Interpretation of kidney stone composition analysis
- Gary C Curhan, MD, ScD
Gary C Curhan, MD, ScD
- Section Editor — Chronic Kidney Disease
- Editor-in-Chief emeritus
- Harvard Medical School
Kidney stones can contain a variety of crystalline and noncrystalline materials. Knowing the composition of a stone influences clinical decisions. Although the 24-hour urine composition provides information on the possible composition of a patient's stone, this is not perfectly predictive of the stone type and, at times, can be misleading. Thus, instructing the patient to retrieve a passed stone or instructing the urologist, who is performing a stone-related procedure, to save and send a stone fragment for analysis is an important part of the approach to prevent recurrent stone formation. The stone composition may also influence the choice of urologic intervention.
The most common crystalline materials found in kidney stones are calcium oxalate, calcium phosphate, uric acid, and struvite. It is not uncommon for a stone to contain more than one crystalline component. Noncrystalline materials found in stones include proteins and blood. Some laboratories report the composition of the nidus (if present) separately from the composition of the body of the stone.
INDIVIDUAL CRYSTALLINE COMPONENTS
Calcium oxalate — Calcium oxalate is the most common component found in kidney stones (approximately 70 to 80 percent). Calcium oxalate can be found in monohydrate (crystal name: whewellite) and dihydrate (crystal name: weddellite) forms (picture 1). Calcium oxalate can also be present in combination with uric acid or calcium phosphate. Because calcium oxalate stones typically grow on a Randall's plaque (composed of calcium phosphate) on the papillary tip [1,2], a laboratory that examines the composition of the nidus may report a stone with an eccentric calcium phosphate nidus (usually 5 percent) and a calcium oxalate body (95 percent).
Urinary risk factors for calcium oxalate crystal formation are lower urine volume, higher urine calcium excretion, higher urine oxalate excretion, and lower urine citrate excretion. Calcium oxalate crystals are pH insensitive in the physiologic pH range of 5 to 8. (See "Risk factors for calcium stones in adults".)
Calcium phosphate — Calcium phosphate is found in approximately 15 percent of kidney stones and can be present in combination with calcium oxalate or struvite. Because of differences in solubility due to urine pH, calcium phosphate is not found mixed with uric acid. The two forms of calcium phosphate include apatite (sometimes reported as carbonate apatite), which is the crystal type found in bone, or calcium hydrogen phosphate (brushite); the frequency of apatite is much greater than brushite. Calcium phosphate crystals in the urine sediment are typically dark and amorphous.
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