Oliguria

Workup:

Urine: UA, sodium, creatinine, osmolality, urea
Plasma: sodium, creatinine, osmolality, BUN, Na, K, Phos, Calcium, CBC, eosinophils
Additional testing (if needed): Complement (C3, C4), ASO titers, ANA, ANCA
Imaging: renal US with doppler flow, voiding cystourethrography (postrenal), radionucline renal scanning (transplant, obstruction), CXR (pulmonary edema), ECHO (heart failure patient)
Other: ECG

Interpretation:

UA:

• few hyaline, fine granular casts; little protein, heme, red cells: prerenal
• heme postive urine with no red cells: rhabdomyolysis or hemolysis
• hematuria, proteinuria: intrinsic renal failure
• brown granular casts: intrinsic renal failure, ischemic or toxic ATN
• red cell casts: intrinsic, acute glomerular nephritis
• white cells, white cell casts, eosinophilia: intrinsinc, interstitial nephritis

Urinary Indices

• Comparine urine and serum creatinine, sodium, osmolality
• resorptive capacity of tubular cells and concentrating ability is preserved in prerenal conditions
• these functions are impaired in instrinsic renal failure due to structural damage
• Be careful with urinary indices; serum and urine indices should be checked prior to fluids, diuretics, mannitol; urine should be free of glucose, contrast, myoglobin

Urine Indices and Prerenal failure

• Specific gravity is high: >1.020; (able to concentrate urine)
• urine: plasma creatinine ratio > 40 (dumping creatinine)
• urine: plasma osmolality high ratio >1.5 (concentrating urine)
• urine sodium low < 20 mEq/L (resorption of Na to preserve intravascular volume)
• FENa, the percentage of filtered sodium that is excreted, is low < 1% (resorption capacity intact)
• be careful; urine indices suggestive of prerenal may in fact be early glomerulonephritis, vasculitis, vascular occlusion, early postrenal failure, contrast nephropathy, rhabdomyolysis; FENa may be falsely elevated in urine high in prerenal failure with high excretion of ketones and glucose

Urine Indices and Intrinsic failure

• Specific gravity is low < 1.020 (unable to concentrate urine)
• urine: plasma creatinine ratio < 20 (unable to dump creatinine)
• urine: plasma osmolality low< 1.1 (unable to concentrate urine)
• urine sodium is high > 40 (unable to reabsorb Na)
• FENa, the percentage of filtered sodium that is excreted, is high >2% (resorption capcity impaired)

BUN/Cr

• marked elevated in BUN: prerenal
• ratio of BUN/Cr > 20: prerenal; this reflects increase in proximal tubular absorption or urea
• definition of ARF is a daily increase in Cr (>0.5-1.5 per day) or BUN (>10-20/day)
• elevations in BUN may also be due to: steroid therapy, TPN, GI Bleed, catabolic states
• spurious elevation in Cr may occur with certain drugs: bactrim, cimetidine, cephalosporins

Sodium

• Hyponatremia is a common finding that is usually dilutional, due to fluid retention and administration of hypotonic fluids

Potassium

• an important complication due to impaired GFR, reduced tubular secretion, metabolic acidosis (as acidosis worsens or pH decreases, K increases), catabolic states
• especially in rhabdomyolysis, tumor lysis syndrome, hemolysis
• symptoms: malaise, nausea, muscle weakness

Phosphate and Calcium

• Phosphate may increase due to impaired tubular secretion
• This can lead to hypocalcemia
• with calcium phosphate deposition in tissues
• Hypocalcemia is induced by hyperphosphatemia-induced impairment of GI absorption of calcium due to inadequate Vitamin D production by the kidney,
• symptoms of hypocalcemia: tetany, seizures, cardiac dysrhythmias

Acid-Base

• impaired excretion of acids, impaired tubular secretion and regeneration of bicarbonate --> results in metabolic acidosis with high anion gap

CBC

• anemia: due to dilution and decrease eryhthropoiesis
• microangiopathic hemolytic anemia with schistocytes and thrombocytopenia: indicates HUS (hemolytic uremic syndrome)
• oliguria + neutropenia and thrombocytopenia: consider SLE
• eosinophilia: allergic interstitial nephritis

Other testing

• complement testing: decreased c3, c4; acute post-streptococcal glomerulonephritis, lupus nephritis, membranoproliferative glomerulonephritis
• elevated ASO: confirms Acute PSGN
• ANA: suggests lupus
• ANCA: suggests vasculitis

ECG:

• Hyperkalemia
• Peaked T-waves, prolonged PR interval, flattened P-waves, widened QRS, ST segment changes, ventricular tachycardia, terminal v-fib