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Nephrology Leukocytes and Other Formed Elements Leukocytes in the urine may be caused by glomerular or tubulointerstitial inflammation, infection, or an allergic reaction Remember: . Sterile pyuria (pyuria and a negative urine culture) suggests Mycobqcterium tuberculosis, interstitial cystitis, or inter- stitial nephritis. . Eosinophiluria suggests AIN, postinfectious GN, atheroembolic disease of the kidney, septic emboli, or small vessel vasculitis. DON'T BE TRICKED . Absence of eosinophiluria does not rule out AIN, postinfectious GN, atheroembolic disease of the kidney, septic emboli, or small-vessel vasculitis. Patients with hemolysis and rhabdomyolysis test positive for blood on dipstick urinalysis in the absence of erythrocytes on urine microscopy. Urine Iipids and fat are almost always associated with heavy proteinuria or the nephrotic syndrome. These may appear as free lipid droplets, round or oval fat bodies, or fatty casts. Casts are cylindrical aggregates of Tamm Horsfall mucoproteins that trap the intraluminal contents and appear in the urine. Different Szpes of casts are associated with specific disorders: . Erlthroc).te casts indicate glomerular disease. . Leukocyte casts indicate inflammation or infection of the renal parenchyma. . Muddy brown casts are associated with ATN. . Broad casts are associated with CKD. lmaging The three main modalities of kidney imaging are ultrasonography, CT, and MRI. Ultrasonography is used to look for: . nephrolithiasis o kidney size and cortical thickness (increased echogenicity implies parenchymal disease) . renal cysts and tumors o obstruction and hydronephrosis . bladder size, postvoid residual, and the prostate in bladder outlet obstruction CT is used to look for: . nephrolithiasis (noncontrast abdominal helical CT) . renal tumors and cysts (contrast abdominal CT) . causes of unexplained nonglomerular hematuria (CT urography) MRI is used: o to characterize renal masses, cysts, and renal vein thrombosis . to look for renal artery stenosis using MRA with gadolinium contrast 261