CKD is silent and asymptomatic at earlier stages and quite often undiagnosed, but can be detected by estimated Glomerular Filtration rate (eGFR) or/and albumin-to-creatinine ratio, as a marker of kidney damage.
An important question that must be addressed is whether all elderly patients classified as having CKD based on a single reduced eGFR value and without other evidence of kidney damage, should be considered as having a disease? [
13]. It is important to avoid false-positive diagnoses of CKD in elderly, because this may lead to financial, social and psychological consequences [
14]. Early recognition and identification of elderly patients with CKD may prevent this growing social-economic problem, as early referral to nephrologists has been associated with arrest or reversal of CKD and decreased mortality [
15–
17].
The Kidney Disease Outcomes Quality Initiative (KDOQI) has described a definition and a staging system of CKD relied on eGFR. At present, eGFR is considered the best indicator of kidney function and CKD is defined by a reduction of GFR < 60 mL//min and/or evidence of kidney damage, if there is proteinuria (albuminuria > 30 mg/g of creatinine), renal hematuria, or abnormal renal imaging and renal pathology for 3 months or longer [
18].
Thus, CKD has been classified in 5 stages, and the same criteria are used for the diagnosis of CKD in older and in younger patients. This classification uses an estimated value of GFR rather than the measured (mGFR). mGFR can be assessed by measuring the urinary clearance of inulin,125 I-iotholamate, iohexol, or other exogenous filtration markers. However, all these techniques are expensive, complicated and difficult to be performed, and their use is confined to the research setting. Thus, for all of these reasons formulas based on serum creatinine have been used for the calculation of eGFR in clinical practice ().
| Table 1.Equations for eGFR calculation, based on serum creatinine (Scr) |
Creatinine Clearance (Ccr), which is often used in clinical practice, overestimates GFR due to the secretion of creatinine by the renal tubules. Moreover, Ccr is susceptible to urine collection errors especially in elderly patients [
19] and is a poor screening test for CKD as it underestimates renal failure in this subpopulation [
20]. Swedko et al [
20] reported that a serum creatinine level greater than 1.7 mg/dL (>150 μmol/L) had an overall sensitivity of only12.6% for the detection of CKD (GFR ≤ 50 mL/min). Physicians using only serum creatinine, fail to diagnose CKD in older patients [
20,
21]. Branten et al have also reported that hypoalbuminemia influences the tubular secretion of creatinine leading to errors in estimation of GFR. Thus, serum creatinine is a poor marker of GFR in disease states with heavy proteinuria as in nephrotic syndrome [
22].
Cockcroft-Gault formula is an equation used to estimate the endogenous creatinine clearance as follows:
Ccr is expressed in milliliters per minute, age in years, body weight in kilograms and serum creatinine (Scr) in milligrams per deciliter. This equation provides an estimate of Ccr but it is not equivalent to GFR due to the effect of tubular secretion of creatinine [
24]. Moreover, in the Cockcroft-Gault equation body weight is considered as a surrogate for muscle mass, so it overestimates Ccr in edematous states and in obese patients [
25]. Verhave et al [
26] have reported that the Cockcroft-Gault equation underestimates GFR in patients over 65 years old. In addition, Cirrilo et al [
27] have found that the Cockcroft-Gault equation systematically under-estimated GFR in the elderly.
The Modification of Diet in Renal Disease (MDRD) study equation was developed using data from 1628 patients with a GFR below 60 ml/min, for the estimation of GFR adjusted for 1.73m
2 [
28]. The MDRD equation was re-expressed with a standardized serum creatinine assay [
29] as follows:
The MDRD equation does not require a body weight variable and it has been recommended by the KDOQI study group for the diagnosis and classification of CKD [
18]. Nevertheless, it is important to note that the use of MDRD equation leads to errors in the classification of CKD due to variable degrees of bias, imprecision and inaccuracy [
24]. Therefore it is important to investigate how gold the used gold standards actually are [
30]. It is worth noting the existence of differences between various laboratories regarding the calibration of the creatinine assay that leads to differences in GFR estimation [
31]. Lamb et al [
32] reported a similar effect of the creatinine assay calibration on eGFR estimates in older patients. They found that the effect of the calibration of creatinine assay led the Cockroft-Gault formula to underestimate the eGFR, whereas the MDRD Study equation overestimated it [
32].
However, the MDRD equation has been considered as more accurate for the elderly in comparison with the Cockroft-Gault formula [
33] and for this reason it has been recommended by National Kidney Foundation [
18]. The MDRD equation was evolved in 1999 and was recommended by NKF in 2002, for the diagnosis and the classification of CKD but has received a lot of criticism recently. In addition, both equations overestimate mGFR in pathological states, as in nephrotic syndrome, hypoalbuminemia and in CKD at stage 5 [
34].
