Table 3: Summary of characteristics and main results of the 17 previous studies included in this review.

Serials	Author/year	Study design	Purpose of the study	Results and recommendations
1	De Loor et al.37	Pilot study	To evaluate whether urinary Chitinase 3-like protein 1 (YKL-40) can predict AKI stage ≥ 2 in ICU patients compared with NGAL.	The concentration of UCHI3L1 within 12 hours of AKI stage ≥ 2 was increased with good performance on AUC-ROC curve (0.792, 95% CI), similar to UNGAL AUC-ROC (0.748, 95% CI), and after 24 h, UCHI3L1 showed AUC-ROC twice as high (95% CI: 1.3-3.1) as controls.
2	Huen et al.38	Review	Focus on future phenotyping of AKI regarding NGAL and YKL-40.	NGAL and YKL-40 are important novel biomarkers involved in moderate renal tubular protection after AKI.
3	Schmidt et al.39	Cohort (comparative) study	To evaluate the role of urinary and blood levels of YKL-40 in allografts after renal transplantation.	Urinary YKL-40 increased early on, within 18 h after surgery (131.3 ± 155.2), with AUC 0.86 ± 0.07; blood YKL-40 retarded to 1 day after surgery (623 ± 285.9), with AUC 0.59 ± 0.08
4	Hall et al.40	Observational cohort study	To measure YKL-40 levels in the urine of clinically hospitalized AKI patients.	Urinary YKL-40 levels were detectable (≥ 5 ng/ml) within 1 h and gave better prognostic value (P = 0.04) with NGAL.
5	Tatar et al.41	Cohort study	To define relationship between YKL-40 and proteinuria in renal transplant recipients.	Mean serum YKL-40 and proteinuria levels were 66 ± 46 ng/ml and 0.77 ± 1.15 g/day respectively without any apparent correlation.
6	Maddens et al.36	Clinical and experimental study	Measurement of urinary and plasma levels of Chitinase 3-like protein 1 and -3 in mice and patients with and without septic AKI.	Urinary CHI3L1 higher in septic-AKI patients than in non-AKI (P < 0.05), but in septic-AKI mice models, CHI3L1 and -3 were found to be high.
7	Malyszko et al.42	Review article	Illustration of candidate biomarkers in cases of delayed graft function as a form of acute kidney injury.	Elevated YKL-40 in both urine and serum levels of patients with DGF, 2 days after transplantation.
8	Muramatsu et al.32	Experimental study	To test CYR61 in the urinary levels of mice and rats after immediate renal ischemic reperfusion injury.	CYR61 protein increased first within 1 h and appeared in urine 3-6 h after ischemic renal injury.
9	Lai et al.43	Experimental study	To investigate the role of CYR61 after unilateral IRI in mice.	CYR61 was significantly induced at renal and urinary levels after IRI.
10	Xu et al.44	Experimental study	To indicate CYR61 expression in renal cell lines under hypoxia	Enhanced expression of renal CYR61 in response to hypoxic ischemic injury.
11	Kim et al.45	Cohort study	To determine possible influence of AKI on serum and urinary levels of Klotho, S100A8/A9 and NGAL	Urinary Klotho levels were 13.21 ± 17.32 versus 72.97 ± 17.96 pg/ml (P = 0.002) in pre-renal and intrinsic AKI respectively.
12	Torregrosa et al.46	Cohort study	Assessment of urinary Klotho levels in patients after cardiac surgery or coronary angiography.	Klotho levels did not behave as a good early biomarker of AKI.
13	Castellano et al.47	Experimental study	To investigate whether or not complement components affect Klotho levels after IRI.	Complement activation result in remarkable decline in renal Klotho levels, 24 h after IRI.
14	Liu et al.48	Case-control study	To evaluate serum Klotho levels at different time intervals after cardiac surgery.	Serum Klotho levels were 101.97 ± 16.93 versus 121.64 ± 19.87 (P < 0.01) in AKI and non-AKI group respectively at 0 h and continued until 4 h. After 3 days, serum Klotho values were 120.50 ± 13.17 versus 128.67 ± 18.84.
15	Seo et al.49	Retrospective cohort study	Assessment of renal Klotho levels in human samples instead of animal models.	Renal Klotho levels were significant reduced with AKI severity.
16	Hu et al.29	Experimental and case-control study	To estimate Klotho at urinary and plasma levels, investigating probable protective ability.	Urinary Klotho values (pmoles/l) were 2.52 ± 0.76 in AKI versus 20.66 ± 1.81 in non-AKI, with P < 0.01.
17	Sugiura et al.30	Experimental study	To explain the physiological relevance of renal Klotho after IRI in rats.	Renal Klotho levels were significantly reduced in IRI rats, 24 h after ischemia.