Loin pain (renal colic)

• The terms “renal colic” and “ureteric colic” are used synonymously.

• The pain is usually unilateral, experienced in the loin and tends to radiate to the groin.

• A ureteric calculus is by far the most common cause, but other causes include the passage of a sloughed papilla or blood clot.

• Non-Enhanced CT (NECT) is the gold-standard imaging test for the diagnosis of renal/ureteric colic.

• It is more accurate than intravenous urogram (IVU) 1, 2 or US 3-6.

• However, patient outcome is the same whether CT or ultrasound is used as the primary diagnostic imaging test in patients presenting with typical symptoms of uncomplicated ureteric colic. US was associated with lower cumulative IR doses but no other significant difference in patient outcomes between the use of CT and US 7, 8.

• A recent study suggests that Point-Of-Care US (POCUS) is associated with shorter length of stay in the Emergency Department without differences in outcome compared to CT 9.

• A 2022 study found that the use of an US-first strategy resulted in a modest decrease in the number of CT scans performed 10.

• US performed by radiologists, compared to POCUS, is less likely to result in follow-up CT scans, but increases visit times within the ED 7.

• There is some evidence that supplementing US with plain film KUB is beneficial 11 in the initial investigation. The latter is often omitted 12, however, a plain KUB may be useful for follow-up

  Plain radiographs.

• CT in the emergency department rarely alters immediate management 13, 14.

• Adverse events are rare 15.

• Despite the introduction of low-dose CT protocols, because many patients are young and have recurrent episodes of renal colic, there is concern about cumulative radiation dose. Therefore US, despite its lower sensitivity, has been recommended by several authorities as the initial imaging modality 12, 16-18, with no sacrifice in patient outcome, thus avoiding the need for CT in about 70% of cases 19.

• Despite the above, expert consensus guidelines differ in their recommendations as to whether US or CT should be the initial imaging procedure in patients with first presentation with symptoms of acute ureteric colic (SEE BELOW).

• In practice, in uncomplicated renal colic, the choice of US or CT will depend on several factors 20, including

• Age of patient/ionizing radiation: Although ionizing radiation (IR) exposure may be minimised by the recommended use of low-dose, non-enhanced CT (LDCT) protocols, it is preferable to avoid radiation in younger patients who are more susceptible to its effects. It is also an important consideration as many patients may go on to have recurrent episodes of renal colic and require recurrent imaging with resultant accumulative IR dose. In older patients, IR exposure is less of an issue and, in addition, it is likely that alternative diagnoses will be more common 21; therefore, CT is a preferable primary modality in this age group.

• Local availability of modalities and/or expertise:  US has the advantage of being widely available and able to be performed at point-of-care by personnel trained in the techniques, including members of Emergency Departments.

• Body habitus: US may be limited by patient habitus and bowel gas. US is less accurate in patients with increased BMI. It may be advisable for those patients with BMI >30 to undergo CT rather than US.

• Uncertain clinical diagnosis: CT is more accurate in detecting alternative diagnoses and is therefore preferable where diagnosis is in doubt.

• Pregnant patients: IR should be avoided. Therefore, US or MRI should be the primary investigation of choice. Non-calculous causes of acute loin pain (particularly acute pyelonephritis) are more common than ureteric calculi.

• In patients with a clinical suspicion of complications, either at presentation or developing later, CT (low-dose non-enhanced followed, where necessary, with post-IV contrast including delayed images i.e. CT-IVU) is preferable to US. clinical suspicion of complications.

EXPERT CONSENSUS GUIDELINES

• European Association of Urology 22 (2024) recommendations:

• Non-enhanced CT (NECT) is the standard, but LDCT can be performed to reduce IR dose

• NECT should follow the initial US assessment

• However, it is unclear from the recommendations whether NECT should routinely follow even if the US examination is positive. If so, this would appear contentious given that:

• Although US has only moderate sensitivity in ureteric colic, it is highly specific

• The majority of patients will pass their calculi without intervention

• The addition of CT when US is positive subjects the patient to unnecessary IR exposure

• The National Institute for Health and Care Excellence (NICE) (UK, 2019) 23, 24 states:

• “There is uncertainty about which imaging modality should be the first-line investigation in the acute setting of suspected renal colic” 24, however:

• NICE recommends:

• Urgent (within 24 hours of presentation) US as first-line imaging for children and young people with suspected renal colic. Note that “young people” are not defined.

