01Purpose and principlesWhat the assessment is for and the core concepts behind it.
Composition is a clue to the mechanism that allowed crystals to form and persist. Calcium oxalate may reflect concentrated urine, excess calcium or oxalate, reduced citrate or combinations of these. Calcium phosphate directs attention to alkaline urine, hypercalciuria and disorders such as hyperparathyroidism or renal tubular acidosis. A mixed stone can contain more than one mineral and can change between episodes; management should follow the actual result and biochemical pattern rather than a permanent label assigned after one event.
Serum and urine tests answer complementary questions. A raised blood calcium concentration may indicate a systemic disorder, whereas high urinary calcium can occur with a normal serum calcium. Low urinary citrate removes an inhibitor of crystallisation and can accompany metabolic acidosis or hypokalaemia. Urinary pH changes the solubility of particular minerals: persistently acidic urine promotes uric acid, while very alkaline urine can support infection-related minerals. A result gains meaning through its units, collection quality and the physiological state in which it was measured.
Basic assessment should not be withheld until a second stone. NICE says to measure serum calcium in adults and consider stone analysis; EAU recommends a reliable analysis and basic metabolic assessment for all stone formers, then reserves a more specific programme for high-risk patients. Risk includes recurrence and the consequences of recurrence. A solitary kidney does not itself necessarily increase the tendency to crystallise, but avoiding another obstruction is particularly consequential. CKD, nephrocalcinosis and certain hereditary or bowel disorders also change the investigation threshold.
Specific urine testing is most useful after the acute disturbance has settled. Vomiting, IV fluids, infection and an unusual hospital diet can distort the everyday biochemical exposure that prevention is meant to change. EAU 2026 advises two consecutive 24-hour collections no earlier than twenty days after an acute episode and preferably around three months, when stable, infection-free and on usual intake. The collection method and preservative must match the requested laboratory tests; a well-labelled but incomplete container is not a reliable twenty-four-hour result.
Key points
- NICE recommends serum calcium measurement in adults with renal or ureteric stones and consideration of stone analysis. EAU recommends reliable stone analysis and basic metabolic evaluation for all stone formers, with specific metabolic testing for high-risk patients.
- Use infrared spectroscopy or X-ray diffraction for recovered stone material. Radiopacity, CT density and urine pH provide clues but do not establish a definitive composition on their own.
- EAU specific metabolic evaluation uses two consecutive 24-hour urine collections when stable, infection-free and on the usual diet and fluid intake: no earlier than twenty days after the acute episode and preferably around three months.
- Interpret urine volume, calcium, oxalate, urate, citrate, sodium and pH with serum calcium, renal function and the clinical history. High serum calcium needs a PTH-based assessment; calcium phosphate disease can reflect hyperparathyroidism, distal RTA or infection.
- Pure uric-acid stones are associated with acidic urine and may occur without excess urate excretion. Ammonium urate and infection stones have different biochemical contexts, so their prevention cannot be inferred from the word urate alone.
- Early onset, family history, recurrence, cystine, urate, infection or brushite stones, nephrocalcinosis, bowel disease and reduced renal reserve require a more directed assessment. Children need expert metabolic assessment rather than an adult routine pathway.
02Indications, selection and cautionsWhen it is useful, when urgency changes and important limitations.
Ask about recurrence, family history, dietary patterns, supplements and hypercalcaemic symptoms. Calcium oxalate monohydrate and brushite can resist SWL, but procedure resistance does not identify the metabolic cause. Calcium phosphate may be brushite without UTI or carbonate apatite associated with infection; the distinction matters.
Low urine pH, metabolic syndrome, gout or chronic diarrhoea can support a uric-acid mechanism, sometimes without hyperuricosuria. Ammonium urate instead suggests another context, including infection, malabsorption, ileostomy, laxative abuse, nutritional deficiency or hypokalaemia. Check culture and the clinical history before proposing the same alkalinisation strategy.
Urease-producing organisms generate ammonia and alkaline urine, favouring struvite and some associated minerals. A mixed calcium and struvite result may indicate infection of an existing metabolic stone. Both residual infected material and an underlying metabolic abnormality can remain relevant after the acute infection is controlled.
