24-hour urine
The 24-hour urine test, marker by marker
A 24-hour urine collection is the standard way of working out why you form stones rather than simply that you do. This page explains what each number on the report measures. It does not tell you what your numbers mean — that is your clinician’s job, and we are careful about the difference.
In short
You collect every drop of urine you pass over a full 24 hours into a container the laboratory provides, keep it cool, and send it back. The laboratory measures how much of various substances you excrete in a day and calculates how close your urine came to the point where crystals form. Most stone panels report between twelve and sixteen values.
One collection is a snapshot of one ordinary day. Two collections, months apart, say considerably more — which is why the tool below is built around comparing them rather than reading one in isolation.
What each marker measures
16 markers on a standard stone panelCollection
The total volume of urine collected over 24 hours. Everything else on the report is a concentration, so volume affects all of them.
A waste product excreted at a fairly steady rate for a given body size. Laboratories use it to check whether a collection was complete — an unexpectedly low value usually means some urine was missed rather than anything about your kidneys.
Stone risk
How much calcium is excreted in urine over 24 hours. Calcium is a component of the most common stone types.
How much oxalate is excreted. Oxalate combines with calcium to form calcium oxalate, the most common stone material.
How much citrate is excreted. Citrate binds calcium in urine and slows the growth of calcium crystals.
How much uric acid is excreted. Uric acid can form stones on its own and can also act as a surface for calcium oxalate to grow on.
How acidic or alkaline the urine is. Different stone materials form more readily at different pH values.
Diet markers
How much sodium is excreted, which tracks how much salt you ate during the collection.
How much potassium is excreted, which tracks fruit and vegetable intake during the collection.
How much magnesium is excreted. Magnesium can bind oxalate in the gut and in urine.
How much phosphorus is excreted. Phosphate is a component of calcium phosphate stones.
How much sulfate is excreted, which tracks animal protein intake during the collection.
How much ammonium is excreted. Laboratories read it alongside sulfate and pH when looking at acid balance.
Supersaturation
A calculated ratio describing how close the urine is to the point at which calcium oxalate would crystallise. Calculated by the laboratory from the values above.
The same calculation for calcium phosphate (brushite).
The same calculation for uric acid.
Deliberately absent above: reference ranges. Laboratories differ, and printing ours next to yours would quietly replace the range on your own report with a number we chose. Use the range your laboratory printed.
I have not had one
How to get a 24-hour urine test
Three routes, what each costs, and a letter you can hand your doctor so you do not have to find the words in a seven-minute appointment.
I have results
Organise and trend your results
Enter each test, see every marker against your own lab’s range, and track the direction across collections. Stays in your browser.