What are calcium oxalate crystals?

Calcium oxalate crystals are the most common cause of kidney stones — hard clumps of minerals and other substances that form in the kidneys. These crystals are made from oxalate — a substance found in foods like green, leafy vegetables — combined with calcium. Having too much oxalate or too little urine can cause the oxalate to crystalize and clump together into stones.


Oxalate comes from many of the foods in our diet. The main dietary sources of oxalate are:

  • spinach and swiss chard
  • sweet potatoes
  • rhubarb
  • wheat bran
  • almonds
  • beets
  • beans, especially black, cannellini, great northern, navy, pinto, and white beans
  • chocolate
  • French fries and baked potatoes
  • nuts and seeds
  • soy products - tofu, soy milk, soy yogurt, tempeh, soy nuts, edamame 
  • tea - black, green
  • oranges, grapefruit, clementines, tangelo,
  • pomegranate, guava, dried apricots, 
  • raspberries, boysenberries, goji berries, elderberries

When you eat these foods, your GI tract breaks them down and absorbs the nutrients. The leftover wastes then travel to your kidneys, which remove them into your urine. The waste from broken-down oxalate is called oxalic acid. It can combine with calcium to form calcium oxalate crystals in the urine.

The main symptoms of calcium oxalate crystals in the urine are:

  • pain in your side and back that can be intense, and may come in waves
  • pain when you urinate
  • blood in your urine, which can look red, pink, or brown
  • cloudy urine
  • foul-smelling urine
  • an urgent and constant need to urinate
  • nausea and vomiting
  • fever and chills if you have an infection

Urine contains chemicals that normally prevent oxalate from sticking together and forming crystals. However, if you have too little urine or too much oxalate, it can crystalize and form stones. Reasons for this include:

  • not drinking enough fluids (being dehydrated)
  • eating a diet that’s too high in oxalate, protein, or salt

In other cases, an underlying disease causes the crystals to form into stones. You’re more likely to get calcium oxalate stones if you have:

Your doctor might use these tests to find out if you have calcium oxalate stones:

  • Urine test. Your doctor may request a 24-hour urine sample to check levels of oxalate in your urine. You’ll have to collect your urine throughout the day for 24 hours. A normal urine oxalate level is less than 45 milligrams (mg) per day.
  • Blood test. Your doctor can test your blood for the gene mutation that causes Dent disease.
  • Imaging tests. An X-ray or CT scan can show stones in your kidney.



You can prevent calcium oxalate from forming crystals in your urine and avoid kidney stones by following these tips:

  • Drink extra fluids. Some doctors recommend that people who’ve had kidney stones drink 2.6 quarts (2.5 liters) of water each day. Ask your doctor how much fluid is right for you.
  • Limit the salt in your diet. A high-sodium diet can increase the amount of calcium in your urine, which can help stones form.
  • Watch your protein intake. Protein is essential to a healthy diet, but don’t overdo it. Too much of this nutrient can cause stones to form. Make protein less than 30 percent of your total daily calories.
  • Include the right amount of calcium in your diet. Getting too little calcium in your diet can cause oxalate levels to rise. To prevent this, be sure you’re getting the appropriate daily amount of calcium for your age. Ideally, you’ll want to get calcium from foods like milk and cheese. Some studiesTrusted Source have linked calcium supplements (when not taken with a meal) to kidney stones.
  • Cut down on foods that are high in oxalate, like rhubarb, bran, soy, beets, and nuts. When you do eat oxalate-rich foods, have them with something containing calcium, like a glass of milk. This way the oxalate will bind to calcium before it gets to your kidneys, so it won’t crystallize in your urine. Learn more about a low-oxalate diet.