Renal Pathophysiology

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RENAL PATHOPHYSIOLOGY: THE ESSENTIALS

resuspended by gently flicking the side of the tube. The sediment should be poured or transferred with a pipette onto a slide and covered with a cover slip. Both the unspun urine and the sediment are now ready for detailed analysis. Evaluation of the unspun urine begins with a dipstick that can test for the following, as well as for protein excretion: „ pH —The pH of the urine normally ranges between 5 and 6.5, depending pri marily on dietary intake. Measurement of the urine pH is generally of little clinical importance except in two settings. First, a urine pH > 7.5 to 8 suggests a urinary tract infection with a urea-splitting organism, and the nitrite test should also be positive. The metabolism of urea can raise the urine pH by driving the reaction—NH 3 + H + ↔ NH 4 + —to the right, thereby lowering the free hydrogen concentration and raising the urine pH. Second, the urine pH should be < 5.3 (maximally acid) in a patient with metabolic acidosis because excreting more acid will tend to normalize the extracellular pH. A urine pH > 5.5 in this setting suggests an impairment in the acidification process, due most often to one of the forms of renal tubular acidosis (see Chapter 6). „ Glucose —Glucose is detectable in the urine primarily in patients with hy perglycemia due to inadequately controlled diabetes mellitus. In this setting, the filtered glucose load is increased to a level that exceeds proximal glucose reabsorptive capacity, resulting in glucosuria. Rarely, glucosuria is noted with a normal plasma glucose concentration; this finding, called renal glucosuria , is indicative of a proximal tubular defect in glucose reabsorption and may be seen in combination with other proximal tubular defects (bicarbonatu ria; see Chapter 6). However, it is now much more common to see glycosuria with normal or minimally elevated serum glucose levels in patients treated with SGLT2i. These medications are now widely utilized for the treatment of diabetes and have beneficial effects on renal and cardiovascular outcomes. „ Ketones —Patients with uncontrolled diabetes mellitus also may have ke toacidosis. β -Hydroxybutyric acid is the primary ketone formed, but ace toacetic acid and acetone are also present. Only the latter two compounds are detected by the dipstick, which will therefore tend to underestimate total ketone excretion. „ Nitrite —Dietary nitrate is normally excreted in the urine. If, however, bac teria are present and there is adequate contact time (as in a specimen ob tained when the patient first voids in the morning), then urinary nitrate can be partially converted to nitrite. Thus, a positive dipstick for nitrite is a reasonably good screening test for a urinary tract infection. „ Heme —A positive test for heme is usually indicative of red cells being pres ent in the urine, a finding that must be confirmed by examination of the urine sediment. In addition to hemoglobin in red cells, the dipstick can detect free heme proteins as with hemoglobinuria due to intravascular hemolysis and myoglobinuria due to skeletal muscle breakdown (rhabdo myolysis). In the latter two conditions, however, the supernatant will be heme positive, but there will be few or no red cells in the urine sediment. „ Protein —A positive test for protein indicates the presence of albumin in the urine. Other urinary proteins are not detected by the dipstick.

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