Urinalysis, Urine Microscopy and Casts
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Direct answer
Urinalysis combines a dipstick test with microscopy of fresh urine, and the casts it reveals localise kidney disease to a particular part of the nephron. Red cell casts indicate glomerular injury, muddy-brown granular casts indicate tubular necrosis, and white cell casts point to tubulointerstitial inflammation. Dipstick results must be read alongside microscopy, because the dipstick misses several important findings.
What you must remember
- Red cell casts: the defining sign of glomerulonephritis, pointing to a nephritic process rather than a simple bleed from the lower tract.
- Muddy-brown granular casts: the hallmark of acute tubular necrosis, often with renal tubular epithelial cell casts.
- White cell casts: suggest pyelonephritis or acute interstitial nephritis, and eosinophiluria is a weak marker of the latter.
- Waxy casts: broad, homogeneous and refractile, seen in advanced chronic kidney disease and urinary stasis.
- Oval fat bodies: fat-laden tubular cells, with Maltese crosses under polarised light, in nephrotic syndrome.
- Dipstick protein: detects albumin well but misses light chains, so Bence Jones proteinuria needs electrophoresis or immunofixation.
- Dipstick blood with few red cells: suggests myoglobin or haemoglobin in the urine, as in rhabdomyolysis or haemolysis.
- Dysmorphic red cells: acanthocytes, often reported above about 5 per cent, point to a glomerular source of bleeding.
A comparison of casts and crystals
Urine sediment is read by matching the cast or crystal to its classic setting. A single cast rarely settles a diagnosis, but a cast type alongside the clinical picture usually narrows the differential considerably. Casts form in the tubule lumen from a protein matrix, so their contents reflect the segment of the nephron where they were formed.
| Cast or crystal | Typical finding | Classic setting |
|---|---|---|
| Red cell cast | Intact red cells in a matrix | Glomerulonephritis |
| White cell cast | Neutrophils or eosinophils in a matrix | Pyelonephritis, interstitial nephritis |
| Muddy-brown granular cast | Degenerated tubular cells | Acute tubular necrosis |
| Waxy cast | Broad, homogeneous, refractile | Advanced chronic kidney disease |
| Oval fat bodies | Fat-laden tubular cells | Nephrotic syndrome |
| Calcium oxalate crystal | Envelope or dumbbell shape | Ethylene glycol poisoning |
Hyaline casts are non-specific and can appear in concentrated urine in healthy people, so they carry little weight alone. The cast contents matter more than the cast shape, which is why a cellular cast is always more informative than a hyaline one.
Where the examiner tries to catch you
Three traps recur in urinalysis questions, and each depends on knowing what the dipstick cannot detect.
- Trap: reading a positive dipstick blood as haematuria. Correct: a positive dipstick with few red cells on microscopy suggests myoglobin or haemoglobin, as seen in rhabdomyolysis or haemolysis.
- Trap: calling every pyuria a urinary infection. Correct: sterile pyuria with a negative urine culture suggests genitourinary tuberculosis, and early morning samples are examined for acid-fast bacilli.
- Trap: using a dipstick to exclude myeloma. Correct: the dipstick detects albumin, not light chains, so urine electrophoresis or immunofixation is required when light chain proteinuria is suspected.
Indian viva questions often ask for the crystal of ethylene glycol poisoning, which is calcium oxalate, and the answer is found on sediment examination.
Frequently asked questions
Which urinary cast indicates glomerulonephritis?
Red cell casts are the cast type that indicates glomerulonephritis, because they form only when red cells leak through the glomerulus into the tubule. Dysmorphic red cells on microscopy support the same glomerular origin of bleeding.
What does a muddy-brown granular cast indicate in urinalysis?
Muddy-brown granular casts indicate acute tubular necrosis, reflecting degenerated tubular epithelium within the cast matrix. They are often accompanied by renal tubular epithelial cell casts, and their presence supports intrinsic renal injury over a prerenal cause.
Why does a dipstick miss Bence Jones proteinuria?
Dipstick protein tests detect albumin well but do not reliably detect light chains, so Bence Jones proteinuria is commonly missed. Urine electrophoresis or immunofixation is needed when multiple myeloma or light chain disease is suspected.
What is sterile pyuria and what should a student think of first?
Sterile pyuria is white cells in the urine with a negative routine urine culture, and genitourinary tuberculosis is the first diagnosis to consider. Early morning urine samples are examined for acid-fast bacilli, and a culture for mycobacteria is the confirmatory test.
Which crystal suggests ethylene glycol poisoning on urinalysis?
Calcium oxalate crystals, with an envelope or dumbbell shape, suggest ethylene glycol poisoning when seen in the urine of a patient with acidosis and renal failure. The crystals form because the metabolites of ethylene glycol precipitate calcium oxalate in the renal tubules.