Young age, affected relatives, repeated bilateral stones or nephrocalcinosis should prompt specialist consideration of cystinuria or primary hyperoxaluria and other inherited disease. Ask about bowel resection, bariatric surgery and malabsorption because enteric oxalate absorption can rise. Reconcile medicines and supplements; a suspected drug effect requires a specific exposure assessment rather than assuming every radiolucent stone is uric acid. Separate direct crystallisation of a drug, such as indinavir, from a drug-induced change in urine chemistry, such as with topiramate. Rare 2,8-dihydroxyadenine stones can indicate adenine phosphoribosyltransferase deficiency; xanthine stone formers usually have low serum urate. These rare compositions carry high recurrence risk and require reliable analysis and specialist investigation rather than an assumed uric-acid label.
03Method and interpretationA systematic approach to the test and its findings.
Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.
- 01
Reliable analysis of recovered materialFirst step - Why
- Identify the mineral composition that guides risk and treatment.
- Interpretation and limitations
- Send a passed or removed fragment for infrared spectroscopy or X-ray diffraction, with appropriate clinical information. Chemical wet analysis is considered obsolete by EAU. Repeat analysis can be appropriate with recurrence despite prevention, early recurrence after clearance or a late new episode because the composition may have changed.
- 02
Serum calcium and targeted blood tests - Why
- Identify systemic causes and interpret urinary findings safely.
- Interpretation and limitations
- Measure calcium in adults, accounting for albumin or using ionised calcium as appropriate, alongside renal function and selected electrolytes, phosphate and urate. Elevated calcium requires PTH assessment; an inappropriately unsuppressed PTH during hypercalcaemia can be important even if it falls within the laboratory reference range. Repeat borderline findings in a suitable state rather than assigning idiopathic hypercalciuria prematurely.
- 03
Two consecutive 24-hour urine collections - Why
- Characterise habitual stone-promoting and inhibiting exposures in high-risk disease.
- Interpretation and limitations
- Coordinate with the laboratory on collection completeness, refrigeration or preservative and requested tests. Quantify volume, calcium, oxalate, uric acid, citrate, sodium and other indicated constituents using the correct daily units. Collect at the stable post-episode time described by EAU; a spot concentration cannot simply be relabelled as daily excretion.
- 04
Urine pH, culture and selected specialist tests - Why
- Distinguish acid-base, infectious and inherited mechanisms.
- Interpretation and limitations
- Repeated fresh pH measurements are useful where alkalinisation is contemplated. A24-hour pH above 6.2 may suggest distal RTA after UTI is excluded; it is not diagnostic from one dipstick. Cystine can be identified by stone analysis or quantified in urine, while marked oxalate excretion with an appropriate phenotype prompts specialist primary-versus-enteric evaluation. Paediatric spot ratios use age-specific interpretation when full collections are impractical.
04Clinical next stepsHow the result changes management or prompts escalation.
01Worked caseFind a systemic driver before treating urinary calciumFirst stepA forty four year old woman has a second calcium-phosphate-containing stone and no current infection.+
- 1Stone analysis by infrared spectroscopy shows mixed calcium phosphate and calcium oxalate. Her albumin-adjusted serum calcium is 2.78 mmol/L. The clinician arranges repeat calcium with PTH and reviews supplements, renal function and symptoms before calling the problem idiopathic hypercalciuria or starting a calcium-lowering urinary drug.
- 2Repeat calcium is 2.76 mmol/L and PTH is 9.1 pmol/L against the supplied laboratory range 1.6–6.9. Kidney function is preserved. Two properly collected stable-state urine samples also show high calcium excretion. The concordant results support PTH-dependent hypercalcaemia, and endocrine assessment confirms primary hyperparathyroidism after evaluating relevant alternatives.
- 3The endocrine and surgical teams complete an individual assessment and she undergoes parathyroid surgery. Histology identifies an adenoma. Subsequent calcium is 2.39 mmol/L and follow-up assessment shows improvement in urinary calcium; the team explains that treating this systemic driver does not remove every future stone risk.
- 4The final investigation record links the recovered composition, repeated serum abnormality, hormonal result and observed response to treatment. General stone-prevention advice and surveillance remain in place, while an unnecessary empirical thiazide prescription for presumed idiopathic disease has been avoided.
