Amorphous phosphates

Amorphous phosphates is the name given to a granular precipitate containing calcium and phosphate in an alkaline urine. Calcium phosphate crystals, regrouped under the term apatite, have mineralogical names that differ according to their chemical composition. The CaH2PO4*(2H2O) is called Brushite, the calcium hydroxyl phosphate is called hydroxyl-apatite, the calcium bicarbonate phosphate is called Dahlite or carbonate-apatite.

The main cause of this crystalluria is the alkaline pH that decreases the solubility of the calcium phosphate and entails a precipitation of the former. The alkaline pH can be caused by the diet (vegetarian, rich in phosphates..) but can also represent a pathological situation. Usually, the presence of these crystals is non significant. The distinction between amorphous urates and amorphous phosphates is often made on the urinary pH basis. With a simple examination of the centrifuge pellet, the precipitate of calcium phosphate is white, while that amorphous urate is pink.

Triple phosphates
(Magnesium ammonium phosphates)
(Struvite)

Triple phosphates are found in urines whose pH is superior to 6,5. The former crystallize in the orthorombic system. The crystal is slightly birefringent and often shows a polarization color. The classic shape is the pyramid, that reminds a coffin lid. The crystalluria is usually polymorphous. The rosette-shaped crystal shown in the picture bank has been observed in a specimen with a pH of 6,5.

The primary factor to the triple phosphate crystals formation is the ammonia concentration. Alkalanisation of a urine specimen with ammonia generates triple phosphates while alkalanisation with sodium hydroxide does not. A normal freshly voided specimen contains little free ammonia; this substance is mainly generated by urea splitting bacteria. Triple phosphates are usualy associated with bacterial growth. With a first-morning fresh specimen, triple phosphates can indicate urinary tract infection. Otherwise, triple phosphates are of little clinical value.

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