• If there is still uncertainty about the diagnosis of renal colic after US for children and young people, consider low-dose non-contrast CT.

• Canadian Association of Radiologists (2024) recommends 17:

• In younger adults US +/- abdo KUB should be the initial imaging investigation. If further imaging is required, CT should be performed

• In older adults CT should be performed as the initial imaging

• Canadian Urological Association (2021) recommends that US with KUB X-ray should be considered the initial modality of choice 12

• American College of Radiology Appropriateness Criteria 25 (2023) state:

• CT is “usually appropriate”

• US “may be appropriate (disagreement)”

• Australian College of General Practitioners (2021) 18 recommends:

• US should be used in younger patients (<50)

• British Association of Urological Surgeons (2018) 26 recommends:

• Non-enhanced CT is the initial investigation of choice

• The American Urological Association (2013) 27 recommends:

• CT

• Faget et al in 2021 28 recommended the use of US for uncomplicated renal colic followed by CT only if US (and plain KUB in their series) are negative but there is a “STONE” score >10.  The STONE score is based on gender, duration of pain, ethnicity, presence of nausea and vomiting, and haematuria on the dipstick. This strategy led to a sensitivity and specificity of >95%, and identified 84% of stones requiring urological management, while markedly reducing the number of CT examinations

IN SUMMARY:

Therefore, ultrasound would seem to be a reasonable policy for first presentation of suspected uncomplicated ureteric colic to employ a selective policy with regard to initial imaging (leaving aside questions of local availability and expertise) which will reduce the number of patients undergoing CT:

• Younger patients* to undergo initial US 17, 18, 23, 24, 29

• In view of the high specificity of a positive US and the probability that stones will be passed spontaneously in a large proportion of patients 30, to proceed to CT if:

• US is technically unsatisfactory or uncertain

• There are suspected complications

• CT is required to determine management (surgical intervention or determination of stone size and/or composition)

• Non-enhanced CT (NECT) should be performed if US is negative and there is continued high clinical suspicion of renal colic

• In older patients, where IR is not as large a consideration, NECT should be the initial imaging

<>* Age cut-off for US versus CT as primary imaging investigation at first presentation is arbitrary - not based on hard evidence. 50 years of age is the cut-off recommended by the Australian College of GPs 18, the Australian College of Emergency Medicine 31 and the Choosing Wisely campaign, while 45 years is the cut-off recommended by the Kaiser Permanente group 29. It is recommended that primary imaging choice based only on age should regard the cut-off point for US or CT as flexible.

• In patients with a clinical suspicion of complications, either at presentation or developing later, CT (low-dose non-enhanced followed, where necessary, with post-IV contrast including delayed images i.e. CT-IVU) is preferable to US

• The CT can be tailored to the suspected complication. For example, for suspected infective complications (obstructive pyelonephritis or pyonephrosis) a post-contrast PV/nephrographic phase may be indicated, whereas for suspected collecting system rupture and/or urinoma a post-contrast excretory phase (i.e. CT-IVU) will be required

• For fever in the presence of a solitary kidney, a non-contrast CT is advisable

• If renal failure (in the presence of a solitary kidney or bilateral calculi) is a complication, a non-contrast CT alone is indicated

• Non-enhanced CT is useful when intervention is planned (surgical intervention, lithotripsy or medical dissolution), in order to determine:

• The exact size of the stone 

• Its location

• Its likely composition – calcium content, urate stones

• Where CT is available, conventional (radiographic) IVU has been essentially superseded

• If neither US nor CT are available, there may still be a role for IVU as a primary imaging investigation

• If US, but not CT, is available, and US is equivocal or intervention is planned, IVU may be indicated

The US diagnosis of ureteric colic is based on the following findings:

• Visualisation of calculi

• The sensitivity of US (+/- colour Doppler) for the detection of ureteric stones varies according to stone size and location, with reported sensitivities being very variable - usually in the range of 60-90% 3, 32-34

