Cystatin C as a Marker of Renal Function in Critically Ill Patients at Risk for or with Acute Renal Failure

Patients at Risk for or with Acute Renal Failure

A.A.N.M. Royakkers, M.J. Schultz, and P.E. Spronk

Introduction

Acute renal failure is a common complication of critical illness [1, 2]. Of all intensive care unit (ICU) admissions, 15 – 20 % develop acute renal failure and 4 – 6 % require some form of renal replacement therapy [3]. Causes of acute renal failure include direct renal toxicity due to medication or radiocontrast agents, hypovolemic hypo- tension, and shock. Acute renal failure frequently accompanies sepsis – its incidence varies from 20 % in patients with moderate sepsis to 8 50 % in patients with septic shock [2, 4]. Acute renal failure carries a high mortality rate, in particular in patients with sepsis – in patients with acute renal failure alone mortality is 45 %; in patients with acute renal failure and sepsis, mortality is reported to be as high as 70 % [4]. The most frequently used form of renal replacement therapy is continuous venovenous hemofiltration (CVVH), an expensive and laborious treatment. CVVH, however, permits efficient control of fluid balance and azotemia in ICU patients with acute renal failure [5].

Acute renal failure can be defined as a sudden fall in the glomerular filtration rate (GFR). The actual GFR, though, is difficult to measure in the ICU setting. In addi- tion, GFR may change rapidly in critically ill patients. Presently, detection of acute renal failure is primarily based on an increase in plasma creatinine or urea concen- tration. However, there are limitations to the use of creatinine and urea for estimat- ing GFR. Determination of residual renal function in CVVH-treated patients is fur- ther hampered by the fact that these molecules are cleared by CVVH itself. As an alternative, a proposed classification scheme for acute renal failure including criteria for plasma creatinine and urine output, has been proposed, known as the Risk of renal dysfunction, Injury to the kidney, Failure of kidney function, Loss of kidney function, and End-stage kidney disease (RIFLE) criteria [6].

At present, biomarkers of renal function in critically ill patients other than

plasma creatinine and urea are seldom used. Such biomarkers, however, may be use-

ful for detection of acute renal failure in an early stage, e.g., before evident clinical

signs of kidney injury (the I – and F – criteria in RIFLE) have developed. Secondly,

biomarkers of renal function may be useful to detect recovery of renal function in

patients treated with continuous renal replacement therapy, such as CVVH. Cystatin

C has been in use as an endogenous marker of renal function in patients outside the

ICU for more than 15 years; serum cystatin C concentrations have been found to

relate to early renal impairment and correlate well with (changes in) plasma creati-

nine. In this chapter, the literature on serum cystatin C in critically ill patients is

reviewed and discussed.

Determining Glomerular Filtration Rate

There are several gold standard methods for determining GFR, including inulin clearance and isotope clearance techniques (Table 1). Unfortunately, these tech- niques are expensive and laborious and, therefore, not routinely used in clinical practice. Plasma creatinine and urea concentrations, as well as clearance of creati- nine based on 24-hour urine collections, are frequently used indicators of GFR.

However, plasma creatinine concentrations are affected by muscle mass [7] and diet [8], and vary with age and gender [9]. In addition, as plasma creatinine concentra- tions rise, its tubular secretion increases, leading to overestimation of GFR in patients with moderate to severe decreases in renal function [10]. Similarly, plasma urea concentrations are affected by various disease states, hepatic function, and diet [11]. An additional problem arises in patients with acute renal failure treated with continuous renal replacement therapy, such as CVVH. Indeed, creatinine and urea are removed by the hemofilter. Consequently, plasma creatinine and urea concentra- tions are useless for determining GFR in these patients.

A more accurate and practical indicator, preferably an endogenous marker of GFR is needed. An ideal marker of GFR is solely eliminated from the human body by glomerular filtration. In addition, this marker should be freely filtered in the glo- merulus, and be neither secreted nor reabsorbed by the renal tubules. If an endoge- nous marker, it should be produced by the human body at a constant rate, not being influenced by disease state or medication. Furthermore, such a marker should not be removed by means of hemofiltration. Finally, ideally a biomarker of renal func- tion should rapidly follow changes in GFR.

