Acute kidney injury and adverse outcomes of critical illness: correlation or causation?

Abstract

Critically ill patients who develop acute kidney injury (AKI) are more than twice as likely to die in hospital. However, it is not clear to what extent AKI is the cause of excess mortality, or merely a correlate of illness severity. The Bradford Hill criteria for causality (plausibility, temporality, magnitude, specificity, analogy, experiment & coherence, biological gradient and consistency) were applied to assess the extent to which AKI may be causative in adverse short-term outcomes of critical illness. Plausible mechanisms exist to explain increased risk of death after AKI, both from direct pathophysiological effects of renal dysfunction and mechanisms of organ cross-talk in multiple-organ failure. The temporal relationship between increased mortality following AKI is consistent with its pathophysiology. AKI is associated with substantially increased mortality, an association that persists after accounting for known confounders. A biological gradient exists between increasing severity of AKI and increasing short-term mortality. This graded association shares similar features to the increased mortality observed in ARDS; an analogous condition with a multifactorial aetiology. Evidence for the outcomes of AKI from retrospective cohort studies and experimental animal models is coherent however both of these forms of evidence have intrinsic biases and shortcomings. The relationship between AKI and risk of death is maintained across a range of patient ages, comorbidities and underlying diagnoses. In conclusion many features of the relationship between AKI and short-term mortality suggest causality. Prevention and mitigation of AKI and its complications are valid targets for studies seeking to improve short-term survival in critical care.

Keywords: AKI; critical care; epidemiology; outcomes; review.

FIGURE 1

Short-term mortality in patients with AKI, relative to those without, adjusted for confounders. Factors adjusted for include the following. Barrantes et al. [2]: not specified—selected from univariate analysis; Harris et al. [66]: ethnicity, CKD, APACHE score, sepsis, mechanical ventilation, liver failure; Masewu et al. [67]: organ systems affected, oxygen saturations, tachypnoea; Reddy et al. [68]: age, APACHE score, admission creatinine, sepsis, RRT; Ralib and Nor [69]: age and Sequential Organ Failure Assessment score.

FIGURE 2

Forest plot demonstrating the relationship between AKI stage and short-term mortality. Stages 1–3 equate to AKIN/KDIGO 1, 2, 3 and RIFLE, respectively. ORs relate to comparison between the relevant stage of AKI and no-AKI after adjusting for confounders. Factors adjusted for include the following: Mandelbaum et al. [3]: age, non-renal SOFA score, comorbidity; Thakar et al. [76]: age, comorbidity, admission diagnosis, source of admission, 11 laboratory test results; Clec’h et al. [77]: non-renal SOFA score, McCabe class, admission source, organ failure; Hoste et al. [23]: admission source, serum creatinine on admission to ICU and non-specified ‘fixed predictors’; Ostermann et al. [78]: age, SOFA score, number of organ failures, gender, chronic illness, mechanical ventilation, AKI categories, RRT and anaemia. SOFA, Sequential Organ Failure Assessment.