Objective: To evaluate the validity of the International Classification of Diseases, Tenth Revision (ICD-10) code N17x for acute kidney injury (AKI) in elderly patients in two settings: at presentation to the emergency department and at hospital admission. Design: A population-based retrospective validation study. Setting: Southwestern Ontario, Canada, from 2003 to 2010. Participants: Elderly patients with serum creatinine measurements at presentation to the emergency department (n=36 049) or hospital admission (n=38 566). The baseline serum creatinine measurement was a median of 102 and 39 days prior to presentation to the emergency department and hospital admission, respectively. Main outcome measures: Sensitivity, specificity and positive and negative predictive values of ICD-10 diagnostic coding algorithms for AKI using a reference standard based on changes in serum creatinine from the baseline value. Median changes in serum creatinine of patients who were code positive and code negative for AKI. Results: The sensitivity of the best-performing coding algorithm for AKI (defined as a ≥2-fold increase in serum creatinine concentration) was 37.4% (95% CI 32.1% to 43.1%) at presentation to the emergency department and 61.6% (95% CI 57.5% to 65.5%) at hospital admission. The specificity was greater than 95% in both settings. In patients who were code positive for AKI, the median (IQR) increase in serum creatinine from the baseline was 133 (62 to 288) μmol/l at presentation to the emergency department and 98 (43 to 200) μmol/l at hospital admission. In those who were code negative, the increase in serum creatinine was 2 (-8 to 14) and 6 (-4 to 20) μmol/l, respectively. Conclusions: The presence or absence of ICD-10 code N17x differentiates two groups of patients with distinct changes in serum creatinine at the time of a hospital encounter. However, the code underestimates the true incidence of AKI due to a limited sensitivity.
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