Table 1. Methods for determining/estimating glomerular filtration rate (GFR)

Methods Advantage(s) (potential) Disadvantage(s) In practice

) inulin clearance

) clearance of isotopes ) precise measurement of GFR

) considered gold standard methods

) laborious and expensive ) (almost) never used in daily practice ) Cockgroft-Gault

) MDRD (modification of diet in renal disease) – equation

) creatinine clearance on 24-hour urine collection

) cheap and easy estimation of GFR

) influenced by several fac- tors, including muscle mass, age, gender, diet ) inaccurate with higher

plasma concentrations ) plasma concentrations influenced with hemofil- tration

) most often used in daily practice

) serum cystatin C measurement

) produced at a constant rate ) not influenced by

muscle mass, age, gender, diet ) serum concentrations

only slightly influ- enced by hemofiltra- tion

) expensive (?) estimation of GFR

) hardly used

in daily

practice

Cystatin C

Cystatins are a superfamily of cysteine proteinase inhibitors found in plants and ani- mals. Cystatins comprise a group of proteinase inhibitors, widely distributed in tis- sues and body fluids. Cystatin C, one molecule of this family, is of interest from a medical point of view. It has a molecular weight of 13 kDa, is composed of 120 amino acids, lacks carbohydrate and has two disulfide bridges located near the car- boxyl terminus (Fig. 1). The concentration of cystatin C is independent of age, sex, body mass, hydration status, and infection [12]. Cystatin C is completely filtered in the glomerulus and metabolized, but not secreted, in the tubulus. When the GFR decreases, serum cystatin C concentrations rise, even with small reductions in GFR.

Cystatin C appears to be a better marker of renal function than creatinine, especially in the early phase of renal impairment. Therefore, it is advocated as an endogenous marker of GRF [13, 14]. Importantly, commercially available assays measure cystatin C in a few minutes [12]. A limitation of the use of cystatin C in the ICU is the poten- tial influence of glucocorticosteroids and thyroid function on serum cystatin C con- centration [15]. Another limitation may be costs involved with measuring cystatin C.

Indeed, it is currently 15 times more expensive to measure plasma cystatin C con- centrations then plasma creatinine concentrations.

Serum Cystatin C in Non-ICU Patients

Over the last decades, numerous studies have been carried out to evaluate the accu- racy of serum cystatin C concentrations as a marker for GFR in non-ICU patients (over 50 studies have been published, for a complete reference list see [16]). Studies have been performed in pediatric as well as adult patients, including those at risk for or with established renal disease, transplants, and liver disease. Some studies have limitations because they examined changes of GFR in small patient groups (causing type II errors), or because of the choice of the reference standard for GFR (i.e., no golden standard method for determination of GFR was used).

Fig. 1. Three-dimensional structure of cystatin C. From [31] with permission

Fig. 2a. Scatter plot of correlation coefficients for the reciprocal of serum cystatin concentrations (squares) and the reciprocal of plasma creatinine concentrations (triangles) from 33 and 29 data sets, respectively;

the horizontal lines represent the cumulative mean correlation coefficient of all studies. b Scatter plot of receiver operating characteristic (ROC)-plot area-under-the-curve (AUC) values for serum cystatin C and plasma creatinine concentrations from 14 data sets; the horizontal line represents the cumulative mean.

From [16] with permission.

In a meta-analysis by Dharnidharka et al., the accuracy of serum cystatin C and plasma creatinine concentrations in relation to a reference standard of GFR were compared [16]. Combining the results of 36 data sets for serum cystatin C concen- trations and 29 data sets for plasma creatinine concentrations, the overall coefficient of correlation, r, was significantly greater for the reciprocal of cystatin C concentra- tions (mean r = 0.816 [95 % confidence interval (CI) 0.726 – 0.758]) in comparison to the reciprocal of creatinine concentrations (r = 0.742 [0.804 – 0.826]) (Fig. 2a).

Nevertheless, correlation coefficients reflect only a linear relation and may not translate into agreement or diagnostic accuracy. More meaningful tests are compari- sons of sensitivity, specificity, and positive and negative predictive values. These val- ues, however, are highly dependent on the cut-off values chosen. The cut-off values used for serum cystatin C concentrations were widely disparate in the studies, pre- cluding meaningful analysis of these parameters. Another method for assessment of diagnostic accuracy is receiver operating characteristic (ROC) analysis. Combining the results of 11 data sets, ROC – plot area under the curve (AUC) showed a greater divergence of values for the reciprocal of creatinine concentrations than for the reciprocal of cystatin C concentrations (AUC for 1/creatinine 0.837 vs. AUC for 1/

cystatin C 0.926); 95 % CI did not overlap ([0.796 – 0.878] vs. [0.892 – 0.960]) (Fig. 2b) [16]. These results demonstrate the superiority of serum cystatin C concentrations over plasma creatinine concentrations with respect to GFR determination in non- ICU patients.

