PENK and acute kidney injury: They screened 31 studies from 2356 studies from various databases including Medline, PubMed, Embase, and Cochrane and evaluated the diagnostic value of 7 biomarkers for persistent acute kidney injury i.e., urinary C–C motif chemokine ligand 14 (CCL14), tissue inhibitor of metalloproteinase-2 and insulin-like growth factor-binding protein-7 (TIMP-2 and IGFBP7), neutrophil gelatinase-associated lipocalin (NGAL), plasma cystatin C (pCysC), soluble urokinase plasminogen activator receptor (suPAR), proenkephalin (PENK A) and urinary dickkopf-3:urinary creatinine (uDKK3:uCr).