If you’ve had a calcium oxalate kidney stone, getting a 24-hour urine test is one of the best ways to understand your risk factors and figure out how to prevent future stones. In this post, we’ll walk through which numbers matter, why they matter, and what you can do about them.
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What is a 24-Hour Urine Test?
A 24-hour urine test measures various components of your urine collected over a full day. This test gives insight into what might be contributing to stone formation and helps identify which dietary or medical changes are right for you.
Everyone who has had a kidney stone should have at least one 24-hour urine test done—often more than one. While collecting all your urine for an entire day might feel inconvenient, the information it provides is incredibly valuable.
How to Prepare for the Test
If this is your first test, try to eat and drink like you were at the time you formed your stone. This can be hard, because many people start making dietary changes as soon as they’re diagnosed. But we need to capture a baseline of your “stone-forming” state. Only then can we compare your results to future tests after you’ve made changes.
Ideally, you’d complete two tests—one on a weekday and one on a weekend—to get a more complete picture of your typical patterns.
Key Metrics to Watch
Supersaturation Levels
Supersaturation refers to the concentration of stone-forming substances in your urine. When the concentration is too high, these substances can start forming crystals.
The test will report supersaturation values for:
- Calcium Oxalate (most relevant if you have calcium oxalate stones)
- Calcium Phosphate (still important because many calcium oxalate stones start as calcium phosphate crystals)
- Uric Acid
Target for calcium oxalate supersaturation: Ideally less than 6
If your supersaturations are high, your stone risk is higher. But even if they fall within the lab’s “normal” range, you may still need to lower them—especially if you’re actively forming stones. In that case, the general recommendation is to aim to cut your supersaturations in half.
Urine Volume
Producing more urine helps dilute stone-forming substances. This is one of the most universal and effective strategies for all stone types.
Target: Typically 2.5–3.0 liters of urine output per day, but check with your doctor on their recommendation based you your unique circumstances
This typically requires drinking 3.0–4.0 liters (96–128 oz) of fluid daily. Not all the fluid you drink ends up in your urine—you also lose some through breathing and sweating. If you sweat heavily (e.g., due to exercise or a physical job), you may need to drink even more.
Urine Calcium
High urine calcium is a major risk factor for calcium stones. Anything over 200 mg/day is considered high.
Target: Less than 200 mg/day in the urine
Many people mistakenly think they should eat less calcium to lower urine calcium. But that often backfires. When you eat too little calcium, your body pulls it from your bones to maintain normal blood levels—this can increase urine calcium and harm bone health.
So, high urine calcium usually isn’t caused by eating too much calcium—it’s more often related to sodium intake (more on that below)..
Note: Some people have a condition called primary hyperparathyroidism, which causes very high calcium in the urine and requires medical treatment. Make sure you’re working with a doctor who can rule this out.
Urine Sodium
This is one of the biggest drivers of high urine calcium. When you eat more sodium than your body needs, your kidneys get rid of the extra sodium—and they take calcium with it!
This calcium loss happens even if you aren’t eating much calcium. That’s because the body pulls calcium from your bones to keep blood levels stable.
Target: Less than 100 mmol/day (which equates to ~2,300 mg of sodium per day in your diet)
Your urine report may list sodium in mmol/day. Use this guide to translate:
- 150 mmol = 3,450 mg
- 125 mmol = 2,875 mg
- 100 mmol = 2,300 mg
- 75 mmol = 1,725 mg
- 65 mmol = 1,500 mg
- 50 mmol = 1,150 mg
Even if your result is technically in the “normal” lab range, anything above 100 mmol may still be too high for stone prevention.
Urine Citrate
Citrate binds with calcium in the urine and prevents it from forming crystals with oxalate or phosphate. Higher citrate levels help prevent stone growth.
Target: More than 400–450 mg/day (this varies by lab)
Low citrate is common in people who form calcium-based or uric acid stones. If your level is low, your provider might recommend:
- Mixing ½ cup lemon juice into 2 liters of water and sipping throughout the day (boosts citrate + fluid)
- Eating more fruits and vegetables
- Cutting back on protein, if your intake is high
- Exploring prescription options with your doctor
Medical conditions like chronic diarrhea or renal tubular acidosis can also lead to low citrate levels. If you have symptoms, talk to your doctor.
Urine Oxalate
Oxalate comes from certain foods and is also produced by your liver. It binds with calcium in the urine to form calcium oxalate stones.
Target: Less than 25 mg/day in the urine
Conditions like Crohn’s disease, short bowel syndrome, and gastric bypass can increase oxalate absorption so be sure your doctor knows about these conditions.
For most people, eating adequate calcium will effectively lower the urine oxalate number. Why? Because calcium and oxalate bind in the digestive tract and pass through your stool—before oxalate ever reaches your kidneys.
Aim for: 1,000–1,200 mg of calcium from food each day (not supplements!)
If you do this well, you may not need to avoid most oxalate-rich foods. I usually only recommend avoiding very high-oxalate foods like spinach (especially raw), at least to start.
What to Do Next
If your 24-hour urine test shows any of these areas are out of range, don’t panic—these are all areas where nutrition and hydration strategies can make a big difference.
Working with a dietitian can help you create a plan tailored to your unique needs. If you’re interested in more tips, recipes, and support, sign up for my free newsletter to stay in the loop!
And if you’re not sure which stone type you have, or you’re waiting on your test results, start with a general approach: drink more fluids, eat balanced meals, and avoid extremes in your diet. Once your results are in, you’ll be better equipped to take targeted action.
