Composition Of Kidney Stones: Oxalate, Phosphate & Urate Calculi

Various types of kidney stones found and their composition are as follows:

  • Oxalate calculus (calcium oxalate)
  • Calculus (usually calcium phosphate)
  • Uric acid and urate calculi
  • Cystine calculi (cystine crystals)
  • Xanthine calculi
  • Indigo calculi

Oxalate stones (calcium oxalate)

  • Popularly known as the mulberry stone, it is covered with sharp projections.
  • These cause the kidney to bleed and altered blood precipitated on the surface of the stone.
  • An oxalate stone, which is usually single, casts an exceptionally good shadow radiologically and this is fortunate because often by virtue of its rough surface, it gives rise to symptoms when comparatively small.
  • A calcium oxalate stone is very hard and on section is laminated concentrically.
  • ‘Envelope’ crystals can be identified in the urine.

Phosphate Calculus

  • Usually calcium phosphate, although sometimes combined with ammonium magnesium phosphate and rarely composed of the latter only, is smooth and dirty white.
  • In alkaline urine, it enlarges rapidly and often fills the renal calyces, taking on their shape (staghorn calculus).
  • Because it is smooth, a phosphatic calculus gives rise to few symptoms until it has attained a large size.
  • By reason of its size rather than its density, it is demonstrated readily by X-rays.

Uric Acid And Urate Calculi

  • They are hard and smooth and because they are usually multiple, they are typically faceted.
  • Their color varies from yellow to reddish brown.
  • Pure uric acid calculi are not opaque to X-rays, but absolutely pure uric acid calculi are uncommon; the majority contains enough calcium oxalate crystals to render them opaque.
  • Calculi of ammonium and sodium urate are sometimes found in children.

Cystine Calculi

  • They are sometimes seen in young girls.
  • It results from a greatly diminished or absent resorption of cystine from the renal tubules.
  • Cystine crystals are hexagonal, white, are translucent and appear only in acid urine.
  • Cystine stones are usually multiple and may assume a cast of the renal pelvis and calyces.
  • These calculi are soft, like bee wax and pink or yellow when first removed. They are radio opaque owing to sulphur they contain.

Xanthine calculi: They are extremely rare. They are smooth and round, brick red in color and show a lamellar structure.

Indigo calculi: They are blue in color and are derived from indicant.