Can The Degree of Hydronephrosis on Ultrasound Predict Kidney Stone Size?


Symptomatic renal colic is a common complaint presenting to the emergency department (ED), with a rate of 126 to 226 per 100,000 ED visits [1]. In the ED, CT is frequently used to make the definitive diagnosis as it allows for determination of stone size and location, degree of hydronephrosis, and evaluation of other pathology that may mimic renal colic. However this is a particularly worrisome approach in patients with recurrent ureteral stones who have been exposed to numerous previous CT imaging studies. Previous data has shown that emergency physician performed ultrasound is accurate at identifying hydronephrosis, which in combination with hematuria, is sufficient for the diagnosis of renal colic [2,3]. Furthermore,  an ultrasound first approach has been shown to be safe and reasonable as an initial evaluation for suspected renal colic [4]. What ultrasound does not tell us about renal colic is the size of the ureteral stone, which can be useful in determining the need for immediate intervention versus medical management. The following study seeks to determine if the degree of hydronephrosis seen on ultrasound performed by emergency physicians, can be predictive of ureteral stone size. 

Can the degree of hydronephrosis on ultrasound predict kidney stone size?

Clinical Question

Can the degree of hydronephrosis on ultrasound predict kidney stone size?

Methods & Study Design

  • Design
    • Retrospective chart review of emergency department (ED) patients at a single academic medical center
  • Population + Inclusion Criteria 
    • Adult patient presenting to the emergency department who had confirmed ureterolithiasis on noncontrast CT and a focused emergency renal ultrasound performed
  • Exclusion criteria
    • No specific criteria
  • Intervention
    • A focused renal ultrasound was performed in the ED by an emergency medicine resident or attending to evaluate for the presence of hydronephrosis as an indicator of obstructive ureterolithiasis
    • All ultrasound examinations were subsequently reviewed for quality assurance by an emergency ultrasound fellowship trained emergency physician
  • Outcomes
    • Each focused renal ultrasound classified the degree of hydronephrosis as none, mild, moderate, or severe and this was compared to the ureteral stone size on noncontrast CT
      • Definitions:
        • Mild hydronephrosis was defined as enlargement of the calices withpreservation of the renal papillae
        • Moderate hydronephrosis was defined as rounding of the calices with obliteration of therenal papillae
        • Severe hydronephrosis was defined as caliceal ballooning with cortical thinning
    • Ureteral stone size was stratified into 2 groups, those 5mm or smaller and those larger than 5 mm, based on the likelihood of successfully spontaneous stone passage


Increasing degree of hydronephrosis seen on focused ultrasound was associated with an increasing proportion of ureteral calculi larger than 5 mm. 113 (87.6%) patients with less severe hydronephrosis  (none or mild) had ureteral calculi 5 mm or smaller. Of the remaining 16 (12.4%) patients with less severe hydronephrosis, none of these patients had ureteral stones larger than 10 mm. There was good interobserver agreement between the degree of hydronephrosis as determined by the performing emergency physician and the quality assurance review (k = 0.847).

Strengths & Limitations

  • Strengths
    • Majority of ultrasound examinations performed by ED physicians making this applicable to point-of-care ultrasound
    • Gold standard was size of ureteral stone on noncontrast CT
    • Good interobserver agreement between ED ultrasound operator and quality assurance review
  • Limitations
    • Retrospective chart review
    • This study only enrolled patients who both a focused renal ultrasound and confirmed ureterolithiasis on noncontrast CT; this would have missed patients who only had either a focused renal ultrasound or noncontrast CT alone (selection bias)
    • No patient centered outcomes data

Authors Conclusion

"In conclusion, our results demonstrate a relationship between the degree of hydronephrosis as determined by emergency physicians on focused emergency ultrasound and ureteral calculi size; patients with less severe hydronephrosis were less likely to have larger ureteral calculi. This suggests that ultrasound can help identify many, but not all, patients who are at lower risk for having larger ureteral calculi.

Our Conclusion

This paper identifies a correlation between the degree of hydronephrosis on ultrasound and ureteral stone size seen on noncontrast CT. Essentially, patients with minimal or no hydronephrosis are very unlikely to have have a large (>5 mm) ureteral stone. Unfortunately, focused ultrasound is not perfect, and in this study  ~12.4% of patients with minimal or no hydronephrosis still had a large ureteral stone. What I found reassuring was that in this group, none of the patients had a ureteral stone > 10 mm, which at most institutions is the cut off for allowing a trial of passage. Even dissecting the data further, of the patients with moderate hydronephrosis, only 2 out of 43 (4.6%) patients had a stone > 10 mm.

