An important controversy and uncertainty still exists around the optimal timing to start renal replacement therapy (RRT) in ICU patients with acute kidney injury (AKI) that do not present with absolute indications for RRT. Recent RCT's comparing "early" and "late" initiation of RRT provided contradictory messages, potentially related to the difference in study population, design (single-versus multicenter), illness severity, modality of RRT and definitions of early and late [1][2][3]. Reliable quantitative tools that predict whether AKI patients will recover would allow avoiding unnecessary RRT. Moreover, by identifying a more severe subset of patients with AKI, an evaluation of the advantages and disadvantages of early starting of RRT could be performed in an enriched population that would probably need RRT sooner or later during their ICU stay. Biomarkers of kidney damage have been extensively studied for their accuracy in early diagnosis of AKI. Their role in distinguishing patients with high versus low likelihood of renal recovery is less clear.In a recent article in this journal, Klein et al. report the results of a systematic review which meta-analyzed the ability of biomarkers to identify patients with and without subsequent need for RRT [4]. The largest body of evidence exists for NGAL that showed only fair discrimination. The markers of cell cycle arrest, the product of [TIMP-2]* , showed the best discrimination with a pooled area under the ROC curve (AUROC) of 0.857 (0.789-0.925). This result (based on four studies with 280 patients and 50 RRT received) was rather homogeneous (I 2 = 24.9%), increasing the confidence of this estimate.On the other hand, the homogeneity of this result probably relates to the more homogeneous patient populations with clearly defined insult (mainly surgery) in the included studies whereas studies on NGAL included more heterogeneous populations where the timing of the renal insult is less clear. A summary of the most important findings can be found in the Supplementary Table (Table S). This enormous amount of work, for which the authors are to be congratulated, cannot overcome the limitations of the included original studies. The most important limitation is the absence of a gold standard for the endpoint (RRT): substantial practice variation exists in the initiation of RRT and many of the included studies did not use predefined indications. In 85% of the studies, the prediction of RRT was not the main purpose. The study populations are heterogeneous and not all patients had AKI at the time of biomarker sampling, which could explain the large variability of RRT need (Table S). The delay between sampling of the biomarker and the start of RRT, when reported, was variable, increasing the observed betweenstudy heterogeneity and biomarker performance (Fig. 1). Biomarkers were not compared with readily available clinical parameters nor was their additive value assessed, for example with the net reclassification index.Finally, we share the authors' interpretation that the use of bi...