Renal Pathophysiology



Correlation of Urinalysis With Differential Diagnosis The different types of renal disease are reviewed in the following chapters. However, it is useful at this time to review briefly how the urinary findings can point toward a particular disease. As can be seen from Table 8.2, different patterns of urinary findings are associated with different diseases; in some cases, the changes seen may be virtually diagnostic for a single disorder. Ex amples include red cell casts for glomerular disease or vasculitis and, in AKI, renal tubular epithelial cells and multiple granular and epithelial cell casts for acute tubular necrosis. Even a relatively normal urinalysis is helpful by excluding a number of disorders, particularly glomerular diseases. Urine Sodium Excretion Estimation of the rate of sodium excretion is used in a variety of clinical settings, including the differential diagnosis of hyponatremia (see Chapter 3) and distin guishing between prerenal disease and acute tubular necrosis as the cause of AKI (see Chapter 11). The basic principle is that with intact tubular function, sodium retention is the appropriate renal response to decreased systemic and renal perfusion. As a result, the rate of sodium excretion should be low (usually < 25 mEq/day), with effective volume depletion causing hyponatremia or AKI. In comparison, sodium excretion is normal (equal to intake) or even elevated when the patient is normovolemic (as with hyponatremia due to the syndrome of inappropriate ADH secretion) or when renal tubular function is impaired (as with AKI due to acute tubular necrosis or with diuretic therapy). Two different methods are used to estimate sodium excretion from a random urine specimen: measurement of the urine sodium concentration and calculation of the FENa. Urine Sodium Concentration The urine sodium concentration is usually < 25 mEq/L with volume depletion and > 40 mEq/L with normovolemia or acute tubular necrosis. There is, how ever, substantial overlap, particularly at values between 25 and 40 mEq/L. Fractional Excretion of Sodium Calculation of the FENa allows sodium handling to be looked at directly with out the confounding effect of the rate of water reabsorption. The FENa re flects the percentage of the filtered sodium load that is excreted (the concept of fractional excretion can be applied to any substance simultaneously mea sured in the urine and blood but is most often used for sodium): 3 At a given rate of sodium excretion, what additional factor will influence the urine sodium concentration?

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