The MARKISIO study was designed to evaluate the ability of a panel of both routine and novel biomarkers to predict the need for RRT within 72 hours following the onset of severe AKI. Among the 256 patients enrolled, 39% met at least one criterion for RRT initiation according to rigorous criteria or died within 72 hours post-inclusion and 56.2% presented a major adverse kidney event 60 days after inclusion. No biomarker achieved an acceptable predictive performance for these outcomes.
Previous studies, which were gathered in a meta-analysis published in 2018, have reported the potential utility of various biomarkers in predicting the initiation of RRT in patients with AKI [4]. Our study refutes these conclusions. This can be largely explained by to the more rigorous definition of the criteria for initiation. Indeed, we took advantage of the results of RCTs that were not available at the time when the vast majority of studies on biomarkers were issued. These recent RCTs allowed the use of well-defined and reproducible RRT initiation criteria that are now considered by many as the standard of care [14, 15]. In contrast, previous studies assessed the utility of biomarkers to predict the physician's decision to initiate RRT; decisions varied widely from one country to another, from one hospital to another, and sometimes within the same team [5]. Historically, there has been a tendency to initiate RRT based on the perceived severity of a patient condition rather than on objective criteria (such as metabolic complications of AKI). This approach not only muddies the predictive power of biomarkers but also creates a circular logic that the ancient Greeks would recognize as “Ouroboros” also known as “the snake that bites its own tail”.
The KDIGO guidelines [10] illustrate this paradox by defining stage 3 AKI as "initiation of RRT." This creates a self-perpetuating cycle in which the severity of a patient condition (Stage 3 AKI) is partly defined by the decision to initiate RRT, which, in turn, has historically been influenced by the perceived severity of patient condition. This feeds back into itself, obscuring both the predictive utility of biomarkers and objective assessment of a patient's need for RRT. This kind of aporia likely explains the consistent efficacy demonstrated by the majority of biomarkers in predicting the specific clinical endpoint of RRT initiation. They were probably primarily assessing the overall severity which is relatively straightforward to evaluate.
The biomarkers chosen for this study were selected based on their potential relevance to the diverse pathophysiological mechanisms implicated in AKI and the potential necessity for RRT. Specifically, urinary KIM1 was included for its presumed role in both kidney injury and recovery. The process of kidney cellular regeneration after toxic or ischemic injury involves undifferentiated proximal tubular epithelial cells that disperse onto denuded areas of the basement membrane and reconstitute a continuous epithelial layer[16]. The KIM1 protein is expressed in these dedifferentiated cells and may play a role in their adhesion to the basement membrane and dispersion onto damaged areas[16, 17]. Nicotinamide and its metabolites, (N2PY and Methylnicotinamide) were examined for their involvement in metabolic pathways directly impacting renal function recovery. In the context of AKI, marked diminutions in nicotinamide adenine dinucleotide (NAD+) concentration, for which nicotinamide is a precursor have an adverse impact on energy synthesis, thereby undermining the primary renal function of selective solute filtration. In contrast, elevated NAD + levels may confer resilience to renal tubules when exposed to a range of acute stressors, making this mechanism one of the most promising directions for therapeutic intervention. Indeed, investigation of this pathway is highly relevant in this particular setting [18–20]. The marker cDPP3 was assessed for its role in the regulation role in Renin Angiotensin System (RAS) known for its implication in glomerular hemodynamics renal inflammation and fibrotic process [21–23]. PenKid was assessed for its previously described potential ability to detect early AKI and renal recovery[24, 25]. Lastly, we evaluated urinary CCL14, given that recent research has suggested its potential as a predictor of persistent AKI and, consequently, lack of renal recovery[3, 26].
In contrast to our study, RUBY [3] found urinary CCL14 to be a promising marker for persistent AKI and subsequent renal non-recovery. In that study, the primary outcome was persistent stage 3 AKI within 72 hours. This outcome was met in more than 50% of cases because RRT was initiated in the absence of pre-specified criteria, creating a circular reasoning as explained above. Although urinary Neutrophil Gelatinase-Associated Lipocalin (NGAL) has been widely claimed a potential biomarker for AKI, we consciously chose not to include it in our panel for this study. This decision was supported by the disappointing performance of NGAL in previous studies, including the ELAIN study on the timing of RRT initiation, where this biomarker failed to discriminate between varying stages of AKI severity[27]. Similarly, the STARRT-AKI study, the largest study in this field to date which initially considered NGAL, opted not to include it in the main study because of its very poor performance in its pilot phase.[8, 28].
One of the principal strengths of our study lies in its methodological rigor, informed by the latest evidence from randomized controlled trials on RRT timing[7, 8] [6, 9]. This allowed us to apply recognized criteria for initiating RRT, thereby reducing the variability often observed in clinical practice. Additionally, the diversity of biomarkers assessed enhances the comprehensiveness of our findings. However, several limitations should be acknowledged. The most significant constraint is the relatively small sample size, which limits the statistical power and generalizability of our results. It is worth noting, however, that this limitation is not unique to our study; the RUBY trial and many other studies on biomarkers also operated under the same constraint of limited sample sizes. Furthermore, the cross-sectional nature of our biomarker measurements could be seen as a limitation, as it provides only a snapshot of a dynamic physiological process. This issue is also present in most other studies, including the RUBY study, due to the practical challenges of obtaining longitudinal biomarker data in a clinical setting. We also could not evaluate the NephroCheck biomarkers (Insulin-like Growth Factor Binding Protein 7 [IGFBP-7] and Tissue Inhibitor of Metalloproteinase 2 [TIMP-2]) or perform the NephroCheck test itself due to the unavailability of kits from the manufacturer, which may have affected the comprehensiveness of our biomarker assessment. In summary, we found that no biomarker demonstrated significant predictive accuracy for the need for RRT within 72 hours after severe AKI onset in ICU patients. In the context of rigorous RRT initiation criteria, these biomarkers may not be suitable for guiding RRT initiation nor informing renal prognosis in patients with severe AKI potentially requiring RRT.