Crystals are frequently found in the urine, however, usually they are not present in fresh urine. The presence of crystals in fresh urine is considered to be an artifact. Crystals are formed by concentration and saturation of the urine chemical constituents, or by their reduced solubility, which is observed if the urine is stored at a temperature lower than body temperature.
On urinary sediment examination, it is important to identify the type of crystals, i.e. whether they are normal, diagnostically irrelevant crystals, or crystals indicative of a disease. The following crystals without clinical significance are mostly found in the urine:
at acidic pH: uric acid, amorphous urates, calcium oxalates
at alkaline pH: amorphous phosphate, triple phosphate, calcium oxalate,
alcium carbonate, ammonium urate
crystals found in urinary sediment due to pathologic processes: cystine, tyrosine, and leucine.
| Uric acid crystals
Uric acid crystals are of a varying size and shape. They may be rhomboids, quadrangles, hexahedral prisms, or shaped like a spindle, rosette, barrel, or rarely like a needle or spear. They are yellowish to yellow-brown in color. Uric acid crystals usually are of a characteristic yellow color.
Color intensity depends on crystal thickness, so that thin plates are colorless, while massive crystals are brownish. Uric acid crystals are clinically relevant only if found in fresh urine. These crystals are found in a highly concentrated urine, in elevated body temperature, gout, diseases characterized by increased degradation of cellular nuclei, e.g., leukoses, or in patients receiving cytostatic therapy for malignant diseases, and occasionally in renal tubular diseases. Uric acid crystals may occur due to metabolic or pathologic events.