We aimed to develop a new examination tool which can effectively screen and predict DPN. The current study has exploratory nature to aim to discover the accurate score system for DPN including SUDOSCAN, and therefore we cannot calculate accurate sample size. Previous studies including T2DM (n=47) and healthy control (n=16) [14], or including type 1 diabetes mellitus (n=45) and healthy volunteers (n=25) [11] showed that feet ESC yielded the diagnostic accuracy of areas under the receiver operating characteristic curve (AUC) 0.705 and 0.64, respectively. We only enrolled subjects with T2DM and did not include healthy control. In this circumstance, the difference of feet ESC levels between subjects with DPN and without DPN might be less profound than previous studies. In this study, we enrolled 235 subjects with T2DM from a single tertiary hospital and analysed the data for 144 of these patients from 2017 to 2019 in whom we can assess the presence of confirmed DPN according to the definition of Toronto Diabetic Neuropathy criteria [8]. We included subjects aged 20 to 79 years and whose anti-hyperglycaemic medications had not changed in the past 3 months to exclude the possible effect of a change in medication on ESC. We also used the information contained in a comprehensive medical history taking and biochemical study to exclude other cause of neuropathy such as thyroid dysfunction and vitamin B12 deficiency. Other key exclusion criteria were renal dysfunction, estimated glomerular filtration rate (eGFR) of <60 mL/min/1.73 m² to exclude uremic neuropathy, pregnancy, or steroid use. When using SUDOSCAN, we also excluded patients who could not stand by themselves or who had active wounds on their hands or feet. This study was approved by the Institutional Review Board of Seoul National University Bundang Hospital (IRB Number: B-1611-372-305). All participants provided written informed consent before participation.

After an overnight fast, the participants visited the endocrinology outpatient clinic. Body weight and height were measured using electronic scales. Body mass index (BMI) was calculated by dividing body weight (kg) by height squared (m²) and is expressed as kg/m². Smoking history was categorised as never, former, and current. Never smoking was defined as having smoked <100 cigarettes in a lifetime. Alcohol intake was evaluated according to average daily alcohol consumption as follows: high (>30 g/day for men or >20 g/day for women), moderate (<30 g/day for men or <20 g/day for women), and none. Positive physical activity was defined as exercising for >150 min/week. Fasting plasma glucose concentration, glycosylated hemoglobin (HbA1c) level, blood lipid concentrations, and renal function test were measured at the central laboratory.

We diagnosed the confirmed DPN if NCS was abnormal with the presence of a symptom or a sign of neuropathy [8]. The NCS was performed at a Department of Rehabilitation Medicine. A trained examiner performed the tests for median nerve, ulnar nerve, peroneal nerve, sural nerve, and tibial nerve. Abnormalities of peroneal nerve velocity, amplitude, and distal latency, tibial compound muscle action potential, and sural amplitude were marked according to the reference values [20]. Abnormal NCS results were defined as more than one abnormal test in two separate nerves. The final diagnosis of DPN was confirmed by certified physicians who were not involved in this study. The MNSI-Questionnaire (MNSI-Q) was previously translated into Korean and Korean version was evaluated by the Neuropathy Study Group of the Korean Diabetes Association [21], and considered to be abnormal if MNSI-Q was 3 and higher [22]. A trained research nurse performed the MNSI-Physical Examination (MNSI-PE). MNSI-PE was consisted of an inspection of feet (appearance and ulceration), ankle reflexes, and vibration sensation assessed by using a 128 Hz tuning fork. The maximum value was 8 for both feet and, we considered abnormal value if MNSI-PE was >2.0 [22,23]. A 10-g monofilament test (Semmes-Weinstein 5.07 monofilaments) was performed at 10 sites on each foot and abnormal value was adopted from the previous Korean study (fewer than seven) [22]. All participants also underwent the SUDOSCAN test. They were instructed to place their palms and soles on the electrodes, and the ESC was measured automatically within 3 minutes. To reduce observer bias, the research nurse performed tests in the order of questionnaire, physical examination, and SUDOSCAN. The score was lastly calculated after SUDOSCAN.

We classified the participants as having or not having DPN and compared the neuropathy examination results. For the SUDOSCAN data, the appropriate cut-off value for feet ESC was determined first and used to categorise the participants. We created a score for each parameter according to the odds ratio (OR).

The data are presented as mean±standard deviation, median with interquartile range (IQR), or as number and percentage. Clinical and biochemical characteristics were compared between participants with and without DPN. Differences between two groups were identified using Student’s t-test for parametric variables, the Mann-Whitney U test for non-parametric variables, and the chi-square test for categorical variables. Spearman correlation analysis was conducted to assess the relationship between two continuous variables. The AUC was calculated to measure the predictive power. The cut-off values for feet ESC and a scoring model were determined using the maximum sum of sensitivity and specificity using the Youden index [24]. The association between parameters and the presence of DPN was assessed by logistic regression analysis. To test discrimination power, we calculated AUC of our new model in total and in each gender group. We compared AUCs of each model using the method of DeLong et al. [25]. Statistical analysis was conducted using IBM SPSS Statistics for Windows version 22.0 (IBM Co., Armonk, NY, USA) and R version 3.6.1 software (R Foundation for Statistical Computing, Vienna, Austria). Two-sided P values <0.05 were considered to be significant.