In this study, we investigated the association between albumin-adjusted plasma free thiol levels in relation to (sepsis-associated) AKI in critically ill patients as a biomarker for oxidative stress. Most importantly, we observed that patients admitted to the ICU with AKI had significantly lower levels of plasma free thiols as compared to patients without AKI. However, plasma free thiol levels upon admission were not significantly different between patients with and without new-onset AKI or progression of AKI within 48 hours. Furthermore, patients with sepsis had significantly reduced levels of plasma free thiols upon admission compared to patients without sepsis. If we separated the AKI patients into groups with and without sepsis, we found patients with sepsis-associated AKI had lower levels of albumin-adjusted free thiol levels compared to patients without sepsis and AKI. Additionally, we observed that plasma free thiol levels were associated with age, CRP, serum albumin, serum creatinine and serum NGAL. In contrast, however, calprotectin did not correlate with plasma free thiol levels.
Oxidative stress plays an important role in the pathogenesis of AKI in critically ill patients (3). The excessive production of free radicals overpowering the antioxidant machinery results in oxidative stress, which is in turn responsible for extensive cellular and molecular damage. Furthermore, AKI itself is a stimulus for increased oxidative stress, due to mitochondrial dysfunction (3) (24). Over the past decade, multiple biomarkers for AKI have been studied and proposed (25,26). However, no adequate and clinically applicable biomarker for the early prediction of AKI development in critically ill patients is currently available. In clinical practice, creatinine levels are nowadays used as a biomarker to diagnose AKI, but these levels only change once renal failure has occurred (27). In contrast, several other AKI biomarkers have been proposed such as NGAL, IL-18 and urine calprotectin (25,26). NGAL is a biomarker with a high predictive and diagnostic value for AKI which is released during ischemia. However, it is also released during systemic inflammation and therefore lacks specificity (28,29). Given the fact that oxidative stress is a key player in the aetiology of AKI in critical illness, extracellular free thiol levels may be of potential diagnostic and/or predictive value (15). In our study, we demonstrated that patients with AKI upon admission had significantly reduced levels of albumin-adjusted plasma free thiol levels compared to patients without AKI upon admission. In addition, plasma free thiol levels significantly correlated with indicators of renal function, including the change in creatinine upon admission. Previous research demonstrated that patients with hospital-acquired AKI had lower levels of unadjusted plasma free thiols as compared to critically ill patients without AKI and healthy subjects (30). In another study, paediatric patients with AKI also had lower levels of unadjusted plasma free thiols as compared to healthy controls (31).
NGAL and calprotectin are associated with neutrophil activation (11) (28). Calprotectin is a protein heterodimer derived from neutrophils and monocytes which both play a key role during inflammation by inducing activation of other immune cells and enhancing their ROS production, thereby augmenting oxidative stress (11,32). Patients who develop AKI after cardiac surgery had higher levels of plasma calprotectin as compared to patients who did not develop AKI (33). In the present study, we found no correlation between serum calprotectin and plasma free thiol levels. Given the association between calprotectin with inflammation and oxidative stress, the absence of the correlation between calprotectin and free thiols in the present study was against our expectations. This could potentially be explained by the fact that neutrophil activation is of lesser importance in the case of AKI-associated oxidative stress. NGAL is another biomarker of systemic inflammation that is released from neutrophil granules. Both, NGAL and calprotectin, were correlated with each other. However, NGAL is also rapidly induced and released from injured kidney tissue, and is therefore a less specific biomarker of neutrophil activation (28)(3,25). Further, it is an important contributor to free radical generation (3). As described earlier, higher levels of NGAL are associated with AKI (25,26). Our results showed higher plasma levels of NGAL in patients with sepsis-associated AKI compared to controls. Further, in our study, plasma levels of NGAL correlated with plasma free thiols. Together, plasma free thiols are associated with NGAL, but apparently not with calprotectin levels. This may implicate that a decrease in plasma free thiol levels would be more associated with a different source of free radicals than neutrophil activation.
Since plasma free thiols were decreased in AKI, it might also identify patients at risk for the development of AKI. However, we did not detect a significant difference in albumin-adjusted plasma free thiol levels in patients that developed AKI compared to patients who did not develop AKI during their ICU stay. Furthermore, albumin-adjusted plasma free thiol levels showed no significant difference between patients with AKI progression and without AKI progression during ICU admission. Based on these findings, it may be questioned if albumin-adjusted plasma free thiol levels could be used as an early indicator of AKI development as was originally hypothesized. This suggests that AKI itself is associated with a decrease in free thiols.
Strengths of this study comprise the large and extensively characterized study population consisting of patients who were admitted to the ICU in our university medical centre. There was no selection bias, since all patients admitted to the ICU as subsequent admissions were included between January 2014 and April 2014. All relevant demographic, clinical and biochemical information was available for each patient from admission upon to 8 days. Until date, to our best knowledge, no other study focused on unraveling the relationship between (albumin-adjusted) plasma free thiols and AKI. However, our study also has several limitations that have to be considered. For instance, we did not have plasma samples available from every patient in our cohort, which downsized our sample size for the plasma free thiol levels analyses to 83,4% of included patients. Further, most patients stayed for a short amount of time at the ICU, therefore we lost patients after day 1. Since our study was performed in a tertiary academic care centre, patients enrolled in our study more frequently had complicated pathologies including complex oncological disease and post-transplantation complications. This may limit the generalizability of our results to other studies on critically ill patients.