This study suggests that focused renal ultrasound can be used to screen patients with suspected renal colic and potentially avoid an unnecessary CT scan. As with any focused ultrasound, the decision to obtain a CT should not be based solely the degree of hydronephrosis but also in conjunction with the clinical history, physical exam and other pertinent factors (previous ureteral stone, previous need for stone intervention, other concerning diagnoses on differential, pain control, institutional culture, urinalysis, etc). 

The Bottom Line

Ultrasound can be used to identify many, but not all, patients who are at lower risk for having larger ureteral calculi. 


This post was written by Michael Macias, MD, Ultrasound Fellow at UCSD.


    1. Teichman JMH. Clinical practice. Acute renal colic from ureteral calculus. N Engl J Med 2004;350:684-93.

    2. Rosen CL, Brown DFM, Sagarin MJ, et al. Ultrasonography by emergency physicians in patients with suspected ureteral colic. J Emerg Med 1998;16:865-70.

    3. Gaspari RJ, Horst K. Emergency ultrasound and urinalysis in the evaluation of flank pain. Acad Emerg Med 2005;12:1180-4.

    4. Smith-Bindman R, e. (2018). Ultrasonography versus computed tomography for suspected nephrolithiasis. - PubMed - NCBI . Retrieved 3 March 2018, from
    5. S, G. (2018). Can the degree of hydronephrosis on ultrasound predict kidney stone size? - PubMed - NCBI . Retrieved 3 March 2018, from

Case # 11: Look and You Will Find

A 32 year old healthy female with no past medical history presents to the emergency department with left sided flank pain x 2 days.

Vitals: T 98.6 HR 72 BP 126/82  RR 12 O2 98% on RA

A bedside ultrasound of the left kidney is performed, what is the next best step in management?

Q40_Simple cyst

Answer and Learning Points


The ultrasound image demonstrates a simple cyst located in the cortex of the kidney. The cyst can be described as anechoic, homogenous, with thin and smooth walls, and would be a type I lesion according to the Bosniak classification system (image below). There is no evidence to suggest obstructing hydronephrosis. The Bosniak classification for renal cysts was developed in the 1980s as an attempt to standardize the description and management of complex renal lesions. Based on classification of the renal lesion, the likelihood of malignancy can also be predicted. While the Bosniak classification was initially described and validated with CT imaging, newer data suggests that ultrasound may be sufficient to follow renal cysts that are minimally complex (Bosniak I & II).

Learning Points


    • Given the bedside ultrasound demonstrates a Bosniak I lesion in the left kidney, the patient can be reassured that this finding is very unlikely to be malignant and she can be referred to a primary care provider for follow up in several weeks for formal outpatient renal ultrasound.
    • Incidental findings are frequently found on point of care ultrasound and while most of them are benign it is of utmost importance to ensure proper follow up when identified. Specifically with renal cysts, this is a common occurrence and most can be followed with renal ultrasound as long as they are simple (Bosniak I or II).
    • One pitfall to be aware of is that renal cysts can be mistaken for hydronephrosis and lead to unnecessary imaging and work up (especially in patients presenting with acute flank pain). Therefore it is critical to note the differences between a simple renal cyst and hydronephrosis. As seen in the comparison above renal cysts tend to (but not always) be located in the renal cortex and are both spherical and very well circumscribed. On the other hand, hydronephrosis is centrally located, and tends to branch outwards like a tree. If there is uncertainty, I recommend performing evaluation in both transverse and longitudinal planes to fully characterize your finding. The opposite kidney in the patient can also be used for comparison.


This post was written by Michael Macias, MD, Ultrasound Fellow at UCSD.


1. Muglia VF, Westphalen AC. (2014) Bosniak classification for complex renal cysts: history and critical analysis. Radiol Bras 47(6): 368–373.

2. McGuire BB, Fitzpatrick JM. (2010) The diagnosis and management of complex renal cysts. Curr Opin Urol 20:349–354.

3. Case courtesy of Dr Matt Skalski, From the case rID: 20989

Case # 9: A Transplant Dilemma

A 52 year old male with a h/o kidney transplant presents to the emergency department with pain over his transplanted kidney site (right pelvic region). He also notes increased weakness, nausea and a significant decrease in urine output. He denies any fever. He states he is compliant with his anti-rejection medications.

