Link Between Calcium Oxalate Crystals in Urine and Kidney Stones

Medically reviewed by Matthew Wosnitzer, MD

Calcium oxalate is a crystallized compound that forms when oxalate from foods like leafy green vegetables and nuts binds to calcium. When the levels of calcium and oxalate in your urine are persistently high, the crystals can form into a solid mass known as a renal calculi or kidney stone.

Calcium oxalate stones are the most common type of kidney stone. Roughly 75% of stones are composed either partially or fully of calcium oxalate.

This article explains what causes calcium oxalate crystals to form and under what conditions crystals can develop into stones. It also describes how calcium oxalate stones are diagnosed and treated, including ways to manage pain as a stone is passed.

Calcium Oxalate Crystals in Urine: Are They Stones?

Having calcium oxalate crystals in your urine increases your risk of kidney stones, but doesn't necessarily mean that you have stones. In the human body, the formation of calcium oxalate crystals is very common and generally of little significance.

It is only when other factors contribute that the crystals can start to stick together in your urine and grow into larger masses, known as kidney stones.

Formation

Calcium oxalate is created from the binding of oxalate to calcium. Oxalate, also known as oxalic acid, is either obtained from foods you eat or produced when the liver breaks down (metabolizes) vitamin C and amino acids. It is generally thought of as a waste product.

Under normal circumstances, oxalate binds to calcium in the intestine and is excreted in stool. Any excess, unbound oxalate will be absorbed into the bloodstream and excreted through the kidneys in urine.

Calcium (derived solely from foods or supplements) also leaves the body in urine and stool. Of the calcium that passes through the kidneys, around 98% is reabsorbed into the bloodstream for reuse in the body. About 2% is excreted in urine.

Calcium oxalate stones can form if these systems are disrupted. This can happen if one or more of the following occurs:

• There is too much oxalate in your urine.

• There is too much calcium in your urine.

• There is too little calcium in your intestine for oxalate to bind to.
  

• Your urine volumes are decreased, increasing calcium and oxalate concentration.
  

Oxalate Stones vs. Other Kidney Stones

Calcium oxalate stones may be the most common type of kidney stone, but there are others. These include calcium phosphate stones, which account for 5% of all calcium stones, and mixed calcium stones (composed of oxalate and phosphate), which account for 45%. Only half of all calcium stones are pure oxalate.

Among the other types of kidney stones are:

• Struvite stones: Also known as "infection stones," these account for 10% to 15% of all renal calculi. These are caused by chronic urinary tract infections (UTIs) like those caused by the bacterium Proteus mirabilis that make the urine more acidic, causing phosphate, ammonium, and magnesium to bind together.

• Uric acid stones: These account for 3% to 10% of kidney stones and are largely caused by diets rich in purines. Purines are compounds found in foods like organ meats, seafood, and beer that can raise uric acid levels, causing the formation of uric acid crystals.

• Cystine stones: These account for less than 2% of all renal calculi and are the result of a genetic disorder that causes the amino acid cystine to build up in the urinary fluids. Kidney stones form because cystine does not dissolve in urine.

• Drug-induced stones: These account for less than 1% of all kidney stones and are linked to medications like guaifenesin, sulfa drugs, and certain human immunodeficiency virus (HIV) medications. Substances created by the breakdown of these drugs can accumulate in the kidneys and clump together.

It is possible to have more than one type of stone at once. Many of the same factors that give rise to other stones (such as a high purine diet) also contribute to the formation of calcium oxalate stones.

What Causes Calcium Oxalate Crystals?

Certain conditions can promote the formation of calcium oxalate stones by increasing calcium and oxalate levels and/or reducing urine volumes. Oftentimes, multiple factors are involved.

These include:

• Diet: This includes eating an excessive amount of oxalate-containing foods. A diet high in protein, sodium, and high fructose corn syrup also boosts the urinary excretion of oxalate.

• Dehydration: A lack of fluid concentrates calcium and oxalate in urine. Dehydration can occur when you drink too little fluid and/or exercise or sweat excessively without adequate hydration.

• Obesity: A body mass index (BMI) over 30 increases oxalate production in the liver and increases compounds like uric aid that promote calcium oxalate crystalization.

• Inflammatory bowel disease (IBD): Crohn's disease and ulcerative colitis block the absorption of fat. Because calcium preferentially binds to fat, oxalate is left unbound and is excreted in urine.

• Hyperparathyroidism: An overproduction of parathyroid hormone, which regulates calcium production, can decrease calcium levels and increase oxalate absorption in the gut.

• Gastric bypass surgery: This weight-loss surgery can affect intestinal calcium levels and promote the overabsorption of oxalate in the gut.

• Primary hyperoxaluria type 1 (PH1): This rare genetic disorder increases oxalate production in the liver by up to twelvefold. Kidney stones can start from a very early age.

• Dent's disease: This is another rare genetic disorder that causes proteinuria (high protein in urine), which, in turn, promotes calcium excretion and hypercalciuria (high urine calcium).
  

Symptoms: How Do Oxalate Stones Feel?