Recently, Levey et al [
35] using data from 16 studies developed a new equation to estimate GFR, the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equation. The CKD-EPI equation was developed in an attempt to create a more accurate equation than this proposed by the MDRD Study. The equation was expressed as follow:
Scr is serum creatinine expressed as mg/dL, k is 0.7 for females and 0.9 for males, α is −0.329 for females and – 0.411 for males, min indicates the minimum of Scr/k or 1, and max indicates the maximum of Scr/k or 1.
The prevalence estimate of CKD in US by using the CKD-EPI equation was 1.6% lower than that obtained by the MDRD equation (11.5% compared to 13.1%) [
36]. Moreover, the CKD-EPI equation had lower bias, especially at eGFR ≥ 60 ml/ min/ 1.73m
2 [
35]. Juutilainen et al [
37] from Finland have also reported that the MDRD equation augmented the trend of increasing the prevalence of CKD in the general population compared with the CKD-EPI formula. Murata et al [
38] have shown that the CKD-EPI equation improves performance in the healthier populations at the expense of slightly reduced performance in more diseased populations, whereas the CKD-MDRD formula provides more reliable results regarding renal function. Earley et al [
39] have recently reviewed the performance of the CKD-EPI and MDRD equations and reported that both were suboptimal for all populations and GFR ranges. The MDRD Study equation was developed in a study population with CKD and a mean GFR of 40 mL/min per 1.73 m2, whereas the CKD-EPI equation was developed in a more diverse study population, including participants with and without CKD, with a mean GFR of 68 mL/min per 1.73 m
2.
It is important to note that even by the CKD-EPI formula, the prevalence of CKD in elderly remained high. The authors reported a limited number of participants older than 70 years and they also reported incomplete data on measures of muscle mass and other conditions or medications that may influence the serum creatinine [
35]. Matsushita et al [
40] in a meta-analysis of data from 1.1 million adults have reported that the CKD-EPI equation classified fewer individuals as having CKD and more accurately categorized the risk for mortality and ESRD than did the MDRD equation across a broad range of populations, lowering the prevalence of CKD in all cohorts except for the elderly. Stengel et al [
41] in a prospective population-based cohort study from France reported that the CKD-EPI and the MDRD equations provided very similar prevalence and long-term risk estimates in the elderly (>65 years) and an eGFR<45 ml/min/1.73m
2 was associated with worse outcomes.
Rule et al [
42] developed the Mayo Clinic Quadratic (MCQ) equation in an effort to create a better formula for estimating GFR especially in patients with preserved kidney function. The equation was expressed as follow:
In a recent study, Carnevale et al [
43] suggested that it is possible that the MCQ equation systematically overestimated GFR in patients aged over 85 years, by including age.
Cystatin C
Cystatin C is an alternative agent for estimation of GFR. It is filtered by the glomeruli and is totally reabsorbed and degraded by tubules, so it could be used as a novel agent for GFR estimation [
44]. Cystatin C concentration is less dependent upon muscle mass, weight or status disease and it is dependent on kidney function, age, sex, smoking and inflammation [
45,
46]. New estimating equations based on cystatin C have been studied in patients with CKD, diabetes, anorexia nervosa and cystic fibrosis [
47–
49] (). Moreover, Stevens et al [
50] reported that an equation incorporating both cystatin C and serum creatinine provides a better estimation of GFR ():
| Table 2.Equations for eGFR based on serum cystatin C (CysC) and creatinine (Scr) in various clinical presentations. |
Peralta et al [
51] have also examined the potential benefits of adding information from a cystatin-based measure of estimated GFR and albuminuria to the current standard of CKD-EPI eGFR estimation. This triple-marker approach for predicting all-cause death and kidney failure was found to be superior compared with eGFR alone [
51]. However, this approach has several limitations and can not be adopted so easily due to its additional costs and complexity for the laboratories [
52]. Nevertheless, the idea of cystatin C based equations to estimate GFR seems very promising.
Limitations of the present classification system for the elderly
There are various limitations of the present classification system because it leads to over- and misdiagnosis of CKD especially in elderly. It is well known that a decline in GFR occurs with aging, so in the case of the absence of any evidence of a complication of aging, a decline in eGFR below 60 mL/min/1.73m
2 should not be considered as CKD in the elderly [
53–
55].
