Renal Pathophysiology

CHAPTER 8 Urinalysis and Approach to the Patient With Renal Dysfunction

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of nephrotic syndrome. White blood cells and white blood cell casts can be seen in interstitial nephritis, and granular casts with renal tubular epithelial cells are sug gestive of tubular injury. Crystals of calcium oxalate and urate are common and may correlate with kidney stone formation. Urine sodium excretion and FENa can be helpful in assessing volume status (renin-angiotensin system activation) but can be misleading in patients with normal GFR. CASE-1 DISCUSSION The patient presented at the beginning of the chapter has severe kidney injury associated with renal failure as manifested by marked elevations in the BUN and plasma creatinine concentration. The normal plasma creatinine con centration 3 months previously suggests that this process is relatively acute. Easy fatigability, anorexia, weight loss, and anemia can be induced by renal failure. However, the concurrent presence of persistent back pain raises the question of an underlying malignancy. This patient had a 4 + sulfosalicylic acid test and high protein/creatine ratio. It is important to note the discrepancy with the dipstick, which only showed trace protein. The urine dipstick only detects albumin whereas patients with paraproteinemia (multiple myeloma in this case) have large amounts of circulating light chains, heavy chains, or intact immunoglobulin molecules. Other possible causes of acute renal failure with a bland urine sediment in clude prerenal disease, urinary tract obstruction (which is associated with dila tation of the collecting systems that can be detected by ultrasonography), and hypercalcemia (Table 8.2). Both urinary tract obstruction and hypercalcemia may be induced by an underlying malignancy, and hypercalcemia can contribute to the decline in renal function in multiple myeloma (see Chapter 11). CASE-2 DISCUSSION This patient has baseline chronic kidney disease from diabetes and hypertension. He is presenting with AKI in the setting of volume depletion. This is discussed in more detail in Chapter 11. The urinalysis is notable for large amounts of glucose but only a mild degree of serum glucose elevation. This reflects the glycosuria induced by the diabetes medication, empagliflozin, which is an SGLT2i inhibitor that blocks glucose reabsorption in the proximal tubule. The finding of a few red blood and white blood cells is nonspecific. The high specific gravity is consistent with volume depletion but is difficult to interpret in the setting of glycosuria, which will raise the urine specific gravity. The absence of cellular casts with the finding of hyaline and granular casts is suggestive of prerenal (decreased renal perfusion) reduction in GFR. This picture is also consistent with early or mild acute tubular necrosis, which indicates tubular damage; the classic findings of muddy brown casts may not be seen and are more common with severe and sustained injury. Administering intravenous fluids to correct the volume deple tion will lead to rapid improvement in renal function if this is entirely prerenal in origin. If there has been a tubular injury, recovery may be delayed or incomplete until the tubules can recover.

CHAPTER 8 Urinalysis and Approach to the Patient With Renal Dysfunction

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