Assembly of the cohort
A longitudinal cohort study involved adults ≥ 65 years (N = 2,057) who were retrospectively identified from the case management care system of the hospital. Diagnosis of diabetes mellitus (DM) was determined by the International Classification of Disease 9th version, Clinical Modification (ICD-9-CM) code of 250.x during out-patient visits, or at least once during in-patient care between February 2, 2010 and November 26, 2015. After applying the following exclusion criteria: age < 65 years, severe neurologic disorders, < 30 days of CKD diagnosis, death within 30 days or an inadequate follow-up length of < 6 months, 921 patients with diabetes mellitus (DM) were enrolled (Fig. 1). The diagnosis of CKD from this database has been validated by the ICD-9-CM code of 585.x and the following criteria: an estimated glomerular filtration rate (eGFR) < 60 ml/1.73m2/min, urine albumin/creatinine ratio (ACR) > 30 mg/g [14] and a urine protein/creatinine (PC) ratio > 0.2 mg/g [15]. Afterwards, the 560 subjects with CKD and 361 subjects without CKD were classified, with index dates defined as the day of receiving comprehensive geriatric assessment (CGA) for all participants. Subjects were prospectively followed up until June 19, 2018. Subsequently, a comparative study regarding DM in non-CKD and DKD patients was investigated. The study was approved by the Institutional Review Board of Taichung Veterans General Hospital (TCVGH, No. CF20293A).
Study Variables
Clinical data, including demography, self-reported comorbidities (dementia, hypertension and hyperlipidemia), chronic obstructive pulmonary disease (COPD, ICD-9-CM codes of 491.X, 492.X, 493.22 and 496) [16], as well as chronic heart failure (CHF, ICD-9-CM codes of 428.0-428.9 and 402.91) were all validated by the ICD-9-CM codes. Additionally, a 2D echocardiogram and N-terminal pro-B-type natriuretic peptide (NT-proBNP) were obtained to diagnose and differentiate HF with preserved and reduced ejection fraction under a standard protocol (American Society of Echocardiography [ASE] or European Association of Cardiovascular Imaging [EACVI] protocol) [17]. In addition, body mass index, Charlson Comorbidity Index and laboratory tests were measured during visits to the in-patient and out-patient departments on the index date. Diabetic severity was measured using both serum glycated hemoglobin and fasting glucose. Diabetes medication, including oral antidiabetic agents (OADs) (α-glucosidase inhibitors, biguanides, meglitinide, thiazolidinedione, sulfonylurea, dipeptidyl peptidase 4 [DPP4] inhibitors and sodium-glucose co-transporter-2 [SGLT-2] inhibitors), insulin and glucagon-like peptide-1 receptor agonists were documented during the study period according to pharmacological and Anatomical Therapeutic Chemical (ATC) classification.
Geriatric Assessment
Cognitive function using the mini-mental state examination (MMSE) was assessed through the Chinese version of the questionnaire. Patient nutritional status was evaluated by the Mini Nutritional Assessment (MNA) [18]. Trunk balance and core activity were measured by the timed up and go (TUG) test using a 46-cm-height armchair and involved regular footwear, any mobility aids, walking a straight line for 3m, turning around, walking back to the chair, and sitting down [17, 19, 20]. Mobility and slowness were determined by the 6-meter walking (6MW) test in which patients were instructed to walk at their self-selected usual pace on a smooth, horizontal walkway [21]. Handgrip strength (HGS) involving the dominant hand was measured and recorded three times, with the maximum value determined by a dynamometer (Smedley’s Dynamometer, TTM, Tokyo, Japan). Rather than using traditional parameters for physical functionality [22, 23], categorized cutoff points were used to define the frailty parameters, including TUG, HGS and the 6-meter walking test (6MW) [17, 19, 24]. TUG values were separated into tertiles (T1, 0 ~ < 14 seconds; T2, ≥ 14 ~ < 21 seconds; T3, ≥ 21 seconds), while the Chi-square test was used to determine the appropriateness of 21 s. The HGS values were divided into fifth (F1, 0 ~ 15.47 kgs; F2, > 15.47 ~ < 20.4 kgs; F3, ≥ 20.4 ~ 22.73 kgs; F4, > 22.73 ~ 26.9; F5 > 26.9 ~ 48.87 kgs in men; F1, 0 ~ < 10.57 kgs; F2, ≥ 10.57 ~ < 12.5 kgs; F3, ≥ 12.5 ~ 14.83 kgs; F4, > 14.83 ~ 17.43 kgs; F5, > 17.43 ~ 24.1 kgs in women), with abnormal HGS being defined as less than the cut-off points of 20.4kg for men and 10.57kg for women.
