be used as a marker of inflammation and ED for the development of DR.

The counts of leukocytes including neutrophils, lymphocytes, monocytes, basophils, and eosinophils are used as classical inflammatory markers, especially in cardiovascular diseases. In recent years, the ratios of these parameters (NLR, MLR, PLR, etc.) have been defined as new inflammatory markers and are frequently studied in inflammation-related diseases [35]. Our study has shown that neutrophil count and NLR value are high in DR, and previous studies on this issue also support our findings [22, 36].

This study has some limitations. First of all, this study had a small sample size. DR grading could not be done because sufficient patients could not be reached. Indicating whether there is a difference between non-proliferative and proliferative DR would have made this study more valuable. Secondly, creatinine levels in the DR group were statistically higher, although within clinically normal limits. While we are not sure whether this affected the results, it might have been better for the health of the study to have creatinine levels similar to other groups.

Conclusıon

MHR was not higher in T2DM patients without DR than that in the control group. However, T2DM patients with DR had higher MHR values than the other two groups. There is a positive correlation between MHR and DR. Based on these results, MHR could be a biomarker for DR.