Improving management of urolithiasis: diagnostic caveats - DVM
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Improving management of urolithiasis: diagnostic caveats


18. Different types of crystals may have similar shapes. When difficulty is encountered in identifying urine crystals by their light microscopic appearance, their composition may be determined by physical methods of quantitative mineral analysis (infrared spectroscopy, X-ray diffraction, etc.). Contact diagnostic laboratories for details about how to prepare and submit samples.

19. Struvite crystals may on occasion resemble cystine crystals, especially in cats. Addition of 10 percent acetic acid will dissolve struvite crystals, but will not dissolve cystine crystals.

20. Don't rely on microscopic evaluation of urine crystals as the sole criterion of the mineral composition of uroliths. Only quantitative analysis can provide definitive information about the mineral composition of the entire urolith. However, interpretation of crystalluria along with other clinical findings often allows one to establish a tentative identification of the mineral composition of uroliths, especially their outer layers.

21. Detection of crystalluria is not synonymous with the presence of uroliths. Crystalluria often is present in absence of uroliths. Conversely, uroliths can be present without concomitant crystalluria.

22. The primary objectives of radiographic evaluation of patients suspected of having uroliths is to determine their site(s), number, density and shape(s). Although properly performed ultrasonography is comparable to survey radiography in detection of uroliths, ultrasonography does not provide information about the relative radiodensity or shape of uroliths. Because the radiodensity, shape and size of uroliths often provides valuable clues as to their mineral composition, we prefer to use survey radiography as the initial imaging procedure when evaluating most patients for uroliths.

23. Once urolithiasis has been confirmed, radiographic and/or ultrasonographic evaluation are important to detect predisposing or complicating abnormalities. Radiographic evaluation of changes in urolith size is also an important index of therapeutic response.

24. Urocystoliths less than 3mm in size are difficult to detect by survey radiography, but may be detected by double contrast cystography or ultrasonography.

25. The radiodensity of uroliths is variable, being dependent on their mineral composition, size, and proportion of matrix. In general, calcium phosphate and calcium oxalate uroliths are the most dense (comparable to the density of bone). In contrast, uroliths composed of urates are often radiolucent (density comparable to soft tissue). Struvite, cystine, and silica uroliths can often be detected by survey radiography, but are substantially less dense than bone.

26. Radiodense uroliths must be differentiated from: a) dystrophic or metastatic calcification of renal parenchyma, b) radiodense ingesta or medications in the gastrointestinal tract, c) calcified mesenteric lymph nodes, d) osseous metaplasia of transitional epithelium or mineralization of a neoplasm, e) radiodensities in the gallbladder and f) large thela of female dogs. Radiodense uroliths within the excretory pathways may disappear or become radiolucent when surrounded by radiopaque contrast agents.

27. Radiolucent uroliths composed of purines (ammonium urate sodium urate, uric acid, and xanthine) can usually be detected by double contrast cystography. Radiolucent uroliths may be distinguished from blood clots with the aid of ultrasonography.

28. Laminations of uroliths detected by survey radiography may be associated with different layers of minerals, or the same type of mineral with different proportions of matrix.

29. The number, location, size and shape of uroliths may rapidly change. For example: infection-induced struvite uroliths may form and grow to a detectable size within one to two weeks. Urocystoliths may also pass into or through the urethra. Likewise, renoliths may pass into the ureters and/or urinary bladder. Following variable periods, new uroliths may form in one or more locations; uroliths may also spontaneously dissolve. Therefore, if significant time has elapsed between the time of diagnostic radiography and the date of initiation of medical or surgical therapy, the number, size, shape and location(s) of uroliths should be reevaluated by radiography or ultrasonography just prior to therapy.

30. In our experience, urocystoliths have been detected in the lower urinary tract in 15 to 20 percent of dogs and cats immediately following cystotomy. If the number of uroliths present in the urinary tract can be accurately determined by survey or contrast radiography, it is usually necessary to obtain immediate postsurgical films to ensure that they have all been removed since they can be accurately counted. However, if the numbers of uroliths detected by radiography are too numerous to count, postsurgical radiographs are indicated to detect uroliths that have been inadvertently allowed to remain in the urinary tract (pseudo-recurrence).

31. Microbes may remain viable within the matrix of uroliths (especially struvite uroliths). In this situation, aerobic culture of surrounding urine may reveal: a) the same type of microbe, b) different types of microbes, or c) no microbes (especially if antimicrobial agents are given prior to collection of urine for culture).

32. Although abnormal serum concentrations of lithogenic metabolites may provide clues about the underlying cause(s) of urolithiasis, normal serum concentrations of potentially lithogenic metabolites are not always associated with normal urine concentrations of those metabolites.

33. The concentrations of lithogenic metabolites in urine is often dependent on the type of diet being consumed, and also whether urine is collected during a non-fasted or fasted state.

34. Although the names of many commercially manufactured diets remain the same over long periods, their composition may change. This fact must be considered when comparing results from one date to another, or between one group of investigators and another.

35. When evaluating the urine concentrations of lithogenic metabolites in urine of patients with urolithiasis, patients should be fed the same type of diet consumed when the uroliths formed, or special diets designed for this purpose.

36. For best results, 24-hour urine collections should be analyzed for concentrations of calculogenic metabolites. Diurnal variation in urinary excretion of lithogenic metabolites decreases the reliability of "spot" urine samples.

37. Laboratory methods designed for measurement of lithogenic metabolites in human urine may not provide reproducible data when used to detect such metabolites in animals. For example, production of ascorbic acid by dogs interferes with commonly used enzymatic tests designed for measurement of urine oxalate concentration in man. Likewise, automated serum chemistry procedures designed to detect relatively high serum uric acid concentrations in man commonly provide irreproducible results of serum uric acid concentrations in dogs and cats.

38. Precipitation of calculogenic metabolites (such as uric acid and ammonium urate) during preservation of urine by refrigeration (or other methods) may result in detection of erroneously low concentrations of these metabolites.

39. If uroliths are removed surgically, consider removing a small biopsy sample for preservation in 10 percent buffered formalin. The sample may be immediately submitted for evaluation, or saved for use in the event that unforeseen difficulties in management of the patient occur.


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