As part of a urinalysis test, urine may be examined under a microscope to check for the presence of crystals. Urine contains many chemicals, some of which can form solid particles, or crystals, under certain conditions. Factors that affect the presence of crystals in the urine include the concentration of the chemical, whether the urine is acidic or alkaline, and the temperature of the urine. Crystals found in urine may indicate an abnormality. However, some types can also be present in normal urine.
Uric acid crystals are diamond or barrel shaped crystals that are typically yellow to orange-brown in color. They can be found in normal urine but may also form when you have a kidney stone or gout. Uric acid crystals may also be present during treatment with certain chemotherapy drugs. Uric acid is a byproduct of a protein digestion. A protein-rich diet can increase uric acid excretion in the urine. Urate crystals are the salt form of uric acid crystals and are typically composed of uric acid plus calcium or sodium. Urate crystals can be needle-like in shape and may also be associated with gout.
Triple phosphate crystals consist of magnesium ammonium phosphate. They are also called struvite crystals. These crystals are colorless and shaped like 3- to 6-sided rectangular prisms. They tend to form in urine with a neutral to alkaline pH. Triple phosphate crystals can be a normal finding but are also commonly associated with urinary tract infections. Certain bacteria that cause urinary tract infections tend to make the urine more alkaline and increase the concentration of ammonia, which favors the production of these crystals.
Calcium oxalate crystals are colorless crystals shaped like a square envelope or an octahedron. They are very common in normal urine at any pH. These crystals are also associated with the presence of kidney stones and ingestion of ethylene glycol, a toxic substance found in antifreeze. It has been suggested that high vitamin C intake is linked to calcium oxalate kidney stone formation, because vitamin C is converted to oxalate. A study involving more than 196,000 adults that was published in March 2016 in the "American Journal of Kidney Diseases" found an increased risk of kidney stones associated with vitamin C intake among men, but not women. Researchers continue to debate whether vitamin C is linked to calcium oxalate kidney stones.
Other Urine Crystals
Other types of crystals are seen less commonly in urine. Calcium carbonate crystals are not associated with disease, but they can sometimes be mistaken for bacteria. They occur in alkaline urine. Certain drugs, including sulfonamide antibiotics or contrast dyes used in medical imaging procedures can also form crystals in the urine. Cysteine crystals are flat, colorless, hexagonal crystals that resemble a stop signs. When these crystals appear in urine, it typically indicates a disease called cystinuria. This is a genetic condition in which the kidneys do not filter amino acids such as cysteine properly. Bilirubin crystals are yellowish or reddish-brown, needle-like or granular crystals that are not normally found in urine but are present with several types of liver disease.
- American Journal of Kidney Diseases: Total, Dietary, and Supplemental Vitamin C Intake and Risk of Incident Kidney Stones
- American Family Physician: Urinalysis: Case Presentations for the Primary Care Physician
- Clinical Chemistry: Theory, Analysis, Correlation, 5th Edition; Lawrence A. Kaplan and Amadeo J. Pesce