Measurement of calcium phosphate crystalluria: influence of pH and osmolality and invariable presence of oxalate.

Calcium phosphate and calcium oxalate urinary crystal concentrations in normal and stone-forming subjects were measured. The urinary crystals were examined by light microscopy and urine samples were analysed for oxalate, pH and osmolality. Calcium phosphate crystal concentrations were clearly related to urine pH but unrelated to urine osmolality. An unexpected finding was co-precipitation of oxalate with calcium phosphate. Consequently, precipitated invariable oxalate increased with rising urinary pH. Possible explanations and implications are discussed.