### Introduction

### Methods

### Patients

^{2}) [5] and patients with end-stage renal disease on maintenance dialysis were also excluded. The local Ethics Committee approved this study to analyze anonymous, routinely collected clinical data and waived the requirement of informed consent.

### Laboratory methods

### GFR measurement and estimation

#### Measured GFR

#### eGFR by serum creatinine

^{−1.009}× age

^{−0.02}(×0.667 if woman)] [5]. The CKD stage was classified according to the recommendation of the Kidney Disease Improving Global Outcomes and National Kidney Foundation-K/DOQI guideline using the K-aMDRD equation [25].

#### eGFR by serum cystatin C

• Larsson A et al [16]: GFR=99.43 × ScytC

^{−1.5837}(mL/minute/1.73 m^{2})• Hoek FJ et al [17]: GFR=80.35/ScytC

^{−4.32}(mL/minute/1.73 m^{2})• Le Bricon T et al [18]: GFR=78/ScytC+4 (mL/minute/1.73 m

^{2})• Filler G and Lepage N [19]: GFR=91.62 × ScytC

^{−1.123}(mL/minute/1.73 m^{2})• Orebro-cystatin (DAKO) [19]: GFR=119/ScytC – 33 (mL/minute/1.73 m

^{2})• Orebro-cystatin (Gentian) [19]: GFR=100/ScytC – 14 (mL/minute/1.73 m

^{2})

### Statistical analysis

*t*test was used for analysis of continuous variables and results are presented as mean±standard deviation (SD). The Chi-square test was used for analysis of categorical variables. Differences in eGFR in each of the CKD stage were compared using a paired

*t*test. The mean difference (bias) between the paired observation is given with SD (precision) and

*P*values. The Bland–Altman plot was used to test the agreement between eGFRs from the K-aMDRD formula and the cystatin C-based equations, as well as between the K-aMDRD and mGFR [26]. Values of

*P*<0.05 were considered statistically significant.

### Results

### Baseline characteristics

^{th}percentile serum cystatin C, respectively, according to the stages of CKD were as follows: CKD 1, 0.87±0.20 mg/L and 1.45 mg/L; CKD 2, 1.20±0.43 mg/L and 2.20 mg/L; CKD 3, 1.94±0.49 mg/L and 3.08 mg/L; CKD 4, 3.13±0.61 mg/L and 4.29 mg/L; CKD 5 4.25±0.74 mg/L and 6.08 mg/L. Other baseline characteristics according to the CKD stage are summarized in Table 1.

### Validation of the aMDRD equation with Korean coefficient

^{2}in CKD 4 and 13.19±6.74 mL/minute/1.73 m

^{2}in CKD 5. The correlation coefficient between mGFR and eGFR was 0.5 (

*P*=0.03). The Bland–Altman plot analysis of the degree of correspondence between the eGFR calculated by the K-aMDRD formula and the mGFR using DTPA scan revealed that the majority of results fell within the 95% confidence interval (Table 2, Fig. 1).

### Cystatin C-to-creatinine ratio according to the CKD stage

### Comparison of cystatin C-based eGFRs

^{2}vs. 75.17±28.26 mL/minute/1.73 m

^{2};

*P*=0.622, bias=0.39±16.09), whereas the Orebro-cystatin (Gentian) equation yielded the most similar eGFRs in Stages 4 and 5 (15.44±9.45 mL/minute/1.73 m

^{2}vs. 15.17±9.05 mL/minute/1.73 m

^{2};

*P*=0.722, bias=0.27±8.87; Table 3). Accordingly, the Bland–Altman plot showed the highest concordance with Filler’s equation in Stages 1–3, and the Orebro-cystatin (Gentian) equation in Stages 4 and 5 (Fig. 4).

### Discussion

^{52}Cr-ethylenediaminetetraacetic acid, Tc-99m-DTPA are considered the true reference standards for determining GFR. Unfortunately, these tests are expensive and laborious, and therefore are not suited to clinical practice. In our cross-sectional retrospective study, we did not measure inulin clearance and performed DTPA renal scans in only 21 patients. Therefore, we were unable to compare the cystatin C-based estimates of GFRs with the true reference GFR. However, the recently proposed K-aMDRD equation is based on inulin clearance [5], and the MDRD equation has been previously validated in patients with CKD [27], [28]. In addition, although the number of patients was limited in our study, the Bland–Altman plot showed agreement between the K-aMDRD eGFR and the GFR measured by DTPA renal scans in 21 patients with CKD Stages 4 and 5. These findings suggest the validity of having used the K-aMDRD equation as a reference rather than as the true reference GFR.

^{2}[31]. By contrast, we found the eGFR from the K-aMDRD equation to be higher than those from the cystatin C-based equations. This may be due to improved accuracy of the modified equation [5], or possibly due to the superior performance of cystatin C as an early marker of GFR decline [32], [33].