All patients should have a basic metabolic analysis:
- Stone analysis if available
- Urine analysis – UTI, pH
- Bloods – creatinine, sodium, calcium, phosphate, urate, magnesium, phosphate, chloride, potassium
Certain patients should then undergo further analysis:
- Serum PTH levels
- 24-hour urine analysis (x 2 preferably)
- Volume
- pH
- Creatinine (control to assess if true 24 hour collection)
- Promotors
- Sodium
- Calcium
- Oxalate
- Uric acid
- Phosphate
- +/- Cystine
- Inhibitors
- Citrate
- Magnesium
Who should get extended work up?
EAU definition of high-risk stone formers:
- Early onset stones (especially children and teenagers)
- Recurrent stone formers
- Familial stone formation
- Short time between stone episodes
- Calcium phosphate stones
- Uric acid stones
- Infection stones
- Solitary kidney
- Systemic illness predisposing to stone formation – MSK, nephrocalcinosis, neuropathic bladder, sarcoidosis, horseshoe or other anatomical obstruction, RTA
When and how to do the 24 hour analysis
2 consecutive 24 hour samples.
Pre-prepared collection device from laboratory with lined with preservative, stored in the fridge between voids.
Normal diet for patient (or after period of dietary modification).
Ideally stone free for 3 weeks.
Can repeat 6- 8 weeks after dietary or medical intervention.
Give patient written information.
Discard first void (or into toilet) on day of collection. Then collect every void after up to and including first void next morning.
Other notes on 24 hour urinalysis:
Creatinine is a by-product of muscle metabolism and its excretion is relatively fixed and stable. Therefore lower than expected creatinine levels in urine (based on patients weight) suggest incomplete sampling.
Most common finding is low urine volume.
Ideal ‘neutral’ pH is 5.5 – 6.5.