Wetzels et al [
56] reported that there is a systematic decline in eGFR and mGFR at a level of 7–10 mL/min/1.73m
2 per decade after the age of 30–40 years. In addition, this age dependent decline in eGFR is not associated with the underlying renal morphology [
57]. However, findings from a longitudinal study showed that the rate of the decline in GFR with ageing was greater in individuals with hypertension than without [
58]. The authors reported a decline in creatinine clearance of 0.75 ml/min/year. However, one-third of subjects enrolled the study had no decline in kidney function and a small number of subjects had even an increase in creatinine clearance [
58].
In addition, a moderate decline in GFR with aging may occur as part of “normal aging” [
56–
58] and for many nephrologists this phenomenon reflects just normal physiology. Although, not all the elderly display a decline in GFR, aged patients with a decline in renal function have probably a genetic predisposition to biological vascular aging and/or increased exposure to cardiovascular risk factors [
59]. On the other hand, Gansevoort and Jong [
30] suggested that “normal physiology” indicates a kind of benefit and the loss of glomeruli among elderly is not beneficial at all. Moreover, these authors have suggested that there is no strong reason to introduce age-specific cut-off values indicating CKD [
30].
Regarding the prevalence of CKD stage 3, around 5–8% of the general population can be defined having CKD stage III (overall higher among elderly), whereas individuals with an eGFR around 45 mL/min/1.73m
2 are at increased risk for all cause mortality in comparison with these with higher eGFR [
60,
61]. Thus, for many nephrologists it should be better to subdivide CKD stage III into two stages: one not necessary pathological being 45–60 mL/min/1.73m
2 and another separate always pathologic stage being 30–45 mL/min/1.73m
2 [
30]. Given all these considerations, various revisions of the current system of defining and classifying CKD have been suggested. Some revisions include lowering the threshold of eGFR below 45 mL/min/1.73m
2 for the definition of stage III [
53], introducing an additional evidence of renal damage in individuals with e GFR ≥ 30 mL/ min/1.73m
2 in order to consider them as having CKD [
54], adding two subcategories to stage III [
62], introducing age- and sex-dependent thresholds after 50 years of age [
60] and setting age- and sex-specific GFR reference values [
60–
64].
Albuminuria and CKD
The current system for the definition and classification of CKD does not include the presence of albuminuria in stages III–V. Reduced eGFR and/or proteinuria (albuminuria > 30 mg/g of creatinine) indicates a higher risk of kidney failure, cardiovascular disease, cognitive impairment and all-cause mortality in the elderly [
65–
69]. Moreover, Hemmelgarn et al [
70] have investigated the importance of macro-abuminuria on the patients prognosis reporting that proteinuria is a worse prognostic factor in comparison with the reduction of eGFR alone and that there is a synergy between them.
According to the NHANES study, the majority (over 90%) of patients with CKD stages I and II, had only microalbuminuria (30–300 mg per day or 17–250 mg albumin/g creatinine, if men, or 25–350 mg albumin/g creatinine, if female) as a diagnostic criterion [
71]. It is well known that microalbuminuria is an independent indicator of cardiovascular disease and morbidity reflecting the systemic endothelial dysfunction [
72]. However, microalbuminuria can also be present, or may be influenced by other conditions not necessary “pathological” such as obesity, fever, exercise, aging and inflammation. Recent studies, have shown that patients with microalbuminuria, have an increased risk for ESRD independent of eGFR levels [
73,
74].
The risk of an elderly person with CKD stage 3 to develop ESRD is around 0.2–0.4% per year and the presence of concomitant proteinuria increases this risk [
74]. However, patients with CKD stage 3 without micro- or macro-albuminuria have no increased risk for cardiovascular or renal events [
75]. It is important to note that this group of patients constitutes 4–6% of the general population and most of them are elderly. Normoalbuminuric patients with CKD stage 3 have a better prognosis than patients with CKD stage 1 with microalbuminuria [
75]. In addition, Tonelli et al [
76] in a large cohort study with > 900.000 participants in Canada has reported that heavier levels of proteinuria, regardless of baseline eGFR (low, intermediate, or high), were strongly and independently associated with worse clinical outcomes.
In conclusion, microalbuminuria should be considered as a risk factor for a systemic vascular disease including kidney disease rather than a sign of CKD [
76,
77]. Thus, it should be re-evaluated whether microalbuminuria alone without other evidence of kidney damage is a diagnostic criterion for CKD, because its presence leads to an overestimation of CKD especially in elderly. Additional information regarding the presence of albuminuria should be incorporated in all stages of the current classification because of its prognostic value [
76–
78]
Our opinion is that since a new more accurate and precise equation has not been yet evolved, elderly patients with a decreased e GFR (≤ 45ml/min) and concomitant proteinuria should be labeled as having definitely CKD.