• Mid-ureteric calculi are particularly difficult to visualize on US

• However, specificity of US for detecting ureteric calculi is 94-100% 3, 35

• Stones missed on ultrasound are typically small 6, 36

• Indirect findings 19

• Hydronephrosis

• Asymmetry or absence of ureteric jet on side of symptoms

• Increased Resistive Index in affected kidney 37

• Colour Doppler ‘twinkling’ artifact incorporating these indirect findings into the analysis significantly increases the sensitivity of US 3, 19, 33, 38

• Perinephric fluid

Advantages of US:

• No ionising radiation (IR), thus safe and repeatable without risk

• Able to be performed at point-of-care by personnel trained in the techniques, including members of Emergency Departments

• Despite the lower accuracy of US compared to CT, US is recommended by various authorities as the initial imaging modality First presentation: Choice of US or CT as primary investigation, with no sacrifice in patient outcome, thus avoiding the need for CT in about 70% of cases 119

• High specificity for the diagnosis of renal colic (see below)

• Patients who undergo US in the emergency department for suspected renal colic undergo fewer cumulative CT scans within the 90 days following their visit than do patients initially undergoing CT 39

Disadvantages of US:

• More operator dependent than CT

• May be limited by patient habitus, bowel gas, etc

• Only moderate sensitivity for ureteric calculi, but high specificity (sensitivity of 45% and specificity of 94% for ureteric stones) 7, 22, 32, 40

• The diagnostic evidence review informing the NICE (UK) guidelines of 2019 found the published US studies (using CT as the reference standard) to be of very low quality but reported US sensitivity and specificity of 0.6 and 0.9 respectively 41

• Less ability than CT to detect alternative causes of symptoms

• US tends to overestimate the size of calculi 32

• Patients who undergo US as the primary investigation, in whom the result of US is negative or uncertain should proceed to CT if:

• There is continued clinical suspicion of renal colic

• There is clinical evidence of complications

• There is an alternative diagnosis that requires clarification

• Where the initial investigation is US and the result is inconclusive (i.e. no hydronephrosis or other indirect signs of an obstructed ureter) or technically inadequate, then the next test is a CT. A Portal venous phase CT with contrast is usually preferable to a non-contrast LDCT, since the absence of US findings (direct or indirect) does dramatically reduce the probability of an acute symptomatic stone and, therefore, the emphasis is to look for an alternate diagnosis e.g.: diverticulitis, appendicitis etc.

• Patients in whom US is positive should proceed to CT if:

• Intervention is planned

• There is clinical/US evidence of complications

• Non-enhanced CT (NECT) is the ‘gold-standard’ for diagnosis of ureteric colic and is used in many institutions as the first-line imaging examination

• It is more accurate than IVU 1, 2 or US 3-6

CT protocol:

• Non-enhanced CT (NECT) - preferably low-dose

• Note that scanning the patient in a prone position helps to distinguish stones at the vesico-ureteric junction from phleboliths and also aids identification of small stones in the bladder (which will sink to the most dependent part of the bladder)

• Follow NECT with contrast-enhanced CT if NECT 

• Suggests complication

• Suggests alternative diagnosis

• Intervention is planned

Advantages of CT:

• High sensitivity and specificity

• Low-dose CT is 98% sensitive and 100% specific for detecting indirect signs of renal colic. In patients with BMI <30, sensitivity of detecting ureteric calculi <3mm and > 3mm in size is 86% and 100%, respectively. For all patients, corresponding figures were 83% and 97-100% 42

• Utility in determining the chemical nature of the stone, stone size and position, thus helping to guide management

• Dual-energy CT increases the ability to characterize all ureteric calculi, discriminating uric acid stones from calcium salt stones 43, 44

• Ability to diagnose alternative causes of symptoms 45

Disadvantages of CT:

• Ionizing radiation, particularly important in younger patients. Also, an important consideration as many patients are young and may have recurrent episodes of renal colic.

Low-dose CT protocols

Low-dose protocols (i.e. <3msV) are essentially as effective as standard-dose CT in diagnosing renal colic.

• A meta-analysis of 7 studies 46 assessing the diagnostic performance of low-dose (<3 mSv) CT for detecting urolithiasis found a pooled sensitivity of 93.1% and a pooled specificity of 96.6%.