Kidney stones can range in size from as small as a grain of sand to as large as a pearl (or bigger).

The shape can also vary. With a calcium oxalate stone, deposits of a type of oxalate called oxalate monohydrate can result in a smoother stone that may be easier to pass. Deposits of another type, called oxalate dihydrate, will result in a jagged stone that is harder to pass.

Kidney stones start to hurt when they cause irritation or urinary tract obstruction. The symptoms can vary by the size and shape of the stone, where the stone is formed, and your anatomy.

Possible symptoms include:

• Severe pain on either side of your lower back

• Vague stomach ache or pain
  

• Urgent and/or frequent need to urinate

• Pain or burning with urination (dysuria)

• Blood in the urine (hematuria)

• Nausea or vomiting

• Fever and chills

• Urine that is cloudy or smells bad

Related: Do I Have Appendicitis or a Kidney Stone?

Diagnosing Crystalized Calcium Oxalate Stones

Kidney stones can be diagnosed with imaging studies, while other tests can help determine whether you have a calcium oxalate stone or some other type of stone.

The determination of the stone type may only be performed after it is passed. Unless there is a reason for an investigation (such as having a kidney stone at a young age), the determination may or may not be relevant to treatment.

The diagnosis of kidney stones typically starts with a physical exam and a review of your medical history, family history, and risk factors. This is typically done by a urologist or nephrologist.

Thereafter, the following tests or procedures may be ordered:

• Abdominal X-ray; A plain film imaging study involving low-dose radiation

• Abdominal ultrasound: A non-invasive imaging study using reflected sound waves

• Abdominal computed tomography (CT): An imaging technique that composites multiple X-ray images for a more detailed, three-dimensional (3D) visualization of an organ

• Urinalysis: An analysis of a urine sample to check for blood, pus, protein, minerals, pH levels, and other features that may indicate which type of stone you have

• Complete blood count (CBC): A blood test that evaluates the cellular composition of your blood and check for abnormalities

• Blood chemistry tests: A blood test to check levels of electrolytes, glucose, and some indicators of kidney function

• Serum oxalate: A blood test that can detect if levels of oxalate are high

• Renal function tests: A battery of blood tests that evaluate the function of your kidneys

Genetic testing may be recommended if type 1 primary hyperoxaluria is suspected in a child with kidney stones or if someone has physical signs of the disease (like changes in eye pigment due to oxalate crystal deposits).

Expert Recommendation

According to the American Urological Association, the current gold standard for confirming kidney stones is a non-contrast CT scan of the abdomen and pelvis.

Dissolving and Passing Oxalate Stones

The main goal of treatment is to pass a kidney stone (meaning to dislodge it and pass it whole or in fragments in urine). In most cases, kidney stones pass without damage but usually with significant pain and discomfort.

Passing calcium oxalate stones can be especially challenging because they tend to be extremely hard and painful to pass without medications or surgery. (Struvite stones, by comparison, tend to be softer and somewhat easier to pass.)

Fluids and pain relievers may be the only treatments needed for smaller stones. Other treatments may be needed for larger stones. In severe cases, surgery may be required.

The determination of treatment is largely based on the size of the stone in millimeters (mm) and how likely it will pass naturally.

Passing a Stone Naturally

Generally, a calcium oxalate stone smaller than 4 mm has a good chance of passing on its own. To aid in the passing, your healthcare provider will likely recommend:

• Ample hydration: Drinking as much as 2 to 3 quarts of water (1.8 to 3.6 liters) per day will increase urination and dilute your urine so that new stones don't form. You can tell if you are drinking enough when your urine is clear or near-clear.

• Pain relievers: Over-the-counter (OTC) nonsteroidal anti-inflammatory drugs (NSAIDs) like Advil (ibuprofen) or Aleve (naproxen) can ease pain and reduce inflammation. If there is blood in your urine, Tylenol (acetaminophen) may be used as it doesn't promote bleeding like NSAIDs can.

• Alpha-blockers: These prescription drugs relax the muscles of the urinary tract and help pass stones quicker and with less pain. Options include Flomax (tamsulosin).

Are There Solutions That Dissolve Stones?

There are no oral solutions you can drink that can dissolve calcium stones. The only solution available works on uric acid stones and can take weeks or months to work. Larger stones or denser stones (like calcium oxalate) do not respond to oral dissolution treatment.

Read Next: The 4 Stages of Passing a Kidney Stone

Surgery and Other Procedures

Only 50% of stones between 4 mm and 6 mm will pass without medical intervention. Anything larger than 6 mm will almost always need medical treatment to help remove them. In cases like these, clinical expertise is needed to determine the best course of treatment, some of which may be more invasive than others.

Options include:

• Extracorporeal shock wave lithotripsy (ESWL): This noninvasive procedure uses percussive shock waves to break down stones so that they can be more easily passed.

• Ureteroscopy: This involves the insertion of a scope, called a ureteroscope, into the urethra (the tube through which urine exits the body) to locate, break up, and remove fragments of stone in the ureter or in the kidney itself. This minimally invasive procedure is usually performed with mild sedation.

