Renal Ultrasonography and Acute Kidney Injury

Acute kidney injury (AKI) is a common occurrence among hospitalized patients, with incidence increasing from 10 to 25 per 1000 discharges over the past 15 years. AKI is defined as an abrupt decline in renal function, indicated either by increased serum creatinine (CR) level (>0.3 mg/dL or 50% above baseline) or decreased urine production (<0.5 mL/kg/h over 6 hours). AKI is significantly associated with increased mortality. Although physicians often prescribe renal ultrasonography (RUS) tests that can determine or eliminate the possibility of an obstructive cause, most cases of AKI are not caused by obstruction. Investigators wished to create a stratification system that would help clinicians ascertain the risk of renal obstruction among those with AKI by designing and validating a decision rule that would identify patients at low risk of obstruction (hydronephrosis [HN] and hydronephrosis requiring intervention [HNRI]) as well as those patients at low risk of obstruction that would require surgical intervention. As a secondary analysis, the authors evaluated the additional value of RUS by assessing the presence of other non-HN but clinically useful findings. The investigators also assessed RUS use at Yale–New Haven Hospital (YNHH), in New Haven, Connecticut, and the effectiveness of RUS screening in terms of number needed to screen. This was a cross-sectional study [Arch Intern Med. 2010;170(21):1900-1907] of 997 US adults >18 years of age admitted to YNHH between January 1, 2005, and May 1, 2009, all of whom were diagnosed as having AKI and who underwent RUS to evaluate elevated CR level. Data on candidate risk factors, as well as demographic data, were abstracted from medical records. A multivariable logistic regression model was developed to create risk strata for HN and HNRI as detected by RUS imaging. There were 100 patients with HN and 100 patients without HN in the derivation sample (mean age, 65.6 years; 56.5% male; and 25.5% black). Overall, patients with HN were more likely to have a previous diagnosis of HN (on RUS or abdominal/pelvic computerized tomography; 3% of patients without HN vs 28% of patients with HN; P<.001); a history of abdominal or pelvic cancer (14% vs 38%; P<.001); previous pelvic surgery (10% vs 19%); a single functional kidney (1% vs 6%; P=.054); or hematuria (4% vs 13%; P=.02) during the selected admission. Patients with a normal RUS result were more likely to have granular casts on urinalysis (28% vs 18%); a white blood cell count >16,000/μL (48% vs 24%; P<.001); a history of congestive heart failure (22% vs 13%; P<.09); documented hypotension during the current admission (55% vs 39%; P=.01); or exposure to either aspirin (>81 mg/day), a diuretic or angiotensin-converting enzyme inhibitor, or intravenous vancomycin during the current admission (63% vs 39%; P=.001). There were 797 patients in the validation sample. Of these patients, 54.6% were male and 22.8% were black. Overall, 10.6% had HN, of which 31.7% required an intervention (3.3% of total sample). Of the 797 patients, 223 (27.8%) were assigned to the low-risk group, of whom 3.1% had HN and 1 patient (0.4%) had HNRI. The prevalence of HN was 10.7% in the middle-risk group and 16.1% in the high-risk group. Since specific factors can identify patients unlikely to have HN or HNRI, the authors said that cost-savings could be realized by limiting RUS to higher-risk patients and curtailing its use in lower-risk patients. They added that “for inpatients with AKI, directing RUS studies toward those at greater risk of obstruction will aid clinical decision-making and decrease the cost of evaluation.”