## 3 Results
### Outcomes Associated with Time Windows: LOS and Mortality  
Average hospital LOS was 4.7 \(\pm\) 4.0 days among admissions where AKI was not detected. Average hospital LOS if AKI was detected using a 24-h observation window was 10.8 \(\pm\) 10.4 days, 48-h expansion was 8.6 \(\pm\) 7.3 days, 48-h to 7-day expansion was 11.8 \(\pm\) 8.3 days, and 168-h expansion was 9.1 \(\pm\) 6.0 days. Compared to patients without AKI, adjusted hospital LOS increases in those with AKI were: using a 24-h observation window, +5.5 days; (95% CI 5.3 to 5.7); using the 48-h expansion, +3.4 days (95% CI 3.2 to 3.6); using the 48-h to 7-day expansion, +6.5 days (95% CI 6.2 to 6.8); and using the 168-h expansion, +3.9 days (95% CI 3.7 to 4.1); all \(p<0.001\), (Figure 2). Incidence of in-hospital death was 1.4% (1801 of 126,367) for all admissions and 0.6% (574 of 104,534) for admissions where AKI was not detected. Incidence of in-hospital death if AKI was detected using a 24-h time window was 9.6% (900 of 9357), the incidence for the 48-h expansion window was 3.4% (234 of 6963), the incidence using the 48-h to 7-day expansion was 2.4% (61 of 2509), and the incidence for the 168-hhour expansion from 48 h to 168 h for both criteria 1a and 1b was 1.1% (32 of 3004). Compared to patients without AKI, the adjusted risk of in-hospital death increased if AKI was detected using a 24-h window (odds ratio [OR] = 16.9), 48-h expansion (OR = 5.5), 48-h to 7-day expansion (OR = 4.2), and the 168-h expansion (OR = 1.6); all are \(p\leq 0.01\), (Figure 3). Adjusted mean difference in days of hospital care (dots) and 95% confidence intervals (lines) were calculated using bootstrap sampled multivariable logistic regression, where the reference category were patients who did not develop AKI. Logistic regression was adjusted for hospital admission status (elective/unknown, urgent, comorbidities of diabetes, anemia, hypertension, congestive heart failure, liver disease, and peripheral and visceral atherosclerosis. Figure 2: **Increased days of hospital care by observation window expansion (_n = 126,367_). AKI, acute kidney injury; KDIGO: Kidney Disease Improving Global Outcomes. Adjusted mean difference in days of hospital care and 95% confidence intervals (lines) were calculated using bootstrap sampled multivariable logistic regression, where the reference category was patients who did not develop AKI. Logistic regression was adjusted for hospital admission status (elective/unknown, urgent, and emergency), estimated GFR in categories of 15 mL/min/1.73 m\({}^{2}\) based on first hospital serum creatinine, gender, hospital type (academic medical center vs. community hospital), and baseline comorbidities of diabetes, anemia, hypertension, congestive heart failure, liver disease, and peripheral and visceral atherosclerosis. **The cases of AKI detected using the 48-h to 7-day expansion or the 168-h expansion occurred later during the hospital stay compared to cases detected using the 24-h or 48-h observation windows (Figure 4). Therefore, a subgroup analysis was conducted among 34,245 admissions with a length of stay \(\geq\)7 days to minimize the influence of when the AKI was detected on associations with in-hospital mortality. In this subgroup, adjusted risk of in-hospital death increased if AKI was detected using a 24-h observation window (OR = 13.6), 48-h expansion window (OR = 5.7), 48-h to 7-day expansion window (OR = 3.3), and 168-h expansion window (OR = 1.8) compared to patients without AKI (all \(p\leq 0.01\)). Figure 4: **Days of hospital care before and after the detection of AKI by observation window expansion (\(n\) = 126,367). ** AKI, acute kidney injury; KDIGO, Kidney Disease: Improving Global Outcomes. The days before AKI were calculated as days from hospital admission to first detection of AKI. If AKI was not detected, the days before AKI were equal to the entire length of hospital stay. Days after AKI were the number of days from first detection of AKI to hospital discharge.