Evaluating Kidney Function in Elderly Population: A Cross-Sectional Study in Primary Health Care

Background: Chronic kidney disease (CKD) is an important condition, with implications in morbidity and mortality that primary care physicians should early detect. The aim of this study was to identify the prevalence of CKD among the elderly population in primary care settings and to investigate the extent to which serum creatinine reflects kidney function. Methods: All the patients aged 65 years old and over who came for laboratory tests in an urban primary health center for six months were invited to participated. Additional data on their age, gender, body weight (kg) and their medical history on chronic diseases were collected. The Cockcroft-Gault equation was used for the estimation of the participants' kidney function. Results: 325 patients participated in the study with mean age 73.9 years old (±6.0, min 65-max 91), out of which 146 male (44.9%). According to the estimated creatinine clearance, which in turn estimates glomerular filtration rate (GFR) in ml/min, eGFR: 60-89 was found in 50.5% (164), eGFR: 30-59 in 26.2% (85) and eGFR: 15-29 in 2.2% (7). Spearman's correlation coefficient (rho0 between age and eGFR was -0.52, p<0.001. Serum creatinine (mg/dl) was ranging from 0.7 to 1.7 (mean 1.1) in patients with eGFR: 30 ml/min to 59 ml/min and 1.6 to 3.2 (mean 2.1) in the group with eGFR: 15 ml/min to 29 ml/min. Conclusion: General practitioners should routinely estimate kidney function in their in the older patients. Since the normal serum creatinine reference interval does not necessarily reflect a normal kidney function, GFR equations may facilitate its estimation in every day practice. Early identification of CKD, dosage reduction of renally excreted drugs and timely referral to a nephrologist is feasible and should be pursued within primary care.


Introduction
Chronic kidney disease (CKD) is an important condition, with implications in morbidity and mortality that every primary care physician should early detect. Ageing and a number of common chronic diseases like hypertension, diabetes mellitus, cardiovascular disease and their treatments are associated with significant modifications of kidney function [1][2][3]. Especially, ageing is frequently associated with loss of muscle mass, but also determines changes in kidney structure, since it is associated with a loss of renal mass by about one fourth from 30 to 80 years of age [4], and a decrease in kidneys' length by 15% from 17 to 85 years of age [5]. In the Framingham Offspring Study, it was shown that for an increase in age of ten years, the odds ratio for developing CKD was 2.56 [6]. Early identification of CKD, appropriate treatment of risk factors, alteration of medication that could be harmful and timely referral to a nephrologist may delay the progression of renal disease, decrease morbidity and mortality and lower costs [7,8].
The measurement of serum creatinine alone can be misleading in the detection of CKD, especially in elderly populations [9,10]. The estimation of creatinine clearance is essential, however a 24-hour urine collection for creatinine clearance is often biased in older patients due to inaccurate or incomplete urine collection [10]. For these reasons, some equations based on serum creatinine, age and sex are used to obtain an estimation of glomerular filtration rate (eGFR) [11,12]. Estimation of GFR is a very practical way for detecting and staging CKD, determining drug dosages, and stratifying risk.
Although estimation of GFR is a recognized necessity in every day practice, a UK study reported that 86% of general practitioners took only serum creatinine concentration into account when prescribing drugs for elderly patients [13]. There are a few published studies in primary care settings reporting on the identification of chronic kidney disease [1,[13][14][15][16][17][18], while no study has been previously published on the prevalence of CKD in Greek primary care patients. The aim of this study was to identify the prevalence of CKD among the elderly population in primary care settings and the extent to which serum creatinine reflects kidney function.
invited to participated. The study was completed in six months, from May until October of 2015. The patients were informed by the general practitioner about the purpose of this study. Inclusion criteria were age (65 years old and over) and willingness and competence to provide their informed consent. Patients who declined to perform their blood tests in the health center's laboratory were excluded from the study, in order to ensure technical homogeneity and comparability of results.

Study variables
All patients underwent serum measurements of urea, creatinine, and uric acid (mg/dl). Moreover, they provided data on their age, gender, body weight (kg) and their medical history on chronic diseases.

Statistical Analysis
Data were entered independently by two researchers and were cross-checked by a third to avoid data entry errors. Continuous variables were summarized with means and standard deviations. Categorical variables were presented with frequencies and the corresponding percentages. For the evaluation of the correlation between age and eGFR, the Spearman correlation coefficient (rho) was used, since both variables were not normally distributed. Data were analyzed using the IBM/SPSS (version 21.0). All p values were twotailed.

Results
Population of the study 325 patients participated in the study with mean age 73.9 years old (±6.0, min 65-max 91), out of which 146 male (44.9%). The description of population of the study in terms of their laboratory results and their medical history is described in Table 1.
The distribution of the study population in the different groups according to their estimated GFR and their medical history (no

Discussion
This cross-sectional study in elderly Greek primary care patients provided further evidence on the necessity of the estimation of GFR for the evaluation of renal function in every day practice. Half of the patients visiting primary care settings were found to have impaired renal function (eGFR between 60 ml/min and 89 ml/min, while one out of three patients was found with eGFR below 60 ml/min. Age was negatively associated with the value of e-GFR, underlining the necessity to routinely estimate GFR in older patients, since kidney function estimate based on simple determination of serum creatinine level is hardly reliable.
Ageing was found significantly associated with CKD, being in line with previous findings [1,16] Since mild and moderate kidney disease is poorly described from serum creatinine alone, clinical laboratories could routinely estimate GFR and report the value when serum creatinine is measured. Such an initiative could significantly help health care providers to early detect CKD among patients with risk factors, but also monitor patients already diagnosed with CKD. In an effort to further facilitate primary care physicians, an interesting multifaceted intervention including the use of a software programme for the estimation of creatinine clearance and recommendation of individual dosage requirements has been used successfully in the setting of small primary care practices in Germany [20]. It was found to be effective in reducing inappropriately high doses of renally excreted medications in patients with CKD [20].
Some limitations of our work should be discussed. First of all, kidney function was estimated by an equation and not as a result of a 24-hour urine collection. However, equations have been proposed in order to provide the physician an easy way of calculation and an accurate estimation of kidney function [12]. However, all of them have their own limitations, and no equation proved to be better than the other ones [12]. Although the Cockcroft-Gault equation [11] may not take into account the variability of creatinine production, and may systematically overestimates the GFR in obese or edematous patients [21], it was the first published and still widely used in every day practice. Another limitation of the study could be that the population of the study was not a random sample from the community, but all the patients who were send for laboratory tests for six months. Since all the patients of the involved primary care setting are asked to undergo laboratory tests once a year, the population of the study can be considered representative of the people who use primary care services, providing useful conclusions for this population.
In conclusion, general practitioners should routinely estimate kidney function in their patients, especially in the older patients, since aging and their co-morbidities increase the possibilities of development of CKD. Since the normal serum creatinine reference interval does not necessarily reflect a normal kidney function, equations may facilitate its estimation in every day practice. Early identification of CKD, dosage reduction of renally excreted drugs and timely referral to a nephrologist is feasible and should be pursued within primary care.