This is a retrospective cohort study of adults presenting to the emergency department (ED) of the Ottawa Hospital with acute kidney injury who underwent kidney PoCUS, between June 1, 2019, and April 30, 2021. This is part of a larger quality improvement project aimed at increasing the uptake of kidney PoCUS in this patient population.

Through our data warehouse, we used ICD-10 coding to identify adults presenting to the ED of the Ottawa Hospital with acute kidney injury, between June 1, 2019, and April 30, 2021. The Ottawa Hospital is a 1335-bed academic tertiary care centre with over 160,000 ED patient-visits per year. We excluded patients who were dialysis-dependent, prior kidney transplant recipients and those who did not meet the Kidney Disease Improving Global Outcomes criteria for stage 1 acute kidney injury (≥ 26.5 umol/L or 1.5 × increase from baseline serum creatinine) [27]. If no previous creatinine was available, patients were included if their creatinine at presentation was ≥ 26.5 μmol/L above the upper limit of normal for their sex (ULN) (ULN is 84 μmol/L for women and 100 μmol/L for men). Finally, we excluded patients who were discharged directly from the ED.

Within our cohort, we identified patients who had undergone kidney PoCUS on presentation. First, encounter notes containing one of 15 keywords synonymous with PoCUSFootnote 1 were identified and patients were included if the PoCUS included the kidney(s). Second, all patient Medical Record Numbers were manually entered into our imaging archiving software QpathE (Telexy Healthcare, Maple Ridge, BC, Canada) to identify exams that may have been missed through our first method. A PoCUS was considered positive if the presence of hydronephrosis was recorded either in the physician note or in the QpathE reporting worksheet. A test was considered negative if the absence of hydronephrosis was recorded in either of these mediums. If a PoCUS scan was archived but no interpretation was documented, the patient was excluded. A PoCUS was considered indeterminate if it was reported as inconclusive or not interpretable. Indeterminate scans were excluded from our diagnostic accuracy analysis. The reasoning behind this consensus decision is that, at our centre, PoCUS providers are taught to fall back on their history and physical examination for clinical decision-making when they obtain an indeterminate scan. This approach is analogous to no PoCUS having been performed and justifies the exclusion of indeterminate tests from our analysis. This process was performed by four independent reviewers. If there was uncertainty about whether a patient should be included, the encounter was reviewed, and a decision was made by the project lead (MGS).

We performed a health records review of our final patient population. We recorded age, sex, baseline and creatinine on presentation, comorbidities by Charleston index, ED diagnosis, admission service, PoCUS date and time If the PoCUS scan was not archived in QpathE, the time of the scan was defined as the time of exam recorded in the encounter note or the time of the physician's initial assessment if the former was unavailable. The PoCUS provider and their credentials were recorded. Emergency physicians were considered credentialed if they completed an introductory PoCUS course, obtained at least 50 supervised or reviewed scans and successfully completed an examination. There was no credentialing process in place for other subspecialities at the time of this review. Finally, a PoCUS expert reviewed all PoCUS scans that had been archived and provided an interpretation (MYW).

We then identified whether patients underwent radiology-performed ultrasound (RADUS) or computed tomography (CT) within 48 h of PoCUS. If radiology-performed imaging was performed first (prior to PoCUS), the patient was excluded. We recorded indication, time of imaging, and imaging result. For patients who underwent both reference standard and index test, we recorded whether Foley was inserted on presentation.

For patients who underwent PoCUS but did not undergo reference standard, a chart review was used to identify if obstructive uropathy had been missed. We identified if imaging was done later (> 48 h) in the index admission, recorded creatinine on hospital discharge, creatinine on post-hospital follow-up, and reviewed clinical notes to determine if an alternate cause of AKI was identified.

The index test was PoCUS of the kidneys, performed by the treating physician. All exams were performed using the Philips Sparq or the Fujifilm SonoSite X-Porte.

The reference standard was computed tomography or radiology ultrasound performed within 48 h after PoCUS. If both computed tomography and radiology-performed ultrasound were performed, the reference standard was computed tomography.

Summary statistics (sum, mean, and median) were generated using Excel. We performed a data distribution analysis to inform choice of the statistic to report for creatinine and time to imaging. Diagnostic accuracy and 95% confidence intervals were determined using the EpiR package in R statistical software. Sensitivity analysis was performed after exclusion of all patients having had a Foley catheter inserted in the ED.

This study was part of a larger quality improvement project aimed at increasing the use of PoCUS in patients with acute kidney injury. We obtained an exemption from the Ottawa Health Science Network Research Ethics Board and registered our project in the IQ@TOH Project Registry prior to the project start.