Master Types of Casts in Urine

Red blood cell casts consist of erythrocytes trapped within a protein matrix formed in renal tubules, indicating glomerular or renal parenchymal bleeding rather than bleeding from the lower urinary tract. Their presence confirms that hematuria originates from the kidney.

The presence of RBC casts in urine indicates glomerular damage due to diseases such as Berger disease (IgA nephropathy) or post-streptococcal glomerulonephritis (PSGN), both of which produce nephritic syndrome. RBC casts may also appear in hypertensive emergencies (BP >180 systolic or >120 diastolic) due to pressure-induced glomerular injury.

White blood cell casts form when neutrophils or eosinophils aggregate within the renal tubules, signifying inflammation or infection of the kidney rather than the lower urinary tract. They are a key finding in conditions involving intrarenal inflammation.

Acute interstitial nephritis is inflammation of the renal interstitium, often caused by drug-induced hypersensitivity reactions (e.g., beta-lactam antibiotics, sulfa drugs, NSAIDs, diuretics), or by systemic diseases such as sarcoidosis and amyloidosis. Patients may present with fever, rash, eosinophilia, and signs of acute kidney injury such as oliguria. WBC casts may be present on urinalysis.

Acute pyelonephritis is a bacterial infection of the renal parenchyma, usually resulting from an ascending urinary tract infection. It occurs more commonly in women and presents with fever, chills, flank pain, dysuria, and nausea. Urinalysis typically shows WBC casts and bacteriuria, confirming an upper (not lower) urinary tract infection.

Also known as oval fat bodies, fatty casts form when lipid-laden renal tubular cells accumulate in the lumen. Under polarized light, they demonstrate the characteristic “Maltese cross” appearance. Fatty casts are strongly associated with nephrotic syndrome and reflect heavy proteinuria and lipiduria.

On polarized light microscopy, fatty casts display a Maltese cross pattern, caused by birefringent cholesterol esters within the cast, a classic finding in nephrotic syndrome.

Nephrotic syndrome is characterized by proteinuria >3.5 g/day, hypoalbuminemia, hyperlipidemia, and pitting edema, along with a hypercoagulable state. Fatty casts and oval fat bodies are typical urinary findings. Common causes include minimal change disease, membranous nephropathy, and focal segmental glomerulosclerosis.

Granular or “muddy brown” casts form from degenerating epithelial cells and granular debris, usually after ischemic or toxic injury to renal tubules. Their brown color and coarse texture are diagnostic clues.

Acute tubular necrosis results from ischemic or nephrotoxic injury (e.g., due to aminoglycosides, radiocontrast agents, or prolonged hypotension). Damaged tubular epithelial cells slough off into the lumen, forming muddy brown granular casts. ATN is a leading cause of intrinsic acute kidney injury (AKI) and may follow untreated prerenal azotemia.

Waxy casts are broad, refractile casts formed from degenerating cellular casts and Tamm–Horsfall protein. Their appearance reflects prolonged tubular stasis in dilated or atrophic nephrons and is associated with chronic kidney disease (CKD) and low urine flow states.

Waxy casts are most commonly seen in end-stage kidney disease (ESKD) or advanced chronic kidney disease (CKD). Their presence indicates long-standing renal parenchymal damage and markedly reduced renal function.

Hyaline casts are composed purely of Tamm–Horsfall protein (uromodulin) secreted by tubular epithelial cells and appear clear and homogeneous on microscopy. They are the most common cast type.

Hyaline casts are non-specific and can appear in healthy individuals, especially with dehydration, vigorous exercise, or mild prerenal azotemia. Their presence alone does not indicate intrinsic kidney disease.