Avaliação da função renal em idosos : um estudo de base populacional Assessment of kidney function in the elderly : a population-based study

Correspondência para: Daisson José Trevisol. Universidade do Sul de Santa Catarina e Centro de Pesquisas Clínicas do Hospital Nossa Senhora da Conceição. Av. José Acácio Moreira, no 787. Tubarão, SC, Brasil. CEP: 88704-900. E-mail: daissont@uol.com.br Tel: (48) 3631-7239. Secretaria Municipal de Saúde do Município de Tubarão e Centro de Pesquisas Clínicas do Hospital Nossa Senhora da Conceição. Avaliação da função renal em idosos: um estudo de base populacional


IntroductIon
Chronic kidney disease (CKD) affects all age groups and its prevalence has been increasing in recent years and it is, therefore, considered a public health problem.It should be noted that CKD is silent and in most cases the diagnosis is not made in the early stages of the disease. 1 CKD is progressive and can lead to kidney failure; however, disease progression can be prevented or delayed.For this reason, it is necessary to identify patients with impaired kidney function in its early stage, particularly those at higher risk of progression to CKD, 2 and start treatment to avoid the most common complications of the disease and prevent premature progression to death. 3n Tubarão, SC, there are 97,235 inhabitants, according to estimates by the Brazilian Institute of Geography and Statistics; 4 in the locality, there are no population studies evaluating renal function in the elderly.
The initial assessment of kidney function can be made by measuring plasma creatinine.Creatinine biological variation is very small, approximately 4%; 5 and the analytical variation is below 2%.This diagnostic method is quite simple; however, when within reference values it does not state normal kidney function, because it is a DOI: 10.5935/0101-2800.20140043late parameter of kidney disease. 6There are formulas developed to estimate glomerular filtration 7 and disease staging 8 based on serum creatinine.The equation used was CKD-EPI (Chronic Kidney Disease Epidemiology Collaboration), pointed out by a recent systematic review 9 as one of the most used in medical practice to calculate kidney function.
The aim of the study was to evaluate the kidney function of elderly patients using the CDK-EPI formula and the presence of factors associated with these changes.For sample make up purposes, we used the elderly enrolled by community healthcare agents acting in different areas of the Family Healthcare Strategy program, which covers over 90%, totaling 9,009 elderly residents in the city of Tubarão.Sample size was calculated considering the prevalence of impaired kidney function by glomerular filtration rate at 25.2%, 10 error margin of 5%, resulting in a minimum sample of 281 individuals for a 95% confidence level, and 474 for a 99% confidence level.Participants were selected by simple random sampling.

Methods
Our study included people aged 60 years (completed in 2010) or more, living in the city of Tubarão (SC), who agreed to participate in the study and provide us with blood samples.
Individuals with cognitive problems (mental or degenerative disease), unable to respond to inquiries or without autonomy to decide on their participation in the study were taken off.

interview
The randomly selected seniors were invited to participate in the study by the community healthcare workers, with their agreement they were submitted to an interview and to answer a questionnaire.The instrument contained sociodemographic and socioeconomic data (age, gender, race, marital status, work and education status) and behavioral data (physical activity, alcohol and cigarettes).At the end, the person was scheduled to attend the Basic Healthcare Unit of their district of residence for blood collection, blood pressure and anthropometric measures.
Those individuals who had consumed 100 or more cigarettes during their entire lives were considered smokers. 11Among seniors who reported using or having used alcohol, the CAGE questionnaire was applied. 12,13This instrument consists of four questions.Two or more affirmative answers is indicative of alcoholism.
Physically active were those individuals who performed at least 30 minutes of physical activity five days a week or more; and a cutoff point was 150 or more minutes of physical activity per week, according to the European recommendation. 14

