Following approval from the Fundación Universitaria de Ciencias de la Salud Medical School, the San Jose Hospital Ethics in Research Committee, and the RTS Ethics Committee, a descriptive, cross-sectional study was undertaken. Participants were patients over 54 years of age according with previous studies (6); attending a CKD secondary prevention program in Bogotá, Colombia, during 2012–2013; diagnosed with CKD in stages 3 to 4 according to the K/DOQI classification  (GFR 30–59.9 and 15–29.9 ml/min, respectively); with at least two nephrologist visits in the program; and with at least one estimation of the GFR. Patients who provided informed consent where included in the study. Patients with signs or symptoms of uremia and those with visual impairment impeding the application of the neuropsychological tests were excluded.
By interview, information regarding age, occupation, civil status, and educational level was collected. Clinical information concerning previous personal history (stroke, hypothyroidism, smoking, Diabetes Mellitus type 2 (DM2), anemia) in addition to data such as body mass index (BMI) using Quetelet’s formula (i.e., weigh/height2), cause of renal disease (DM2, HBP, glomerulopathy, obstructive nephropathy), alcohol consumption (over five drinks per week), and the last estimation of glomerular filtration rate (eGFR) were obtained from the clinical record.
Clinical assessment with specific instruments was performed by a multidisciplinary team composed of nephrologists, a psychiatrist, a neurologist, and a neuropsychologist.
Instruments chosen to be applied in the present study were selected due to simple and rapid application as well as appropriate sensitivity and specificity.
Short Neuropsychological Instrument in Spanish (NEUROPSI)
NEUROPSI is a short neuropsychological assessment specifically developed for Spanish-speaking population, which has been validated in Mexico and may be applied to people with a low educational level or illiterate subjects. The overall repeated test reliability is 0.87, and its sensitivity is 91.5 % . NEUROPSI assesses specific cognitive domains including orientation, attention, concentration, language, memory, visual motor skills, executive functions, reading, writing, and computing. The maximal score is 130. Result interpretation is adjusted for the age and educational level of the subject and is reported as follows: normal, mild disturbance, moderate disturbance, and severe disturbance.
The short version (15 questions) of the Geriatric Depression Scale by Yesavage was applied. The maximal score is 15, and the result may be reported as normal (0–5), moderate depression (6–9), or severe depression (10–15). Sensitivity and specificity of the Spanish validated version are 81.1 and 76.7 %, respectively, for a cutoff point ≥ 5 [23, 24].
International Neuropsychiatric Interview (MINI 5.0.0) by Sheehan and Lecrubier
MINI is a short structured diagnostic interview exploring the main psychiatric disturbances from axis I from the DSM-IV and the CIE-10 [25, 26]. Results from several studies have shown that MINI has high validity and reliability scores and it may be applied in a much shorter time (mean 18.7 ± 11.6 min, median 15 min) compared to other instruments (e.g., the SCDI-P mentioned in the DSM-III-R and the CIDI, a structured interview aimed at non-clinical interviewers, developed by the World Health Organization for the CIE-10) [26, 27]. In this study, MINI was applied by an experienced psychiatrist; as it is known, this improves the operating characteristics of the test (sensitivity 96 %, specificity 88 %, positive predictive value 87 %, negative predictive value 97 %) [25–27].
Assessment of activities of daily living (ADL)
Modified Lawton score, validated in Spanish by Vergara et al. for being applied to older people
The Lawton ADL Scale is a test used to assess functional and physical impairment in patients with dementia and specifically evaluates factors from the onset of disease. The test was applied to patients whose cognitive condition allowed so; otherwise, it was responded with the help of the caregiver. Fourteen instrumental activities of daily living (IADL) in total were evaluated [28, 29]. A score > 7 at the time of the interview was considered as a significant functional impairment or a dementia syndrome. Other disease conditions may alter or reduce the specificity of the test [28, 29].
Operative variables definitions
As per Petersen’s criteria , CI was defined as follows:
A. Normal (N): normal result in NEUROPSI and no significant impairment in Lawton’s test (≤ 7).
B. Mild cognitive impairment (MCI): mild impairment as per NEUROPSI result and no significant impairment in Lawton’s test (> 7).
C. Moderate cognitive impairment: mild or moderate impairment as per NEUROPSI result and significant impairment in Lawton’s test (> 7).
D. Severe cognitive impairment: severe impairment as per NEUROPSI result and significant impairment in Lawton’s test (> 7).
Major depression: Patients with a score > 9 in the Yesavage test and a depression diagnosis as per MINI or patients with a score > 5 and a severe depression diagnosis as per MINI were defined as having major depression. Dysthymia was excluded.
The sample size was calculated using the TAMAMU program®  Simple random sampling from the registry of patients with CKD stages 3–4 was performed. For estimating the prevalence of CI, an expected proportion of 30 % , a 5 % maximal difference, and a 5 % type I error were used. For estimating the prevalence of depression, the expected proportion was 23 % , type I error was 5 %, and maximal difference was 5 %. For estimating the prevalence of CI and depression, 257 and 224 patients, respectively, were required. On the largest sample size, a 20 % adjustment for non-response rate was performed. Consequently, a final sample size of 308 patients was obtained.
Qualitative variables were summarized by descriptive statistics with absolute and relative frequencies; quantitative variables were reported using central tendency and dispersion measures. The prevalence of CI and depression is reported as a proportion and 95 % confidence intervals (95 % CI).
Two logistic regression models were undertaken on an exploratory basis; the first one for assessing factors associated with CI and the second for assessing factors associated with depression. All the demographic and clinical variables were examined using backward stepwise logistic regression analysis, and values of p < 0.05 were considered to be statistically significant. All statistical tests of hypothesis are two-sided. Statistical analysis was made with Stata 13.0 ®.