With the accuracy of cystatin C established, determination of its utility as a mea-

sure of GFR in clinical practice rests also with the rapidity with which serum cysta-

tin C concentrations change with changes in renal function. In most studies, the

number of cystatin C concentration measurements per patient was low, making it

difficult, if not impossible, to draw conclusions with respect to this issue. However,

one study nicely showed a more rapid change in serum cystatin C concentrations as

compared to systemic creatinine concentrations with changes in renal function

(Fig. 3) [17].

Fig. 3. Cystatin C and creatinine kinetics in renal transplant patients (N = 30). Transplant patients were separated into two groups: Normal course (absence of complications;

N = 16) and delayed graft function (DGF) (N = 14), defined as requiring hemodialysis during the first 2 weeks after surgery. Values are pre- sented as medians. * indicates first day value significantly different from the day of surgery. From [17]

with permission

Serum Cystatin C for Early Detection of Acute Renal Failure in Critically Ill Patients

While the serum cystatin C concentration is considered to be a reliable marker of renal function in non-critically ill patients, its value in critically ill patients can be hypothesized to be lower. First, while the generation rate of cystatin C is reported to be stable in non-critically ill patients [18], the production of cystatin C may be influ- enced by critical illness itself. Indeed, several disease states may have an effect on serum cystatin C levels, including thyroid dysfunction [19, 20]. Furthermore, the use of corticosteroids has been suggested to elevate serum cystatin C concentrations [21 – 23]. Second, even if serum cystatin C concentrations do have an excellent corre- lation with plasma creatinine in critically ill patients, it is uncertain whether serial measurement of serum cystatin C concentrations affects clinical practice, and if so, influence patient outcome.

Ahlstrom et al. assessed serum cystatin C as a marker of acute renal failure in 202 ˚ ICU patients and evaluated its power in predicting survival in patients who evetually developed acute renal failure [24]. For this study, serum cystatin C concentrations, plasma creatinine concentrations, and urea concentrations were measured on admis- sion, daily during the first 3 days, and once per 1 – 2 days thereafter until ICU-dis- charge. Acute renal failure occurred in 54 patients (27 %). Serum cystatin C was a good predictor of acute renal failure in critical illness (ROC – AUC: 0.885, 0.893, and 0.901 for day 1 – 3 serum cystatin C concentrations, respectively) and correlated well with plasma creatinine and urea concentrations (r: 0.72, and 0.86, respectively). Serum cystatin C concentrations were not predictive of mortality, however. Importantly, this study showed that abnormal concentrations of serum cystatin C and plasma creatinine appeared in the same time frame. Furthermore, hydrocortisone (100 – 300 mg i.v. per day, prescribed in the majority of cases for suspected relative adrenal insufficiency due to septic shock) did not seem to affect serum cystatin C concentrations.

In a smaller study, Mazul-Sunko et al. investigated 29 critically ill septic patients

[25]. In this study no correlation was found between serum cystatin C concentra-

tions and acute renal failure. Of note, however, only one sample was analyzed (obtained on the day of admission); another dissimilarity with the previous study was urine output in patients with acute renal failure: while most patients in the study by ˚ Ahlstrom et al. [24] suffered from anuria, a higher urine output was observed in this study.

Herget-Rosenthal et al. evaluated serum cystatin C concentrations in 85 patients at high risk for acute renal failure [26]. In this study, acute renal failure was defined according to the RIFLE-classification [6]. In acute renal failure by R-, I-, and F-crite- ria, serum cystatin C concentrations increased 1.5 – 2 days earlier than plasma creat- inine levels; ROC – AUC was 0.82 and 0.97 on the two days before R-criteria were ful- filled. Serum cystatin C concentrations were found to be only moderate predictors of renal replacement therapy in the further course of acute renal failure, but the num- ber of patients fulfilling the F-criteria was rather low (27 patients). Importantly, nei- ther thyroid dysfunction, nor corticosteroid deficiency or excess seemed to affect serum cystatin C concentrations.

Le Bricon et al. compared serum cystatin C concentrations with an isotope clear- ance technique for GFR, plasma creatinine concentrations, 24 hour creatinine clear- ance, and two GFR-prediction equations (the Cockcroft-Gault creatinine clearance and the modified diet in renal disease-estimated GFR) [27]. Twenty-eight surgical ICU patients were followed for 5 days. Serum cystatin C concentrations correlated well with plasma creatinine concentrations, the Cockcroft-Gault creatinine clear- ance, and also with the modified diet in renal disease-estimated GFR. The sensitivity and specificity of serum cystatin C concentrations to detect a GFR ‹ 80 ml/min were 88 % and 97 %.