• Studies using low-dose CT protocols have shown sensitivities of 93-97% when compared to standard dose CT 47 and IR doses equal or less than that of IVU 47, 48.

• A further study confirmed the accuracy of low dose protocols in patients with a BMI <30; low-dose CT was 86% sensitive for detecting calculi <3 mm and 100% sensitive for detecting calculi > 3 mm42

• These low-dose protocols can be effectively used in acute renal colic 27, 49

• In addition, alternative diagnoses mimicking renal colic can be diagnosed 50. Two studies demonstrated no difference in detecting alternative or additional pathology between "low-dose" and "standard dose" protocols 47, 51

• Some studies have reported ultra-low dose CT (ULDCT) protocols  (i.e. doses <1-1.9mSv) with doses comparable to one plain KUB film or slightly more with persisting high accuracy 52, 53. ULDCT allows accurate measurement of stones (the major factor in determining whether stones will pass without intervention) 53. CT identifies alternative diagnoses that can mimic renal colic in about 10% of cases 52

• For patients with BMI >30, discretionary increase in CT exposure parameters may be needed

• Even with low-dose CT, accumulative IR dose is a significant consideration in younger patients with recurrent episodes of renal colic

Plain radiographs of the kidneys, ureters and bladder (KUB) have only limited use in renal colic:

• It is of limited sensitivity (sensitivity and specificity 44-77%)22, 54

• There is some evidence that supplementing US with plain film KUB is beneficial 11 in the initial investigation, but the latter is often omitted 12

• KUB can identify radiopaque stones which are usually calcium-containing, struvite and cystine stones; uric acid stones, however, are usually radiolucent and are unlikely to be detected

• There is no advantage to the use of plain KUB as an initial investigation of renal colic 55

• After a positive US or CT scan, it may be useful to perform a baseline plain KUB to determine whether a stone is radio-opaque, since if the stone is visible on KUB, then follow-up KUB may aid management by determining the progress of the stone (rather than the more resource-intensive repeat CT or US) 56, 57

• However, if a stone is detected by CT scan and is <600HU in density, it is unlikely to be visible on plain KUB

• The majority of stones pass spontaneously, and the likelihood of passage is related to size 30:

• Within 20 weeks, stone passage occurred in:

• 98% of stones < 3mm in size

• 81% of stones 4mm

• 65% of stones 5mm

• 33% of stones 6mm diameter

• Only 9% of stones >6.5mm

• Predictors for spontaneous passage of ureteric stones include:

• Stone size - the majority of stones <5mm 30. Other studies have found stones <10mm (58) to be more likely than not to pass

• Stone location - patients who present with stones in the distal ureter have a higher chance of spontaneous passage

• Left-sided stones are more likely to pass than right30

Passage of stones

• The majority of stones pass spontaneously, and likelihood of passage is related to size 30

• Predictors for spontaneous passage of ureteric stones include

• Stone size - the majority of stones < 5mm 30. Other studies have found stones < 10mm (58) to be more likely than not to pass

• Stone location - patients who present with stones in the distal ureter have a higher chance of spontaneous passage

• Left-sided stones are more likely to pass than right

Alternative diagnoses:

• Conditions that can mimic renal/ureteric colic include:

• Pyelonephritis

• Acute adnexal pathology in women

• Appendicitis

• Diverticulitis

• Abdominal aortic aneurysm rupture or aortic dissection

• Colonic diverticulitis

• Renal vein thrombosis

• Two studies have reported alternative diagnoses on CT in 10% of patients with a clinical diagnosis of renal colic 50, 59

• Low dose CT is also capable of establishing alternative diagnoses that mimic renal colic 50(50). Two studies demonstrated no difference in detecting alternative or additional pathology between "low dose" and "standard dose" protocols 47, 51

• However, it should be noted that US is also capable of identifying most of the above mimickers

Pregnant patients:

• Ureteric stones are uncommon in pregnancy. Non-calculous causes of acute loin pain (particularly acute pyelonephritis) are more common than ureteric calculi

• Ionising Radiation should be avoided in pregnancy. Therefore, CT is not the preferred primary investigation for suspected renal colic

• US (+/- Transvaginal US (TVS)) is by consensus the preferred initial imaging method and the most widely used 60

