STEPTRACKERRENAL | PATHOLOGY + PHYSIOLOGY | HIGH-YIELD REVIEW

Urinalysis & Casts

Casts form in tubules and carry a snapshot of whatever is in them — read the cast, find the segment, name the disease.

FOCUS ON
Casts mean the problem is in the kidney (they form in tubules, on a Tamm-Horsfall protein matrix). RBC casts → glomerulonephritis. WBC casts → pyelonephritis or interstitial nephritis. Muddy brown → ATN. Fatty → nephrotic. Waxy/broad → chronic kidney disease. Hyaline casts alone are usually normal.
FINDING
WHERE · WHY
DISEASE
CLUE
RBC casts
RBCs leak through injured glomeruli into tubules
Glomerulonephritis (nephritic syndrome)
With dysmorphic RBCs, acanthocytes
WBC casts
Inflammation in the tubules/interstitium
Acute pyelonephritis, acute interstitial nephritis, transplant rejection
Cystitis does not cause casts
Granular "muddy brown" casts
Degenerating sloughed tubular cells
Acute tubular necrosis
Also renal tubular epithelial cell casts
Fatty casts / oval fat bodies
Lipid-laden tubular cells in heavy proteinuria
Nephrotic syndrome
Maltese cross under polarized light
Waxy casts
Old casts in slow-flowing, dilated tubules
Advanced or chronic kidney disease; also myeloma (fractured)
Broad casts: dilated atrophic tubules (CKD)
Hyaline casts
Tamm-Horsfall protein alone; concentrated urine
Normal; dehydration, exercise, diuretics, prerenal state
Nonspecific
Hematuria without casts
Bleeding below the kidney
Stones, bladder/urothelial cancer, cystitis, BPH, trauma
Isomorphic RBCs · older smoker → cystoscopy
Dipstick blood, no RBCs
Heme in urine from free hemoglobin or myoglobin
Rhabdomyolysis, intravascular hemolysis
Check CK and hemolysis labs
Nitrite + leukocyte esterase
Gram-negative Enterobacterales convert nitrate → nitrite; WBCs release esterase
Urinary tract infection
Nitrite negative: Enterococcus, S saprophyticus
Sterile pyuria
WBCs without culture growth
Urethritis (Chlamydia, N gonorrhoeae), renal TB, AIN, recent antibiotics, interstitial cystitis
Order NAAT for STIs
Protein: dipstick vs total
Dipstick detects mainly albumin; misses light chains
Negative dipstick + ↑ total urine protein → Bence Jones (myeloma)
Sulfosalicylic acid or immunofixation detects all

Mechanism — how a cast is made

Thick ascending limbSecretes Tamm-Horsfall (uromodulin)
→
Protein gel in tubuleSlow flow, acidic, concentrated
→
Traps contentsRBCs, WBCs, cells, fat
→
Cast in urineProves renal origin

A cast is a mold of the tubule: no cast, no proof the kidney itself is involved.

Urine crystals

CRYSTAL
SHAPE
THINK
Calcium oxalate
Envelope or dumbbell
Ethylene glycol, hyperoxaluria, Crohn
Uric acid
Rhomboid, yellow-brown
Gout, tumor lysis, acidic urine
Struvite
Coffin lid
Urease bacteria (Proteus)
Cystine
Hexagonal
Cystinuria
Calcium phosphate
Wedge-shaped prisms
Alkaline urine, type 1 RTA
Acyclovir, sulfa, indinavir
Needles
Drug crystal nephropathy

How it's tested

COMMON CONFUSIONWBC casts appear in both pyelonephritis and AIN. Fever, flank pain and positive culture = pyelonephritis; rash, eosinophilia and a new drug = AIN.
THE TRAPA negative protein dipstick in a patient with renal failure and anemia doesn't exclude myeloma — the dipstick barely detects light chains.
YOUR APPROACHDipstick (blood, protein, nitrite, esterase, glucose) → microscopy (RBC morphology, casts, crystals) → localize: glomerulus, tubule, interstitium or lower tract.

If you see… think…

RBC casts → glomerulonephritis
WBC casts + fever + flank pain → acute pyelonephritis
WBC casts + rash + eosinophilia → acute interstitial nephritis
Muddy brown granular casts → acute tubular necrosis
Fatty casts with Maltese crosses → nephrotic syndrome
Waxy and broad casts → chronic kidney disease
Only hyaline casts in a dehydrated runner → normal / concentrated urine
Heme-positive dipstick with no RBCs → myoglobinuria or hemoglobinuria
Pyuria, negative culture, sexually active young man → Chlamydia urethritis
Hexagonal crystals in a child with stones → cystinuria