BlOOd pressure measurement
Blood pressure was measured using the OMRON Model HEM -742INT digital sphygmomanometer with the patient sitting.We performed two measurements with a 15 minute interval between them, and we used the arithmetic mean value of two BP measurements to determine the patients BP levels. 15he cutoff line that defined systemic hypertension (SH) considered systolic BP greater than or equal to 140 mmHg and/or diastolic BP greater than or equal to 90 mmHg or the use of anti-hypertensive agents. 16OOd sampling and anthrOpOmetric values 10 ml of peripheral venous blood was collected by technically skilled professionals for serum creatinine and fasting glucose measurements.The patients were instructed to fast for 12 hours and avoid alcohol consumption within 72 hours prior to blood collection.The tests were processed by the Clinical Laboratory of the University of Southern Santa Catarina.
Kidney function was evaluated by serum creatinine, with a glomerular filtration rate (eGFR) estimated by the CKD-EPI 17 formula and then, the kidney function was scored as follows: 18 Normal Kidney Function: > 90 ml/min/1.73m 2 ; Light or functional kidney impairment: 60-89 ml/min/1.73m 2 ; Moderate laboratorial or kidney failure: 30-59 ml/min/1.73m 2 ; Severe or clinical kidney impairment: 15-29 ml/min/1.73m 2 ; End stage kidney failure or dialysis: < 15 ml/ min/1.73m 2 .For analysis purposes, the estimate kidney glomerular filtration rate determined by the CKD-EPI formula was used dichotomously with a cutoff at < 60 ml/min/1.73m 2 for kidney damage and above this value for normal kidney function.
Diabetes mellitus diagnosis was based on the use of oral hypoglycemic agents or insulin, as well as having serum fasting glucose ≥ 126 mg/ dl of blood. 19e measured body weight in kilograms, with the individuals wearing light clothing and without shoes.Height measurements were reported in centimeters with patients standing on the scales, erect and motionless in the center, staring at a fixed point on the horizon, checked by the Wiso W71 machine.Body mass index (BMI) was calculated as the ratio of weight (kg)/height 2 (cm) and classified according to the values set by the World Health Organization, 20 considering as obese those individuals with BMI greater than or equal to 30 kg/m 2 .

statistical analysis
Sample size calculation was performed using the OpenEpi version 2.3.1 (Open Source Epidemiological Statistics for Public Health) software.The collected data was entered into the EpiData software, version 3.1 (Epi Data Association, Odense, Denmark), of public domain; and the statistical analysis was performed using the Statistical Package for Social Sciences (SPSS for Windows v 18, Chicago, IL, USA).The variables are described as measures of central tendency and dispersion in the case of quantitative variables.Qualitative variables were described as absolute numbers and proportions.To assess the association between the variables of interest, we applied the chi-square test for qualitative variables and Student t-test variables when comparing averages.To assess the correlation between the results of the CG formula and age we used the Pearson's correlation test.We also used a modified Poisson regression multivariate analysis as a robust estimator to control for confounding variables.The level of statistical significance was p < 0.05.

results
Figure 1 shows the selection of study participants.
822 elderly with a mean age of 68.6 (SD = 7) years -ranging from 60 to 92 years of age were enrolled.Table 1 presents the sociodemographic characteristics of the participants in the present study.
Of the total, 40.3% of seniors were drinking or had drunk alcohol throughout life.Among those classified as smokers, 22.5% were current smokers and 77.5% were former smokers.Among the seniors studied, 70.4% had a previous medical diagnosis of hypertension.Table 2 describes the presence of comorbidities in this population.
Table 3 presents the classification of chronic kidney disease in the elderly population from the CKD-EPI formula.Table 4 shows crude and adjusted analysis of variables associated with decreased glomerular filtration rate, estimated by the CKD-EPI formula for renal function assessment.
Figure 2 shows the correlation between age and glomerular filtration rate estimated by the CKD-EPI formula.
In comparing mean ages, elderly patients with normal glomerular filtration rate had a mean age

