CMicroscopic findings

i. RBCs. Finding of RBCs that are too numerous to count (TNTC; ie, > 50) suggests renal vein thrombosis or stone.

ii. WBCs. Presence of many WBCs suggests infections.

iii. Eosinophils. Presence suggests interstitial nephritis.

iv. Casts. Granular casts suggest ATN; red cell, acute nephritis; white cell, pyelonephritis.

2. Serum chemistries. Critical for identifying electrolyte abnormalities associated with renal failure, including hyponatremia and hypernatremia, hyperkalemia, low bicarbonate (metabolic acidosis), hyperphosphatemia, and hypocalcemia. If BUN and creatinine are elevated, a BUN-to-creatinine ratio > 20-30:1 could indicate a prerenal state, obstruction, GI bleeding, or catabolic state. A ratio < 10-15:1 more likely indicates an intrarenal cause. However, ratios are less accurate in children because of lower baseline creatinine levels.

3. Urine electrolytes and creatinine. These are key findings in distinguishing prerenal states from other types of renal failure (see Table I-23). Urine sodium < 20 mmol/L suggests a prerenal state, as does fractional excretion of sodium (%FENa) < 1% (or < 2% for a newborn). %FENa is the more accurate test, and excretion < 1% (2% in newborn) is a strong indication to use aggressive fluid replacement. %FENa > 1% (2% in newborn) suggests intrinsic renal damage and more careful use of fluid replacement.

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