What are the different types of kidney stones?

Types of Kidney Stones

Kidney stones are classified into five major types based on their chemical composition: calcium oxalate (most common at 61%), calcium phosphate (15%), uric acid (~12%), struvite (infection stones, ~10%), and cystine (rare). 1, 2

Calcium-Based Stones

Calcium Oxalate Stones

• Comprise approximately 61% of all kidney stones, making them the most common type 1, 2
• Many form on a foundation of calcium phosphate called Randall's plaques present on the renal papillary surface 1
• In primary hyperoxaluria, these stones are typically calcium oxalate monohydrate (whewellite) with distinctive white or pale yellow appearance and disorganized internal structure 3
• Rarely grow in a staghorn configuration 3

Calcium Phosphate Stones

• Account for approximately 15% of kidney stones 2
• Form in alkaline urine conditions 4
• Citrate is a potent inhibitor of calcium phosphate crystallization 3
• Rarely develop into staghorn calculi 3

Uric Acid Stones

• Represent approximately 12% of all kidney stones 2
• Low urine pH (below 5.5) is the primary risk factor, more important than elevated urinary uric acid excretion 5
• Typically radiolucent on plain radiography, limiting fluoroscopic visualization 3
• Have lower CT attenuation values compared to calcium, cystine, and struvite stones 3
• Can grow in a staghorn configuration when pure or mixed with other components 3
• Main causes include tubular disorders (including gout), chronic diarrheal states, or severe dehydration 5

Struvite (Infection) Stones

• Account for approximately 10% of kidney stones and are directly caused by urinary tract infections with urease-producing bacteria 5, 2
• Composed of magnesium ammonium phosphate (struvite) and/or calcium carbonate apatite 3, 6
• Bacteria reside within the stone itself (not just on the surface), making the stone infected 3, 6
• Urease-producing organisms split urea into ammonia and hydroxide, creating an alkaline environment that promotes crystallization 6, 5
• Struvite stones account for the majority of staghorn calculi, which can fill the entire collecting system and destroy kidney function or cause life-threatening sepsis if untreated 3, 6, 5
• Important distinction: E. coli, the most common UTI pathogen, typically does not produce urease and is not associated with struvite formation 6

Cystine Stones

• Rare hereditary disorder caused by impaired tubular reabsorption of cystine 5
• Stone formation occurs due to cystine's low solubility at urine pH below 8.0 5
• Can grow in a staghorn configuration when pure or mixed with other components 3
• Commonly resistant to shock wave lithotripsy due to their structural characteristics, though the degree of resistance is variable 3
• May be barely opaque on standard imaging or fluoroscopy 3
• Stones with rough-appearing external surface on plain film are more apt to fragment with shock-wave energy than those with smooth contour 3
• Patients typically have their first stone event early in life and are prone to recurrent stones 3

Mixed Composition Stones

• Stones from patients with primary hyperoxaluria types 2 and 3 frequently contain mixtures of calcium oxalate and calcium phosphate 3
• Cystine or uric acid stones can occur in pure form or mixed with other components 3

Clinical Significance of Stone Type

Stone composition directly determines treatment approach and prognosis, as different types respond differently to lithotripsy and require specific medical management strategies 3. Stone analysis should be performed at least once when available to guide preventive measures 7.