The National Health and Nutrition Examination Survey (NHANES) is executed by the National Center for Health Statistics (NCHS), which contains these US noninstitutionalized civilian participants. All participants underwent comprehensive measurements, such as physical and laboratory examinations and standardized interview questionnaires, including socioeconomic, demographic, and health-related questions.
In our study, we used public data from two NHANES cycles (2005–2006, 2007–2008). More informations about the data are available on the NHANES website (www.cdc.gov/nchs/nhanes/). Between 2005 and 2008, there were 20,497 participants in NHANES. We first excluded 19,199 participants with no diabetes. The subsequent exclusion criteria were as follows: (I) unknow retinopathy grading (n=414); (II) unknown peripheral blood MLR (n=46); (III) no diabetic retinopathy (n=538). After excluding these factors, 300 participants were included in the final study.
According to the Declaration of Helsinki, this design was approved by the institutional review board of the NCHS. Before examinations, all participants completed informed consents.
Study variables and outcome
The MLR is the monocyte count/lymphocyte count. The lymphocyte count and monocyte count can be obtained directly from laboratory data files. The neutrophil count is calculated from the white blood cell count and neutrophil percentage.
DR was defined by the presence of hemorrhages, hard exudates (HE), cotton wool spots (CWS), microaneurysms (MA), venous beading, intraretinal microvascular abnormalities (IRMA), and retinal new vessels based on the severity scale of the Early Treatment for Diabetic Retinopathy Study (ETDRS). Nonmydriatic fundus photography (TRC-NW6S; Topcon, Tokyo, Japan) was applied for measuring the level of retinopathy in the worse eye. The grades were categorized into no DR, non-proliferative DR, and proliferative DR. Detailed information is listed in the Digital Grading Protocol of the NHANES.
Other covariates included sex (male or female), age, race (non-Hispanic white, non-Hispanic Black, Mexican American, other Hispanic and other), marital status (married, unmarried and other), poverty income ratio (<1, ≥1), education level (less than high school, high school or equivalent, college or above and other), BMI (<25.0, 25.0-29.9 and≥30.0 kg/m2), cotinine (<0.015, 0.015-10, and≥10), HbA1C (<7%, ≥7%). Diabetes are defined as self-reported physician-diagnosed diabetes. Duration of diabetes was calculated from the reported age at screening minus the age of the subject when first told he/she had diabetes. Family history of diabetes was determined by the answer to the following question: ‘Including living and deceased, were any of your biological relatives, that is, blood relatives, including grandparents, parents, brothers, and sisters, ever told by a health professional that they had diabetes?’
All the analyses were performed with the statistical software packages R (http://www.R-project.org, The R Foundation) and Free Statistics software versions 1.3. The differences of continuous and categorical variables were investigated using the independent-test and the chi-squared test, respectively. These logistic regression models were used to determine the relationship of MLR with the presence of PDR. Model 1 was unadjusted. Model 2 was adjusted by age, sex, race, PIR. Model 3 was Model 2 + adjusted by BMI, Cotinine, total cholesterol, HGB, Model 4 was Model 3 + adjusted by HbA1C, duration of diabetes. Subgroup analysis examined the relationship between MLR and PDR according to PIR. Test for interaction in the logistic regression model was used to compare odd ratios (ORs) between the analyzed subgroups.