Laboratory testing is necessary when urgently evaluating any patient with presumed upper urinary tract obstruction. Standard tests include a complete blood count, serum chemistry testing, complete urinalysis with evaluation of the urinary sediment, urine Gram stain, and urine culture. In addition, for patients with fever or findings of pyelonephritis on radiographic imaging, multiple blood cultures should also be obtained. Ideally, cultures would be obtained before antibiotics are started. A complete blood count is obtained predominantly to exclude leukocytosis, which could suggest presence of infection concurrent with the obstruction. Depending on the etiology and duration of the obstruction, the following laboratory findings can be observed with chemistry testing: elevated serum creatinine, elevated blood urea nitrogen, hyperkalemia, and/ or acidosis. Normally, elevated creatinine, acidosis, and hyperkalemia will accompany bilateral upper tract obstruction or patients with an obstructed solitary functioning kidney (i.e., previous nephrectomy patient, contralateral nonfunctioning kidney, renal transplantation patient). Typically, the patients with the most dramatic chemistry abnormalities are likely to have a rather longstanding or chronic source of underlying obstruction in all renal moieties (Gulmi et al. 2002). Even for patients with complete unilateral obstruction (acute or chronic), serum chemistry findings would commonly be normal or just slightly elevated (serum creatinine 1.2-1.6 mEq/l) unless the patient had pree xisting renal insufficiency. A serum creatinine above 2.0 would be considered pathologic and possibly suggest a prerenal component of renal dysfunction or preexisting medical renal disease. Microscopic evaluation of the urine can show a variety of findings in the setting of obstruction including hematuria, pyuria, proteinuria, cast formation, and/or crystal formation. The urinalysis can also show changes in concentrating ability based on the duration of the obstruction and overall condition of the kidneys. In patients with obstruction related to a chronic condition affecting both kidneys, urine electrolytes may provide additional diagnostic value. In this setting, decreased urine to plasma creatinine ratios and elevated urinary sodium concentrations are typically noted in addition to poor concentrating ability. For the patient with acute unilateral obstruction, concentrating ability is less commonly affected and urine testing will typically show increased urine osmolality values and relatively low urine sodium concentrations (Gulmi et al. 2002). For patients with hyperkalemia, an emergent electrocardiogram should be performed and appropriate medical management is required to address the elevated serum potassium levels prior to treating the obstruction. In situations where obstructing is thought to be related to ur-othelial carcinoma, a voided urine cytology can help assist with the correct diagnosis. Lastly, all child-bearing females should have a pregnancy test performed before undergoing any diagnostic radiologic evaluations or treatment, if pregnancy is a possibility.
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