Urinalysis

• Findings of granular muddy-brown casts are suggestive of tubular necrosis. The presence of tubular cells or tubular cell casts also supports the diagnosis of ATN.  Often, oxalate crystals are observed in cases of ATN.

• Reddish brown or cola-colored urine suggests the presence of myoglobin or hemoglobin, especially in the setting of a positive dipstick for heme and no RBCs in the microscopic examination.

• Dipstick assay findings may show the presence of significant proteinuria, which would suggest glomerular or interstitial disease.

• The presence of RBCs in the urine is always pathologic. Eumorphic RBCs suggest bleeding along the collecting system. Dysmorphic RBCs or RBC casts indicate glomerular inflammation, suggesting glomerulonephritis is present. 

• The presence of WBCs or WBC casts suggests pyelonephritis or acute interstitial nephritis. The presence of urine eosinophils is helpful in establishing a diagnosis but is not necessary for allergic interstitial nephritis to be present.

• The presence of eosinophils, as visualized with Wright stain or Hansel stain, suggests interstitial nephritis but can be seen in urinary tract infections, glomerulonephritis, and atheroembolic disease.   

• The presence of uric acid crystals may represent ATN associated with uric acid nephropathy.

• Calcium oxalate crystals are usually present in cases of ethylene glycol poisoning.