Chapter 104. Urinalysis and Urine Electrolytes

1. What useful clinical information can urinalysis provide?

2. What is the diagnostic significance of different urinary casts?

3. What are the appropriate clinical settings in which to measure the fractional excretion of sodium and the fractional excretion of urea?

4. What are the limitations of the different urinary tests?

5. How can the different pieces of information provided by various urinary tests be integrated to assist in making an accurate diagnosis?

Urine tests can be a valuable diagnostic tool, especially since they are easily obtained and can provide a wide range of information. When interpreted in the context of a thorough history and physical examination, the clinician can use the information obtained from urinalysis to narrow the differential diagnosis and guide treatment. For example, in a patient presenting with kidney disease, the presence of significant proteinuria and red blood cell casts in the urine strongly suggests glomerulonephritis as a diagnosis. Unlike most blood tests, basic urinalysis can be performed rapidly by the examining physician.

This chapter will review how important clinical information can be obtained from various attributes of the urine—such as the appearance of the urine, urine specific gravity and osmolality, urine electrolytes, and urine microscopy. Especially when various urine tests are considered together, these tests can shed significant light on the patient's clinical status. However, all urine tests have significant limitations. Moreover, certain urine tests, such as urine osmolality, have wide reference ranges, so a urine osmolality value that may be normal in one patient could be abnormal in another patient. For example, a relatively low urine osmolality of 100 mOsmol/kg may be normal in a patient who takes in a large volume of water, but could suggest diabetes insipidus in a patient who is hypernatremic with polyruia.

Even before any official urinalysis results are available, inspection of the urine can be revealing (see Table 104-1). For instance, urine may be pale yellow or clear in a patient who is well hydrated, whereas dark yellow urine can suggest a patient who is volume depleted. Urine may have a reddish hue in cases of significant hematuria, though a vaginal source or other perineal source needs to be considered. Myoglobinuria and hemoglobinuria, from rhabdomyolysis and hemolysis, respectively, can also cause red urine. Red urine can also result from the ingestion of certain foods, such as beets and rhubarb, and medications, such as senna and doxorubicin. Centrifugation of the urine can help distinguish hematuria from myoglobinuria and hemoglobinuria. Upon urine centrifugation, with hematuria the intact red blood cells (RBCs) should form a red-colored sediment only, whereas if the supernatant is red, that suggests the presence of myoglobinuria or hemoglobinuria. Foamy urine may be seen in patients with significant proteinuria.