• TVS is effective in visualising distal ureteric calculi and helps ascertain the level of a hydroureter 61. TVS also facilitates determination of the presence or absence of ureteric jets (but note that in a small number of patients, particularly in the third trimester, the ureteric jet may be absent without obstruction 62

• Note that normal physiological changes (hydronephrosis) in pregnancy (especially in the second and third trimester) can mimic ureteral obstruction. Physiologic hydronephrosis of pregnancy occurs in >80% of pregnant patients, more commonly occurs on the right than the left, and is generally seen beginning in the second trimester 25

• Expert guideline recommendations for imaging are as follows:

• European Association of Urology 22 recommends ultrasound as the preferred method of imaging, with magnetic resonance imaging (MRI) as a second-line imaging modality. “Judicious use” of low-dose computed tomography should be a “last-line” option in pregnant women

• Canadian Urological Association 12 recommends first-line diagnostic testing for stones in pregnancy is US, but low-dose non-enhanced CT or MRI (without gadolinium in the first trimester) can also be used

• The National Institute for Health and Care Excellence (NICE) (2019) recommends “ultrasound is current practice in place of CT” 41

• The Canadian Association of Radiologists (CAR) (2024) recommends US is recommended as the initial modality 17

•  The American College of Radiology (ACR) (2023) recommends US as “usually appropriate” for the initial or follow-up imaging in pregnancy. There was not agreement by the panel on the use of Colour Doppler. MRI without IV contrast or CT abdomen and pelvis without IV contrast “may be appropriate” 25

• The American College of Obstetricians and Gynaecologists (Committee Opinion 723 (2017)) recommended that CT is justified when medically necessary 63

• If US (+/- TVS) is non-diagnostic and clinical suspicion of ureteric calculi persists, MRI without IV contrast is the preferred second-line imaging investigation. CT should be avoided, especially in the first trimester, but may be used judiciously with a low-dose technique in second and third trimesters 27

• If MRI is unavailable, low-dose CT may be used

• If CT scans are contemplated in pregnancy, a multidisciplinary consensus in consultation with the patient is recommended 63

The following are indications for immediate CT:

• Fever. This may indicate an obstructed infected kidney which requires urgent intervention

• A patient with known solitary kidney. Obstruction of a solitary kidney may obviously lead to renal failure

• Renal failure (acute or chronic)

The CT can be tailored to the suspected complication. For example, for suspected infective complications (obstructive pyelonephritis or pyonephrosis) a post-contrast PV/nephrographic phase may be indicated, whereas for suspected collecting system rupture and/or urinoma a post-contrast excretory phase (i.e. CT-IVU) will be required.

• Non-enhanced CT is useful when intervention is planned (surgical intervention, lithotripsy or medical dissolution), in order to determine:

• The exact size of the stone

• Its location

• Its likely composition – calcium content, urate stones

Recurrent presentations:

The use of imaging and choice of modality depend on the clinical context.

1. If the patient has had previous symptoms of renal colic and has not been documented to have passed the stone spontaneously or been subjected to intervention, it is possible that the recurrent symptoms are related to stone migration or complication.

1. If there are no clinical features of complication and the previous stone(s) is known to be radio-opaque on plain KUB, then plain radiography may be repeated. Otherwise, US may be appropriate

2. If there are clinical features of complications of stone disease, then CT is preferable

2. If there has been previous documentation of stone passage or the patient is known to have been previously stone free (i.e. the patient is, or suspected to be a recurrent stone former), then new typical symptoms of renal colic may be investigated with US, and CT reserved for those patients in whom diagnosis is in doubt, intervention is contemplated or there are suspected complications. That is, US is preferable to reduce IR exposure that would result from repeated CT scans, especially in younger patients 26. If the patient is a known stone former, particularly if a CT KUB has been performed within the last three months, a urinary tract ultrasound and/or a KUB X-ray may suffice 27. The combination of urinary tract ultrasonography and KUB is a viable option for a known stone former who has previously had radiopaque stones.

In recurrent stone formers with symptoms of uncomplicated renal colic, there may be an argument for no imaging 20 or delaying imaging until or unless there is non-resolution of symptoms within a few days with associated failure of documentation of stone passage, or development of complications, or contemplation of intervention.