Delanaye et al. confirmed these findings in a smaller study on 14 patients admit- ted to a medical ICU [28]. In this study, GFR was estimated by creatinine clearance using 24-hour urine collection and the Cockcroft-Gault equation. The ability of cystatin C to detect a GFR ‹ 80 ml/min/1.73 m

was significantly better than that of creatinine.

Finally, Villa et al. measured serum cystatin C concentrations and 24-hour creati- nine clearance in 50 critically ill patients at risk of developing acute renal failure [29]. Half of the patients developed acute renal dysfunction, only five (20 %) of these 25 patients had elevated serum creatinine, whereas 76 % had elevated serum cystatin C levels. This study suggests that even with small decreases in renal dysfunction, serum cystatin C concentrations may be superior to plasma creatinine to detect patients at risk of developing acute renal failure and reflect GFR changes sooner than concentrations of plasma creatinine.

In conclusion, in critically ill patients, serum cystatin C seems to be an early and

efficient marker for renal dysfunction. Especially with mild reductions in GFR it is

a better predictor for the development of renal failure than plasma creatinine. Of

note, in some of the abovementioned studies the numbers of patients were small

[25, 28]. In addition, all study subjects were admitted to one hospital and, therefore,

vulnerable to a center-effect. Finally, the choice of the reference standard for GFR

(i.e., no golden standard method for determination of GFR was used) makes inter-

pretation of results difficult [24, 25]. Only one study used the RIFLE-criteria to iden-

tify acute renal failure [26]. Finally, although it has been suggested that serum cysta-

tin C concentrations may be influenced by severity of illness, thyroid dysfunction, or

the use of glucocorticosteroids [15], this was not confirmed in the studies men-

tioned above [24, 26].

Serum Cystatin C for Detection of Residual Renal Function During CVVH

In patients on CVVH-treatment it is difficult to determine residual renal function.

Indeed, renal function is usually calculated from plasma creatinine and/or urea con- centrations. Yet, creatinine and urea are removed by the CVVH-filter, and thus are useless for calculation of GFR.

If cystatin C is removed by CVVH, then a similar problem arises with the use of cystatin C as a marker for residual renal function while CVVH is applied. However, recently Baas et al. showed that clearance of cystatin C by CVVH is low [30]. These investigators studied serum cystatin C concentrations in 18 patients with oliguric acute renal failure treated with CVVH during three consecutive collection periods.

Serum cystatin C concentrations were measured in blood samples taken from the afferent and efferent lines, and in corresponding ultrafiltrate samples. Serum cysta- tin C concentrations were 2.25 „ 0.45 mg/l in the afferent and 2.19 „ 0.56 mg/l in the efferent samples; cystatin C concentrations in corresponding ultrafiltrate samples were 1.01 „ 0.45 mg/l. The sieving coefficient of cystatin C was 0.52 „ 0.20; clearance of cystatin C was 17.3 „ 6.6 ml/min; removed quantity of cystatin C averaged 2.0 mg/

hour. This quantity is less than 30 % of the production of cystatin C. Therefore CVVH is unlikely to influence serum cystatin C concentrations, suggesting it can be used as a marker of residual renal function during CVVH-treatment.

Of note, as pointed out above, it has been shown that serum cystatin C concentra- tions rapidly decline with recovery of renal function. Indeed, serum cystatin C con- centration has been shown to normalize within several days with recovery of renal function after renal transplantation; in addition, normalization of cystatin C was found to appear approximately 2 days earlier than normalization of plasma creati- nine levels [17]. As yet, no such studies have been performed in critically ill patients.

Conclusion

Early detection of acute renal failure is mandatory to design or apply measures to prevent persistent anuria and consequential need for renal replacement therapy dur- ing critical illness and thereafter. Biomarkers of renal function may not only be use- ful for early detection of acute renal failure, but also in the recognition of recovery of renal function in patients treated with CVVH. Cystatin C is a promising bio- marker of renal function in critically ill patients. Indeed, even before clinical signs of kidney injury are revealed, serum cystatin C levels rise. Notably, cystatin C is not cleared by hemofiltration, which makes it a potential marker of residual renal func- tion during CVVH. In this way serum cystatin C concentrations may be useful in the decision when to stop CVVH.

Presently, three large multicenter studies are exploring the kinetics of cystatin C

in critically ill patients at risk for acute renal failure, patients with established acute

renal failure, and patients who are in need of renal replacement therapy. One ran-

domized study is investigating the effect of using (changes in) serum cystatin C con-

centrations to stop CVVH on duration of CVVH-therapy and length of stay in the

ICU.

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