Risk Factors Predicting the Necessity of Renal Ultrasound in the Emergency Department

INTRODUCTION

Renal ultrasound (US) is widely used in the emergency department (ED) as a tool for assessing patients with suspected renal pathology. These pathologies include renal colic, renal obstruction, pyelonephritis including suspected abscess, and chronic and acute renal failure.

During on-call hours, when medical staff is limited, it is crucial to determine and prioritize which renal US examinations should be performed.

Acute renal obstruction or obstructive uropathy, resulting in acute hydronephrosis, may be considered an indication for emergent urological intervention (within 24 h), while other pathologies may be important to diagnose but are less critical to treat urgently, allowing those examinations to be postponed until the morning shift.

Several studies evaluating the use of US in the setting of acute renal insufficiency have found that it is usually unnecessary to exclude obstruction, unless there is a clinical history that strongly supports it, such as flank pain, urolithiasis, pelvic mass, or recent pelvic surgery.^[1-3] Furthermore, only 5% of cases are acute kidney injuries (AKIs) due to obstructive causes.^[4] However, to the best of our knowledge, there are no publications in the literature concerning identification of predictors for hydronephrosis on US leading to urgent urological intervention in patients from the ED.

The aim of this study was to evaluate patient risk factors to predict the necessity of emergent renal US in the ED.

MATERIAL AND METHODS

This retrospective study was conducted at an academic, referral, trauma center. The study received Institutional Review Board approval, with waived informed consent.

All adult patients (>18 years) referred for renal US from the ED during on-call hours (3 pm - 7 am), for a period of two years (May 2015 to April 2017), were included.

Data collected from a review of patients’ medical records included age, gender, blood creatinine and white blood count (WBC) levels, renal US results, and previous medical diagnoses; specifically those predisposing patients to urinary tract obstruction. The latter included a known diagnosis of abdominal and pelvic mass (benign or malignant), benign prostatic hypertrophy (BPH), neurogenic bladder, history of nephrolithiasis, and presence of a permanent urethral catheter. All examinations were performed in the US unit by trained sonographers or residents in training. The resident on-call took the initial readings, and then the on-call physician (board-certified radiologist with 0.5–20 years of experience) performed the final readings. US findings included hydronephrosis, renal stones, renal cysts, and signs of chronic renal disease manifesting as echogenic and atrophic parenchyma and small kidneys.

The primary outcome was urological intervention within 24 h following the US examination. Indications for urgent intervention were septic patient, acute rise in creatinine, acute imbalance in electrolytes, and indurable pain. Urological intervention included nephrostomy or/and urethral stent.

RESULTS

Four hundred and seventy-seven patients were included in the study. The mean age was 64 ± 19 (range 18–98) years. Table 1 lists patient demographic characteristics and medical history. Fifty-eight (12%) patients had a permanent urethral catheter, 49 (10%) had a history of renal stones, and 87 (18%) had a diagnosis of abdominal or pelvic mass.

US results are listed in Table 2. A hundred and forty patients (29%) had hydronephrosis and 41 patients (9%) had visible stones on US.

Only 17/477 (3.6%) patients required an emergent urological intervention.

A background of malignancy, recent and current chemotherapy administration, personal history of renal stones, history of previous nephrostomy, presence of a permanent catheter, and presence of abdominal or pelvic mass all predicted an emergency urological intervention with P < 0.05. Neurogenic bladder also predicted an emergency urological intervention with P < 0.05; however, there were only three such cases [Table 3].

All of the above listed factors were found to have a high correlation with three risk factors: Permanent catheter and/ or history of renal stones and/or known abdominal or pelvic mass. Other factors did not predict emergency urological intervention, including congestive heart failure, acute and chronic renal failure, hypertension, sepsis, diabetes, cirrhosis, BPH, history of previous abdominal surgery, creatinine >1.3 mg/dl, and WBC >11*10³/μl.

Based on the multivariate analysis, 160/477 (33.5%) patients had a permanent urethral catheter and/or a history of renal stones and/or known abdominal or pelvic mass. Of these, only 17 (10.6%) patients received a urological intervention within 24 h of the US examination. However, 317/477 (66.5%) patients did not have a permanent catheter, history of renal stones, nor a known abdominal or pelvic mass. None of these patients received a urological intervention within 24 h following the US examination. The receiver operating characteristic curve (ROC) for the model was calculated as 0.883, 95% CI (0.84–0.92).

DISCUSSION

Prioritizing imaging examinations are crucial in any medical facility, especially referral trauma centers at night. At those hours, the medical staff in the radiology department usually consists of less experienced residents who are generally overwhelmed by the demand for imaging examinations from the ED as well as from inpatient wards.

US has high sensitivity and specificity for detecting renal stones and acute renal obstructions with known advantages, including the low cost and absence of radiation.^[5]

In our institution, we try to reduce the number of CT examinations, to reduce exposure to radiation, by performing US first, especially during on-call hours when only emergent pathologies with a need for emergent treatment should be performed.

Renal US is often recommended in the evaluation of AKI to exclude hydronephrosis even if the pre-test probability for obstructive uropathy is low.^[4,6] Therefore, the examination is ordered in high volumes, very often for no urgent indication.

Although the American College of Radiology (ACR) has published appropriateness criteria for suspected stone disease and renal failure,^[7,8] these guidelines are insufficient for prioritizing all patients referred to a renal US, especially during on-call hours.

Nevertheless, some authors do advocate a more restricted use of renal US, given that the majority of AKI cases in hospitalized patients are due to acute tubular necrosis or pre-renal etiologies, and US results are not expected to change patient management.^[2,3,5]

In our study, a multivariate analysis showed that not one of the patients that had none of the three risk factors (permanent urethral catheter, history of renal stones, or a known abdominal or pelvic mass) needed an urgent urological intervention. However, 10.6% of the patients that had one of the risk factors needed urological intervention urgently. The ROC for the model was calculated as 0.883, 95% CI (0.84–0.92). This result states that the three risk factors; permanent urethral catheter, history of renal stones, or a known abdominal or pelvic mass, are accurate in predicting the outcome of urgent urological intervention.

Sepsis is a common indication for urgent intervention in patients with urinary obstruction, it was not found to be a predicting factor in our study for emergent intervention. This can be explained by the fact that only a small percentage of the patients presenting to the ER with sepsis have an urgent urinary obstruction, and the cause of their sepsis is of a different origin. Therefore, sepsis is not an independent risk factor that can predict urinary obstruction on US that may mandate emergent intervention.

Our study showed that more than half of the patients underwent emergent renal US during on-call hours with no immediate advantage for patient outcome with regard to renal obstruction. This large number of US examinations constitutes an unnecessary work overload. In times of increased demand and decrease in medical staff, this can have a negative impact on the ability of the on-call residents to give urgent service to those who truly need it.

CONCLUSION

Using only these three variables, permanent urethral catheter, history of renal stones, or known abdominal or pelvic mass as risk factors for an outcome of emergent urological intervention, we can greatly reduce the number of renal US examinations done at on-call hours allowing for prioritization of only the necessary examinations.