dIscussIon
The present study demonstrated that 13.6% of the sample had GFR less than 60 ml/min/1.73m 2 , which was similar to other studies.Bowling et al. 21held a study among elderly in the United States and revealed a kidney injury prevalence in 39% of the participants.Fu et al. 22 led a study in China with the elderly population and found a prevalence of 43% of kidney dysfunction, also using the CDK-EPI formula.In Brazil, there are few studies estimating kidney function impairment in the general population, and they differ in the criteria used for CKD identification.However, it is known that the number of patients who undergo dialysis grows annually in Brazil. 23he present study showed that more than half of the participants had some kidney damage  even if classified as mild.The reason for this high prevalence may be attributed to the presence of comorbidities arising from the aging process 24 which impair kidney function. 25Kidney function tends to decline with advancing age, as presented, being an independent factor for decreased glomerular filtration rate -corroborating data in the medical literature. 26,27This decline in renal function with aging can be explained by a physiological process of aging accompanied by structural changes to the renal system. 25he statistical analysis of this study showed a significant association between hypertension and decreased glomerular filtration rate, agreeing with Zhang et al., 28 who demonstrated that the factors associated with kidney damage are hypertension, diabetes and cardiovascular diseases.
The prevalence of hypertension in the study population was quite high, and the limitations of the present study included the fact that blood pressure measures were performed only once, not taking into account the diagnostic criteria suggested by the Brazilian Guidelines on hypertension, 29 suggesting two measurements at different times.One study 30 held in the same city with a younger population found a prevalence of 40.5% of hypertension in the participants.However, it is noteworthy that most elderly people with hypertension had their medical diagnosis prior to this study, and as age increases, there is an increase in hypertension among this population.
Although the association between diabetes mellitus and renal damage is commonly found in the literature, this studied found no statistical significance in these regards.Studies performed with only diabetic patients demonstrated high prevalence of glomerular filtration rates below 60 ml/min/1.73m 2 and correlation between diabetes and kidney damage. 31,32n the present study, there was no difference in kidney function between the genders, which agrees with data from other autores. 27,33,34n this study there was no statistically significant difference between kidney function and ethnicity.However, in a study involving 5,179 patients, Peralta et al. 35 found that blacks had higher rates of kidney function decline, regardless of sociodemographic characteristics or other traditional risk factors.It is likely that this lack of statistical association between ethnicity and kidney function is due to the fact that there was a predominance of whites in the sample, due to the European colonization in Santa Catarina. 4egarding habits adopted by the elderly, this studied showed no positive association between alcohol consumption and decreased glomerular filtration rate; but smoking was an independent factor for glomerular filtration rate < 60 ml/ min/1.73m 2 , as already documented. 36egarding anthropometric indicators, obese patients had reduced renal function compared to their eutrophic counterparts, agreeing with the literature that obesity is a major risk factor for glomerular hyperfiltration and progression to chronic kidney disease. 37t is worth mentioning some limitations of this study, such as the cross-sectional design that portrayed the patients at a single time, not following CKD guidelines, 8 that define chronic kidney disease as two measures of glomerular filtration rate below 60 ml/min/1,73 m 2 in two scans at least 3 months apart.However, as an outpatient population sample screening, this study contributes to portray the local reality, and can reveal that the elderly actually have declining kidney function; and attention and monitoring the health of this population is needed so that preventive and therapeutic measures can be initiated early.The literature remains scarce regarding the degree of kidney function, especially in the elderly.

conclusIons
This study concluded that the vast majority of the elderly had a change in kidney filtration rate, and 13.6% had moderate or greater degree of dysfunction.Older age, smoking, hypertension and obesity were positively and independently associated with decreased glomerular filtration rate.

Figure 1 .
Figure 1.Selection of the patients participating in the study.
, and those with glomerular filtration rate < 60 ml/min/1.73m 2 had a mean age of 73 years (p < 0.001).

Figure 2 .
Figure 2. Scatter chart showing the correlation between age and the glomerular filtration rate calculated by the CKD-EPI formula.Pearson's R = -0.41,p < 0.001.
This study was approved by Unisul's Research Ethics Committee based on Resolution 196 of 1996 by the National Board of Health, under protocol 09.345.4.01.III.This is a cross-sectional and population based epidemiological study.The sample studied came from the ESITU (Health Study of Tubarão Senior citizens) project, carried out between September 2010 and May 2011.ESITU is a study that aimed to assess the health status and quality of life of seniors residents from the city of Tubarão (SC).

tAble 2 prevalence
Of the cOmOrBidities fOund amOng the elderly whO participated in CI: Confidence interval.

tAble 4
grOss and adjusted analysis Of the variaBles assOciated with reductiOn in glOmerular filtratiOn rate PR: Prevalence ratio; CI: Confidence interval; HAS: Arterial hypertension; DM: Diabetes mellitus.* Variables adjusted for age, BMI, smoking, hypertension and diabetes mellitus.