Stone size, position and likelihood of passage

01Core principlesThe concepts and mechanisms needed to understand the subject.

A passage estimate is a forecast made before the outcome is known. Stone diameter affects the ability to traverse the ureter, while its location indicates the remaining route and may reflect progress already made. Other features, including impaction, anatomical narrowing and the degree of obstruction, can make a simple size label less informative. Read the measured dimensions and location from the image report rather than infer them from where the patient points to the pain.

EAU 2026 explicitly describes the natural-passage evidence as limited. Its cited series report passage in roughly 49–52% of upper,58–70% of mid and 68–83% of distal ureteric stones. When size and location are considered together, the cited estimates for stones below 5 mm are 89% distally and 71% in the upper ureter. These figures support a direction of effect and a counselling range; they should not be presented as a calibrated prediction for every patient, imaging method or duration of observation.

Probability and eligibility answer different questions. A four-millimetre distal stone may have a favourable chance of passage, but infected obstruction, uncontrolled pain or worsening renal function can make observation unsafe. Conversely, an informed patient with manageable symptoms may reasonably choose a monitored interval of observation for a stone that is less certain to pass, provided the urologist considers active removal unnecessary at that point. Shared decision-making includes the burdens of waiting and procedures, not only the percentage.

Keep renal and ureteric natural histories separate. An asymptomatic lower-pole renal stone is not already travelling down the ureter, so a distal ureteric passage estimate does not describe it. Likewise, fragments after a procedure and a newly diagnosed untreated stone may have different follow-up needs. The recommendation to review an observed stone must specify what will demonstrate passage, what will trigger earlier treatment and who is responsible for the decision.

Key points

Smaller and more distal ureteric stones are generally more likely to pass; quoted passage figures describe study populations rather than a guaranteed outcome for an individual.
Observation requires controlled symptoms, no infection and no threatened renal function, together with an informed patient and a practical reassessment plan.
NICE advises treatment within forty eight hours of diagnosis or readmission when adult ureteric-stone pain remains intolerable or the stone is unlikely to pass; this is distinct from emergency infected-obstruction drainage.
EAU 2026 cites limited-data estimates of 89% passage for distal stones below 5 mm and 71% for similarly small upper-ureteric stones; location remains relevant within a size category.
A reported average passage time of about seventeen days, with a six-to-twenty-nine-day range in the cited evidence, is not a safe waiting period for a patient who deteriorates.
Confirm stone passage and restoration of baseline renal function; symptom disappearance, a normal creatinine or insertion of a drain alone cannot prove the stone has cleared.

02Mechanisms and patternsImportant relationships and how to distinguish them.

Read the full stone description

Establish whether the report concerns a renal or ureteric stone, which ureteric segment is affected and how the dimensions were measured. A short rounded headline may hide multiple stones or a larger dimension in another plane. Clarify an ambiguous report before using a treatment threshold.

Measure the clinical cost of waiting

Ask about recurrent pain, sleep disruption, vomiting, absence from work and repeated emergency visits. A patient who can tolerate an initial episode may no longer regard observation as acceptable after several recurrences. These consequences contribute to the intervention decision even if the scan dimensions are unchanged.

Recognise renal reserve and infection

Fever, systemic illness, anuria, a solitary functioning kidney or bilateral obstruction can outweigh a favourable passage estimate. Do not use a normal total urine volume to infer that the affected side drains, and compare creatinine with baseline rather than an isolated laboratory reference range.

Identify a misleading endpoint

Pain can resolve after stone movement, analgesia or changes in pressure without documented expulsion. A patient who has not recovered a stone may still require imaging or another appropriate assessment. A temporary ureteric stent can restore drainage while leaving the original stone in place.

03Interpreting evidenceInformation, measurements and their limitations.

Reasoning sequence

Consider the information, its meaning and its limitations before deciding what follows.

01
Review of the diagnostic CT
Why
Define stone location, dimensions and obstruction before counselling about passage.
Interpretation and limitations
Use the actual image or radiologist’s report to distinguish distal, middle and proximal ureteric disease. Compare multiple measurements consistently on subsequent imaging. Density, anatomy and visibility on plain imaging may also inform eventual treatment selection, but they are not substitutes for the observation safety assessment.
02
Renal function and infection assessment
Why
Determine whether continued observation remains medically acceptable.
Interpretation and limitations
Repeat creatinine when renal impairment or persistent obstruction is a concern, and obtain cultures and systemic investigations when infection is suspected. A small stone with new fever and obstruction moves into the drainage pathway; it should not remain on an outpatient passage estimate while culture results are awaited.
03
Evidence of actual passage
Why
Establish whether the stone has left the ureter and drainage has recovered.
Interpretation and limitations
Ask the patient to strain urine and retain a recovered fragment for analysis. A convincing recovered stone plus a concordant clinical course may be sufficient in selected circumstances; persistent uncertainty, symptoms, prior obstruction or renal concern can require imaging. Confirm restoration of baseline renal function rather than simply documenting that pain stopped.
04
Proportionate interval imaging
Why
Answer an unresolved question without using repeated CT as an automatic surveillance habit.
Interpretation and limitations
Choose ultrasound, plain imaging for an appropriately radiopaque target, or low-dose CT according to the stone and the question. Ultrasound can assess dilation but may not directly show a ureteric stone. If continued obstruction or uncertain passage will change management, select a test that can actually resolve that uncertainty.

