What are the characteristics of urinary crystals in patients with renal stones?
Calcium oxalate monohydrate crystals may be dumbbell shaped, needle shaped, or oval, with the last resembling red blood cells. Calcium oxalate dihydrate crystals are pyramid shaped and have an envelope appearance. Calcium phosphate and uric acid crystals are too small for standard light microscopic resolution and look like amorphous debris. Uric acid crystals are characteristically yellow-brown in color. Less commonly, uric acid dihydrate crystals may be rhomboid shaped or resemble the four-sided diamonds on a deck of cards. If urine is fresh and warm, the crystals suggest a cause of the renal stone. However, all of these crystals may be found in normal cooled urine and are not necessarily diagnostic of disease. An exception is the presence of cystine crystals, which are flat, hexagonal plates resembling benzene rings—this always means cystinuria. Struvite (magnesium ammonium phosphate) crystals are rectangular prisms that resemble coffin lids.