Expansion
Spot ratios have replaced timed collections for most purposes
Expansion
Spot ratios, which have replaced timed collection for most indications
- Albumin to creatinine ratio (ACR): the standard for diabetic kidney disease and chronic kidney disease staging. A first morning sample is preferred. Above 3 mg/mmol is significant; above 70 warrants nephrology referral
- Protein to creatinine ratio (PCR): used where non-albumin protein matters, and in pregnancy for pre-eclampsia, where 30 mg/mmol is the threshold
- Calcium to creatinine ratio: distinguishes familial hypocalciuric hypercalcaemia from primary hyperparathyroidism
Dividing by creatinine corrects for how concentrated the urine is, which is what a timed collection was doing in a more laborious way.
Where a 24 hour collection is still used
- Urinary free cortisol for Cushing’s syndrome
- Metanephrines and catecholamines for phaeochromocytoma, though plasma metanephrines are now often preferred
- 5-HIAA for carcinoid
- Creatinine clearance, where eGFR is unreliable, such as extremes of muscle mass, and before some chemotherapy
- Stone analysis: calcium, oxalate, citrate, urate
- Sodium intake assessment, and some electrolyte studies
Sources of error in a timed collection, which are substantial
- Over- or under-collection, the commonest problem. Adequacy is checked against expected creatinine excretion, roughly 15 to 20 mg/kg per day in women and 20 to 25 in men
- Discarding the wrong sample: the collection starts after the first void and ends with a final void at the same time next day
- Incorrect preservative or container, and failure to refrigerate
- Dietary and drug interference: caffeine, bananas and paracetamol for metanephrines and 5-HIAA, depending on the assay