Vitals: T 99.0 HR 105 BP 165/91  RR 18 O2 98% on RA

A bedside ultrasound is performed, what is the next best step in management?

tx_severe hydro

Answer and Learning Points


Insertion of foley catheter. The clip above demonstrates severe hydronephrosis of the patient's transplanted kidney. A foley was inserted in the emergency department with immediate output of 1.5 L of clear urine. The patient was found to be in renal failure secondary to his urinary outlet obstruction. He was admitted to transplant surgery and his renal function improved over the next day; he was discharged home with a leg bag and urology follow up. Below is a repeat ultrasound of his transplanted kidney after drainage of his bladder: 

Learning Points

    • Urinary obstruction in a transplanted kidney can be missed initially as pain over the patient's graft site and decreased urine output is easily contributed to possible rejection or infection.
    • The differential diagnosis of acute renal failure in the transplanted kidney is broad (see table below) and emergency department management should include a thorough evaluation for prerenal, intrinsic and post renal causes, in consultation with a transplant service.
    • All renal transplant patients presenting with acute renal failure should have a formal renal ultrasound with doppler to evaluate the graft however often this is not available immediately and a bedside ultrasound can assist with rapid identification of acute urinary obstruction.


This post was written by Michael Macias, MD, Ultrasound Fellow at UCSD.


    1. Kadambi PV., Brennan DC., Chon J. (2017). Evaluation and diagnosis of the patient with renal allograft dysfunction. In T.W. Post, B. Murphy, & A. Lam (Eds.), UptoDate. Available from

Case # 5: It’s Not Always Blood

A middle aged male s/p TURBT (transurethral resection of bladder tumor) 1 day ago presented with lower abdominal pain and no urine output from his foley catheter. A bladder scan was performed which was ~ 50 cc. What's the dx?

Vitals: T 98.7 HR 110 BP 117/70  RR 18 O2 98% on RA (2)

Answer and Learning Point


The image above is of Morrison's pouch, demonstrating significant free fluid within the peritoneal cavity. Given the recent TURBT and lack of urinary output from the patient's foley catheter, this suggests that the fluid identified is consistent with urine secondary to intraperitoneal bladder perforation. This was later confirmed by CT cystogram and shortly after the patient was taken to the operating room for definitive repair.

Learning Points

    • Bladder perforation from TURBT is relatively rare with an incidence of clinically significant perforations of 1.3%. Furthermore, intraperitoneal bladder perforation only accounts for ~17% of these, making it quite uncommon [1]. A small number of intraperitoneal bladder perforations are also associated with small bowel or colon injury [2].
    • While around 30% of bladder ruptures from TURBT are detected intraoperatively, the remainder present postoperatively (mean time to diagnosis of 6 days) with lower abdominal pain and/or decreased urine output [2].
    • CT cystogram is the gold standard for diagnosis of bladder perforation and can provide information on location of the perforation as well as whether it is intraperitoneal or extraperitoneal [3].
    • As demonstrated in the case above, ultrasound can be used as an imaging adjunct at the bedside to rapidly detect intraperitoneal fluid to expedite consultation with urologic services and definitive CT imaging.
    • The treatment of extraperitoneal perforation of the bladder is usually conservative via prolonged foley catheter drainage. For intraperitoneal lesions, open-surgical exploration and repair is recommended [1-2].
    • Emergency department management of these patients should consist of rapid diagnosis, broad spectrum antibiotic therapy, fluid resuscitation as needed, and urgent urological consultation.


This post was written by Michael Macias, MD, Ultrasound Fellow at UCSD.


    1. Rausch S, e. (2017). [Transurethral resection of bladder tumors: management of complications]. - PubMed - NCBI . Retrieved 2 September 2017, from
    2. Golan S, e. (2017). Transurethral resection of bladder tumour complicated by perforation requiring open surgical repair - clinical characteristics and oncological outc... - PubMed - NCBI . Retrieved 2 September 2017, from
    3. COMPLICATIONS OF TRANSURETHRAL RESECTION OF BLADDER TUMORS. Eric A. Singer MD, MA and Ganesh S. Palapattu MD. Complications of Urologic Surgery: Prevention and Management, Chapter 25, 295-302
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