Abnormal values of 6MW were separately calculated as quartiles (Q1, 0 ~ 8.95 seconds; Q2, > 8.95 ~ 12.7 seconds; Q3, ≥ 12.7 ~ 16.6 seconds; Q4, > 16.6 ~ 52.0 seconds in men; Q1, 0 ~ < 8.0 seconds; Q2, ≥ 8.0 ~ 11.8 seconds; Q3, > 11.8 ~ 17.51 seconds; Q4, > 17.51 ~ 50.0 seconds in women), with cutoff points of > 8.95 seconds for men and > 17.51 seconds for women, due to men and women walking at different speeds because of different leg lengths.
Calculation Of Frailty
Both the Fried phenotypic model [25] and Rockwood frailty index [26] are currently used to define frailty according to the Asia-Pacific clinical practice guidelines [22]. Cumulative health deficits have examined the association between frailties, as defined by the Rockwood frailty index [26]. A modified Rockwood frailty index (RFI) defining cross-cutting risk factors was used to measure frailty by utilizing cumulative multi-dimensional health deficits collected in health assessments, including four items of CGA (MNA-SF, TUG, HGS, and 6MW), 20 chronic diseases except for DM and CKD, and 19 abnormal laboratory data. Categorization of the modified RFI was determined according to established cutoffs in community-dwelling cohorts to match the Fried physical phenotype: non-frail (0–0.1), pre-frail (> 0.1–0.21), and frail (> 0.21) [27]; however, these categories were not good enough to predict the outcome. Therefore, a Rockwood frailty index ≥ 0.313 for outcome prediction was assessed by Area under the Receiver Operating Characteristic (ROC) curve (AUC) under the nonparametric assumption with 68.8% accuracy, 25.4% positive predictive values and 95.0% negative predictive values.
Study Outcome, Ascertainment Of Oad Use, And Follow-up
The primary outcome of this study was all-cause mortality. All-cause death was determined based on the Clinical Information Research and Development Center, TCVGH, with the accuracy of death being validated by Taiwan’s National Death Registry, according to either ICD-9 (ICD9 001.x-999.x) or ICD10 (A00.x-Z99.x). The index date was the date of DM and CKD diagnosis. CGA was completed around the time of DM diagnosis. Ascertainment of OAD exposure was defined as the cumulative use of at least one type of medicine for ≥ 90 days before and after the index date; a strategy utilized by other pharmacoepidemiology studies [28]. All participants were followed up until either death or June 19, 2018 to prevent lead-time bias.
Statistical Analyses
Statistical analyses were performed with SPSS for Windows version 22.0 (SPSS Institute Inc., Chicago, USA). For continuous variables in the baseline characteristics, we used the Kolmogorov-Smirnov test to determine the normality of sample distributions. Continuous variables were analyzed by Mann-Whitney U tests, generating the median and interquartile range (IQR). Categorical variables, presented as number and percentage, were tested by Chi-square or Fisher’s exact tests, followed by Bonferroni post hoc analysis for multiple testing. During the follow-up period, we used Kaplan-Meier analyses to examine cumulative survival, with a comparison made between with and without CKD groups using the log-rank test. Subgroup analyses in the Kaplan-Meier (KM) plots were generated to compare various cumulative survival rates in different subgroups by the log-rank (Mantel-Cox), as well as pairwise comparisons to evaluate the effect of CKD, frailty and different physical function on long-term mortality. The combined assessment of a previously defined high RFI, CKD or not, and high or low functioning status was delineated between KM analyses and Cox proportional hazard models for predicting clinical outcomes in older adults with diabetes. In addition, Cox proportional hazard models were used to evaluate the effects of CKD, frailty, nutrition, TUG, 6MW and HGS on long-term mortality, independent of the roles exerted by age, gender and the Charlson comorbidity index. P values for nonlinearity were calculated using the null hypothesis test. Statistical significance was set at P < 0.05.