The importance of the urinary output criterion for the detection and prognostic meaning of AKI

Abstract

Most reports on AKI claim to use KDIGO guidelines but fail to include the urinary output (UO) criterion in their definition of AKI. We postulated that ignoring UO alters the incidence of AKI, may delay diagnosis of AKI, and leads to underestimation of the association between AKI and ICU mortality. Using routinely collected data of adult patients admitted to an intensive care unit (ICU), we retrospectively classified patients according to whether and when they would be diagnosed with KDIGO AKI stage ≥ 2 based on baseline serum creatinine (Screa) and/or urinary output (UO) criterion. As outcomes, we assessed incidence of AKI and association with ICU mortality. In 13,403 ICU admissions (62.2% male, 60.8 ± 16.8 years, SOFA 7.0 ± 4.1), incidence of KDIGO AKI stage ≥ 2 was 13.2% when based only the SCrea criterion, 34.3% when based only the UO criterion, and 38.7% when based on both criteria. By ignoring the UO criterion, 66% of AKI cases were missed and 13% had a delayed diagnosis. The cause-specific hazard ratios of ICU mortality associated with KDIGO AKI stage ≥ 2 diagnosis based on only the SCrea criterion, only the UO criterion and based on both criteria were 2.11 (95% CI 1.85-2.42), 3.21 (2.79-3.69) and 2.85 (95% CI 2.43-3.34), respectively. Ignoring UO in the diagnosis of KDIGO AKI stage ≥ 2 decreases sensitivity, may lead to delayed diagnosis and results in underestimation of KDIGO AKI stage ≥ 2 associated mortality.

Figure 1

Euler diagram illustrating the distribution and overlap between AKI diagnosis according to different AKI criteria. Numbers indicate the number of patients in that overlap zone, so who would be diagnosed by different criteria. SCrea: serum creatinine > 4.0 mg/dl or > 2 × baseline, where baseline = SCrea-1 whenever available, otherwise SCrea-2, or SCrea-3 (when neither SCrea-1 nor SCrea-2 are available) with Screa-1 defined as baseline Screa measurement as manually entered in ICIS by the treating physician at ICU admission; Screa-2 defined as the lowest pre-ICU measurement up to 365 days before ICU admission as extracted from the lab information system; Screa-3 defined as a back-calculated baseline Screa using the simplified 4-variable Modification of Diet in Renal Disease (MDRD) Study equation assuming an estimated glomerular filtration rate (eGFR) of 75 ml/min/1.73 m² for every patient; UO-1: total UO during the last 12-h period was ≤ 6 ml/kg; UO-2: total UO during each of the last 12 consecutive 1-h periods was ≤ 0.5 ml/kg.

Figure 2

Cumulative incidence of automated AKI diagnosis over time. (A) Cumulative incidence of automated AKI diagnosis since ICU admission, based only on KDIGO Screa criterion (left) or only on KDIGO UO criterion (right) (black curves). Shaded areas represent cases that were, by each time point, either missed (red) or diagnosed with delay (purple) by ignoring the other criterion. (Dark shades in the left panel indicate missed or delayed cases compared to the UO-2 criterion, while the combination of light and dark shades in the left panel indicate missed or delayed cases compared to the UO-1 criterion). (B) Cumulative incidence of automated AKI diagnosis in patients still hospitalized and without AKI diagnosis by the 12th hour since ICU admission, based only on KDIGO Screa criterion (left) or only on KDIGO UO criterion (right) (black curves). Shaded areas represent cases that were, by each time point, either missed (red) or diagnosed with delay (purple) by ignoring the other criterion.