Crystalluria is defined as the observation of crystals during microscopic examination of a centrifuged urine sample. Occasionally, crystalluria may be heavy enough to be observed macroscopically. Crystalluria represents elimination of a solid (minerals and electrolytes) in a liquid medium (urine). The most common crystals observed in dog and cat urine are struvite (magnesium ammonium phosphate hexahydrate), calcium oxalate dihydrate, ammonium urate, and cystine. Additionally, other ions and certain drugs are capable of forming crystals in urine. Amorphous crystals, which lack a classic symmetrical structure, are commonly observed in dog urine and are sometimes found in cat urine. Typically, amorphous crystals are variant forms of struvite; however,other ions, such as ammonium urate, may appear amorphous. Crystals form when the urine is or has recently been oversaturated with the ions that make up the crystals. With oversaturation, the minerals cannot remain dissociated (dissolved) and they combineto form an organized structure (a crystal). The microscopic appearance of the crystals is dependent upon their composition. The concentrations of minerals, urine volume, urine pH, temperature, and the presence of promoters and absence of inhibitors of crystal formation all influence crystal formation. The concentration of crystallogenic compounds is dependent upon the ion excretion rate and the volume of urine produced. The less urine excreted, the higher the concentrations of solids in the urine. Urinary pH and presence of other ions affect relative concentration and solubility of certain compounds in urine. Temperature influences crystal formation as well. If the urine is allowed to cool below body temperature, crystals are likely to precipitate. Most crystals form in acidic urine (pH < 7.0), while a few (particularly struvite) form when urine pH is > 7.0. The presence of a surface, such as denuded mucosa or a piece of suture, or another mineral, promotes crystal formation. Lastly, there are compounds that inhibit crystal and stone formation. Crystalluria is influenced by both diet and water intake. Dietary influence on crystal formation may include 1) altered excretion of dietary constituents or metabolites, 2) dietary-induced alteration of urinary pH, or 3) changes in urine volume. Alterations in urinary pH, ion excretion, and urine concentration can be observed following the initial consumption of a new food and may not reflect a pathological process. However, in most cases, urine compositionstabilizes in 4-7 days. Time of feeding with respect to urine collection and feeding method can have a significant influence on urine pH and possibly crystalluria. A postprandial alkaline tide is common 3-6 hours after eating, particularly in meal-fed animals. Some pharmacological agents result in crystalluria. Drugs associated with crystalluria include sulfadiazine and its metabolites, ampicillin, and radiopaque contrast agents. Occasionally, foreign material may be mistaken for crystalluria dependingon method of urine collection.
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