Key Clinical Questions
How does interpretation of the urinalysis and urine electrolytes guide clinical decision making?
What are the limitations of urinalysis and urine electrolytes?
When is it helpful to measure the urine osmolality, the fractional excretion of sodium and the fractional excretion of urea?
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 (RBC) casts in the urine suggests glomerulonephritis as a diagnosis.
This chapter will review how the major components of the urinalysis—including the appearance of the urine, urine dipstick, urine microscopy, and urine electrolytes—can guide clinical decision making. Unlike blood tests, which usually have discrete normal ranges, the “normal” range for many urine parameters is a function the patient’s clinical status and any metabolic or volume perturbations to which the kidney may be responding.
URINE COLOR AND APPEARANCE
The gross appearance of the urine may provide clues about volume status, source of bleeding, presence of infection, and medication use, as shown in Table 111-1.
TABLE 111-1Urine Appearance and Associated Medications and Conditions ||Download (.pdf) TABLE 111-1 Urine Appearance and Associated Medications and Conditions
|Appearance of Urine ||Associated Medications ||Associated Conditions |
|Pale yellow || ||Volume replete |
|Dark yellow || || |
|Orange || || |
|Red || |
Ingestion of beets
Ingestion of rhubarb
Ingestion of blackberries
|Brown || ||Povidone-iodine contamination |
|Blue-green || ||Pseudomonas aeruginosa urinary tract infection |
|Milky ||• Propofol (depending on pH) ||Lipiduria Chyluria |
|Foamy || ||Proteinuria |
Centrifugation of the urine can help distinguish hematuria from other causes. A red-colored pellet suggests intact RBCs, whereas a red supernatant may suggest myoglobinuria or hemoglobinuria. The gross appearance of the urine may suggest significant proteinuria (foamy urine) or lipiduria (greasy urine). Foul-smelling urine may suggest infection. Sweet-smelling urine raises the possibility of ketonuria.
URINE REAGENT STRIP (DIPSTICK) TESTING
SPECIFIC GRAVITY (AND URINE OSMOLALITY)
Specific gravity, which is measured using a urine dipstick, is the density of the urine relative to the density of water. This is a measure ...