The presence of carbonate crystals (calcium) generally has little clinical significance. They are found in alkaline urine (pH greater than 6.5). The formation of calcium carbonate crystals can be caused by a combination of factors including decreased urine volume or a condition that alkalinizes (increases the pH of) urine, such as a vegetarian diet, chronic diarrhea, urinary tract infections or certain medications. They are also found in individuals with calcium levels that are too high (immobilization, overactive parathyroid glands, bone metastases, etc.).
In individuals with kidney stones, the repeated presence of carbonate crystals and other calcium salts could indicate the probable nature of the stone. It should be noted, however, that most patients with calcium carbonate crystals will never form stones.