Appendix Table 1.

Properties of Biomarkers of AKI

Biomarker Types	Biomarkers	Sources Tested	Site of Expression	Known Functions	Biomarker Kinetics in Cardiac Surgery Setting	Factors Affecting Biomarker Levels	Assay Platform
Functional	Cys-C (9)	Urine, serum	Nucleated cell	13-kDa cysteine protease inhibitor, freely filtered and further reabsorbed and metabolized in the PT	Detected 12–24 h postinjury, and peak levels at 24–48 h	Albuminuria increase urinary excretion of Cys-C (competitive reabsorption through megalin-facilitated endocytosis); blood Cys-C levels are affected by GFR	Nephelometry, turbidimetry, ELISA (assay analysis is affected by diabetes, large dose steroids, hyperthyroidism, inflammation, hyperbilirubinemia, rheumatoid factor, and hypertriglyceridemia)
Tubular injury	IL-18 (12)	Urine, serum	PT, mononuclear cells, macrophages, fibroblasts, dendritic cells, intestinal epithelia, adrenal cortex	24-kDa molecule cleaved by caspase-1 to generate biologically active 18-kD molecule; upregulation in ischemic kidney injury and is proinflammatory	Elevation within first 6 h after injury, and peak levels at 12–18 h	IL-18 levels are also high in inflammation, sepsis, heart failure	ELISA
	KIM-1 (10)	Urine	PT (KIM-1a: renal; KIM-1b: liver)	38.7-kDa type-1 membrane glycoprotein; phosphatidylserine receptor; upregulated in tubular injury and activates immune cells TH-1, TH-2, and TH-17; promotes apoptotic and necrotic cell clearance and remodeling of injured epithelia	Detected 12–24 h postinjury and peak levels at 48–72 h	KIM-1 is also expressed at high levels in patients with clear cell–type renal cell carcinoma, chronic proteinuria, CKD, and sickle-cell nephropathy	ELISA; Luminex
	L-FABP (20)	Urine	PT, liver, small intestine, pancreas, lung, stomach	15-kDa cytoplasmic protein which transports free fatty acids to mitochondria and peroxisomes for metabolism; protects against damage caused by reactive oxygen species; upregulated during ischemia–reperfusion injury	Detected within 1 h after injury, and peak levels within 6 h	L-FABP levels are also elevated in liver disease, sepsis, polycystic kidney disease, CKD	ELISA
	NGAL (13)	Urine, serum	Kidney (PT>DT), neutrophils, liver, spleen	25-kDa lipocalin protein associated with gelatinase from neutrophils; NGAL chelates labile Fe released from damaged tubules and prevents formation of hydroxy radicals; also upregulates the reno-protective enzyme heme-oxygenase-1	Detected as early as 3 h after injury, peak at 6 h, and can have sustained elevation as long as 5 d	Plasma NGAL influenced by CKD (really?), chronic hypertension (really?), systemic infections, inflammatory conditions, anemia, hypoxia, and malignancies	ELISA
	NAG (19)	Urine	PT	140-kDa proximal tubular lysosomal enzyme, not filtered	Detected 12 h after renal injury	Urinary NAG is inhibited by urea, heavy metals, industrial solvents	Colorimetric (false positive in rheumatoid arthritis, hyperthyroidism, and impaired glucose tolerance; degraded over time even when stored at -80°C)
	α-/π-GST (11,17)	Urine	α-GST: PT; π-GST: DT	47–51 kDa cytoplasmic enzymes; increased after tubular injury	Detected 12 h after renal injury	—	ELISA
Cell cycle arrest	TIMP-2 (23)	Urine	All epithelial cells; kidney tubular epithelial cells	TIMP-2 stimulates p27 expression and IGFBP7 increases the expression of p53 and p21, which block cyclin-dependent protein kinase complexes on cell cycle promotion, thereby resulting in transient cell cycle arrest	Detected within 12 h after renal injury	—	ELISA; Luminex
	IGFBP7 (23)

PT, proximal tubule; KIM-1, kidney injury molecule-1; TH, T-helper cells; L-FABP, liver-type fatty acid–binding protein; NGAL, neutrophil gelatinase–associated lipocalin; DT, distal tubule; Fe, iron; NAG, n-acetyl-β-d-glucosaminidase; α-GST, α-glutathione s-transferase; π-GST, π-glutathione s-transferase; TIMP-2, tissue inhibitor of metalloproteinases-2; IGFBP7, IGF-binding protein 7.