02Perform a useful metabolic collectionPlan and interpret testing after the acute episodeA recurrent stone former has recovered from colic and is referred for a specific biochemical assessment.+
- 1Check that infection has cleared and the patient has returned to usual food and fluids. Use the EAU timing of no earlier than twenty days and preferably around three months, then arrange two consecutive 24-hour collections with the laboratory’s instructions. Explain exactly how missed urine or a collection timing error should be reported.
- 2Interpret both collections alongside creatinine, calcium, electrolytes, medication exposure and dietary history. Repeated low volume suggests a different primary action from isolated hypocitraturia, high oxalate or high urinary calcium. Discordant or implausible results should prompt a collection-quality discussion rather than immediate multi-drug treatment.
- 3Link each important abnormality to an action and a way to assess response. EAU suggests repeat 24-hour urine eight to twelve weeks after starting pharmacological prevention, allowing dose or diet adjustment; once parameters have normalised, annual collection is suggested, with acknowledgement that evidence for precise intervals is limited.
03Recognise a different risk pathwayEscalate childhood, inherited or infection-related diseaseEscalationA young patient, family cluster or unusual stone result changes the expected work-up.+
- 1For children and young people, seek expert metabolic assessment rather than using adult daily-excretion ranges and prescriptions. Early disease, nephrocalcinosis, bilateral recurrence or a strong family history can warrant a specialist hereditary evaluation. A cystine result is high-risk information even when the current stone episode is uncomplicated.
- 2For struvite or mixed infection stones, review cultures and residual stone burden, address infected material and assess metabolic abnormalities after stone removal and infection control. Avoid interpreting an infection-distorted pH as the patient’s stable noninfectious baseline.
- 3For a bowel or medication-associated pattern, verify the actual disease, operation or exposure and coordinate a targeted response with the relevant specialist. Document the confirmed mechanism or remaining uncertainty and the next discriminating investigation, rather than collapsing several rare causes into an unsupported diagnosis.
05Risks, monitoring and follow-upComplications, safety checks and further assessment.
- Check that every recovered stone result reaches the clinician responsible for recurrence assessment; a composition report arriving after discharge should still change the prevention discussion.
- Review collection timing, completeness and units before interpreting 24-hour results, and keep the patient’s usual intake and recent illness context with the report.
- Follow any abnormal serum calcium, potassium, bicarbonate or renal function to a documented explanation and action rather than concentrating only on urinary numbers.
- When prevention begins, define the biochemical and clinical outcomes being followed, including adverse effects, new stone events and renal function; a better urine result is useful but does not guarantee zero recurrence.
06Special situationsVariants, exceptions and circumstances that change the usual approach.
A normal-range PTH can be abnormal in context
PTH should be suppressed when calcium is elevated. A value within the population reference interval can therefore be inappropriate during hypercalcaemia and should be interpreted with repeat calcium and the wider endocrine assessment.
Radiolucent does not mean invisible on CT
Uric-acid stones are radiolucent on plain films but generally visible on noncontrast CT. Some drug stones have different imaging behaviour, so a normal plain film or an assumed density category cannot replace reliable material analysis. Indinavir is a specific exception to the usual noncontrast-CT detection rule: its stones may be missed, so compatible obstruction and a relevant exposure still need assessment when the scan does not show a stone. A drug history distinguishes this possibility from medicines such as topiramate that alter urine composition; imaging alone cannot identify either mechanism with certainty.
Two urine samples reduce a single-day assumption
Daily intake and excretion vary. Two consecutive collections provide a more useful view than one unusually hydrated day, while neither can overcome incomplete collection or a preservative that prevents the intended pH measurement.
Bowel disease can change oxalate handling
In malabsorption, intestinal calcium may bind fatty acids, leaving more oxalate available for absorption. This supports a targeted dietary and specialist strategy; broadly restricting dietary calcium can worsen the mechanism it is intended to treat.
07Common pitfallsFrequent interpretation and management errors.
- 01
Presuming that every calcium stone needs dietary calcium restriction ignores normal calcium requirements and can increase intestinal oxalate absorption.
- 02
Calling a single alkaline dipstick diagnostic of RTA overlooks infection and the need for a coherent acid-base assessment.
- 03
Applying a pure uric-acid alkalinisation plan to ammonium urate or struvite disease can select the wrong biochemical direction.
- 04
Collecting urine during acute vomiting or IV fluid treatment and treating the result as habitual risk can produce an inappropriate long-term prescription.