Assessment of kidney function in ICU patients

Introduction

Kidney function assessed by serum creatinine levels has low sensitivity for detection of decreased kidney function because of false-negative results. The aim of the study was to evaluate other methods of assessment of kidney function and compare these with the golden standard, the measured inulin clearance (GFR).

Methods

The GFR and other kidney function assessments were measured during a 24-hour period. The GFR was compared with 1-hour and 24-hour measured urinary creatinine clearance (Ccr1 and Ccr24), levels of cystatine C and β-trace protein, using five equations for glomerular filtration rate (GFR) based on cystatine C (Hoek, Larsson, Le Bricon, Filler, Sjöström), and equations for GFR based on the creatinine level (Cockcroft Gault, MDRD, MDRDs, Jelliffe). Bias was assessed by the mean difference between GFR and different methods, precision by SD of the bias, and accuracy by the proportion of patients for whom the difference in the GFR was within 10% or 30% of the GFR (Acc10–30%).

Results

Fifty patients were included, 64% were male, age was 56 ± 18 years, and APACHE II score was 19 ± 6. Creatinine was 0.66 mg/dl (0.45–1.58), cystatin C was 1.26 mg/dl (1.00–2.63) and β-trace protein was 0.92 mg/dl (0.67–1.85). The GFR was 64 (20–98) (ml/min/1.73 m²); 72% of patients had GFR <90 and 48% had GFR <60. Bias was lowest for cystatin C-based equations (range: 3–9), was intermediate for Ccr1 and Ccr24 (27 vs 17) and great for the four creatinine-based equations (range 33–196). Precision was low for all methods of GFR assessment (range 28–141). Also, accuracy was low: Acc10% for all methods ranged from 0% to 20%, and Acc30% from 2% to 50%. Cystatin C and β-trace protein were better than creatinine levels for detection of GFR <60 (ROC analysis: AUC = 0.95 vs 0.91 vs 0.87).

Conclusion

Cystatin C and β-trace protein hold promise for assessment of kidney function in ICU patients. Cystatin C and β-trace protein performed better than creatinine for the detection of kidney insufficiency. Also, GFR assessed on cystatin C-based equations were better than Ccr (low bias); however, they also lacked precision and accuracy.