04Applied reasoningWorked examples connecting principles to decisions.

01Worked caseUse favourable odds without promising passageA forty year old has a four-millimetre stone in the distal right ureter.

1CT shows one stone with mild upstream dilation and a normal contralateral kidney. He is afebrile, renal function is at baseline and initial analgesia has controlled pain. He can drink and has transport back to hospital. The team therefore evaluates passage likelihood only after establishing that a period of observation is clinically acceptable.
2The clinician explains that small distal stones often pass and cites the limited-data EAU estimate of about 89% for distal stones below 5 mm, while making clear that it is not his personal guarantee. He chooses observation over an immediate procedure after discussing possible recurrent pain, the need for reassessment and the option to change the plan.
3At the agreed two-week individual review, he has had no further severe pain but has not recovered a stone. Because passage is uncertain and the original scan showed dilation, follow-up imaging is arranged to answer whether obstruction persists; the absence of pain is not used as the final endpoint.
4Before the imaging appointment he catches a stone and supplies it for analysis. Ultrasound then shows resolution of dilation and repeat creatinine remains at baseline. The review records concordant passage and renal recovery, cancels an unnecessary planned stone intervention and begins prevention assessment from the recovered material. The two-week review was an individual plan, not a universal guideline interval.

02Observation becomes unsuitableAct on a change in symptoms or renal functionA patient under observation returns with a reason that active treatment may now be needed.

1Reassess the severity and cause of pain, compare renal tests and establish whether the stone continues to obstruct. Recurrent intolerable pain, persistent obstruction or deterioration of renal function can outweigh the earlier expectation of passage. Infection with obstruction or anuria prompts an emergency drainage assessment.
2For an adult ureteric stone with ongoing pain that is not tolerated, or a stone judged unlikely to pass, apply the NICE recommendation to offer treatment within forty eight hours of diagnosis or readmission. Do not turn the average natural-passage time from a cohort into a reason to postpone treatment in a deteriorating patient.
3Discuss the feasible intervention using size, position, anatomy and patient preference. If a stent or nephrostomy is used to address the immediate problem, keep the original stone and subsequent device removal on the plan; successful drainage alone does not close the stone episode.

03Less favourable passageCompare waiting with a timely procedureA stable patient has a larger or more proximal ureteric stone without infection.

1Explain that increasing size and a more proximal position generally reduce the chance of spontaneous clearance. Avoid applying the 89% small-distal estimate to this setting. Review pain burden, obstruction, renal reserve and whether the stone appears impacted before judging the value of continued observation.
2If active removal is indicated, discuss how ureteroscopy and shockwave lithotripsy differ in the likelihood of achieving clearance with one procedure, retreatment, anaesthesia and complications. An intervention recommendation should reflect the renal-versus-ureteric size table and individual anatomy, not an isolated diameter cutoff.
3If a monitored interval is chosen after discussion, document the planned review and triggers for stopping observation. At review, evaluate actual stone position or passage evidence and renal drainage; repeating the original probability statement without checking the outcome is not follow-up.

05Checking understandingVerify the reasoning, revisit uncertainties and apply feedback.

At each review, record pain episodes, vomiting, fever, oral intake and any emergency attendance, because the tolerability of observation can change before the stone dimensions do.
Check renal function and the status of drainage when there was previous impairment, ongoing obstruction or limited renal reserve; do not monitor symptoms alone.
Ensure the patient knows who reviews pending imaging and what to do if the planned appointment does not arrive; an unowned follow-up request can leave persistent obstruction unrecognised.
Close observation with a documented outcome: recovered or imaged passage with appropriate renal recovery, a continuing justified plan, or actual intervention and device follow-up.

06Special situationsVariants, exceptions and circumstances that change the usual approach.

A range is more honest than a promise

The EAU figures come from limited and heterogeneous natural-history data. Presenting a favourable chance can help a patient choose observation, but the clinician should also describe what would make that estimate inapplicable and when an alternative strategy becomes preferable.

Time to passage is conditional

The reported average of about seventeen days describes observed outcomes in the cited studies. It is not an instruction to wait seventeen days with infection, renal dysfunction or refractory pain, and it does not mean that every stone remaining at that time needs the same operation.

A drain and a stone are different endpoints

A stent can relieve pressure around a retained stone. The patient may feel better while still needing definitive treatment and later stent removal, so records should separately track drainage success, stone clearance and device status.

Small renal stones need their own discussion

NICE permits informed watchful waiting for selected asymptomatic renal stones. That is a different natural-history question from passage of a symptomatic ureteric stone, and the size and location data must not be transferred between these situations without qualification.

07Common pitfallsFrequent interpretation and management errors.

01
Quoting a distal-small-stone passage rate for a proximal or renal stone creates false precision and can distort the treatment discussion.
02
Treating a cohort’s mean passage time as a mandatory conservative-treatment duration can delay care when clinical safety has changed.
03
Calling a patient stone-free because pain has stopped or a stent drains well can miss a retained obstructing or subsequently symptomatic stone.
04
Framing a change from observation to intervention as failure may discourage timely reporting of symptoms; a revised decision can be the appropriate response to new information.

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