• Percutaneous nephrolithotomy (PCNL): This minimally invasive surgery performed under general anesthesia involves the injection of a dye into the kidney to locate the stone, after which a small incision is made in your side or back to break up and extract the stone.

• Laparoscopic lithotomy: This is a minimally invasive surgery in which several small incisions are made to accommodate a narrow, fiber-optic scope (laparoscope) and specialized surgical tools. (Laparoscopic surgery is preferred over traditional open surgery, which is still sometimes used in complicated cases.)
  

Related: Kidney Stone Surgery: Everything You Need to Know

Takeaway

In 2023, The Food and Drug Administration approved Rivfloza (nedosiran), a lactate dehydrogenase A (LDHA)-Directed Small Interfering Ribonucleic Acid (siRNA). Rivfloza blocks the enzyme LDH, lowering urinary oxalate levels and decreasing kidney stone formation. Rivfloza is a once-monthly injection approved for children 9 years of age and adults with (PH1).

Diet Recommendations With Calcium Oxalate Crystals

As diet is a major contributing factor to calcium oxide stones, the avoidance of oxalate-rich foods can help avoid recurrence. Some of these foods should be avoided entirely, particularly if you have a history of recurrence, while others need to be limited.

Foods to avoid include:

• Chard

• Rhubarb

• Spinach

• Star fruit

Foods to limit or restrict include:

• Avocado

• Beets

• Bran

• Canned pineapple

• Carrot juice

• Chocolate

• Cocoa

• Corn grits

• Cream of wheat

• Dates

• Endive

• Figs

• Navy beans

• Okra

• Peanuts

• Potatoes

• Raspberries

• Soy

• Sweet potatoes

• Tea

• Tree nuts

• Vegetable juice

You may be able to enjoy certain oxalate-containing foods by cooking them rather than eating them raw. Boiling is the most effective means, reducing the oxalate content in vegetables by up to 50%. Steaming is far less effective, and baking appears to have minimal effect.

Some experts also recommend the DASH (Dietary Approaches to Stop Hypertension) diet to help guide your eating habits. The diet was created for people with high blood pressure and focuses on vegetables, fruits, whole grains, low-fat dairy, beans, fish, and poultry.

A 2014 study in the American Journal of Kidney Disease found that the DASH diet reduces the supersaturation of calcium and oxalate in the kidneys (meaning these minerals are in high concentration and are prone to form crystals). The diet is also associated with weight loss and a reduction in sodium and protein intake, all of which benefit people with calcium oxalate stones.

Managing Pain During Healing

Even after a stone is passed, you can experience significant pain for several days. With proper rest, hydration, and the appropriate use of painkillers, most people recover quickly.

On the other hand, if you had surgery to remove a kidney stone, the recovery time can take longer. Even with ESWL, some urologists may recommend that you take several days to a week off from work to allow fragments to pass. For PCNL and other more invasive procedures, as many as three weeks may be needed before you can manage without pain.

During this time, the following measures can help ease pain and speed recovery:

• Hydration: Continue drinking plenty of fluid to keep your urine a light, straw color.

• Rest: Sitting can be uncomfortable for several days, so take time to rest and increase activity levels gradually. If your urine is blood-tinged, wait until there is no longer any pink before lifting or engaging in strenuous activity.

• Heat therapy: Applying a heating pad or soaking in a warm bath (when the incision is amply healed) can also help ease pain.

• Oral pain medications: These include OTC painkillers like Advil or Tylenol or prescription drugs like Voltaren XR (diclofenac) or Percocet (oxycodone and acetaminophen).

Why Is Recurrence Possible?

Compared to other kidney stones, calcium stones are the type most likely to recur. This is due to underlying conditions that give rise to them, some of which can be modified (like diet, hydration, and obesity) and others that cannot.

To reduce your risk of recurrence, your healthcare provider will want to manage contributing conditions. This may include treating inflammatory bowel disease with medications like methotrexate, Rinvoq (upadacitinib), and Zeposia (ozanimod) or treating hyperparathyroidism with parathyroidectomy (the surgical removal of the parathyroid gland).

Thiazide diuretics, allopurinol, and calcium citrate supplements are also effective in preventing calcium stones that recur despite lifestyle modifications.

Related: Can Kidney Cleanses Prevent Kidney Stones?

Summary

Calcium oxalate crystals form in the urine when oxalate from foods like spinach or rhubarb binds to calcium. If calcium or oxalate levels are high and/or urine volumes are low, the crystals can stick together and form calcium oxalate stones.

The kidney stones can range in size from a grain of sand to larger than a pea. Passing a calcium oxalate stone can be extremely painful and, depending on the size, may be passed on its own or require surgery or procedures like extracorporeal shock wave lithotripsy (ESWL).

Change in diet, weight loss, and ample hydration can reduce the risk of calcium stone recurrence, Medications like thiazide diuretics and calcium citrate supplements can also prevent recurrence if lifestyle changes don't help.

Read Next: 10 Steps to Prevent Kidney Stones

Read the original article on Verywell Health.