urine microscopy
The urine sediment is free, fast, and diagnostic — but only if you spin and examine it yourself on a fresh sample rather than relying on automated lab reporting. Cast morphology localises the site and nature of kidney injury — RBC casts confirm glomerulonephritis, muddy brown casts confirm ATN, and WBC casts point to interstitial nephritis or pyelonephritis. A bland sediment in AKI narrows the differential to prerenal, obstruction, or hepatorenal syndrome.
cast interpretation
| cast type | diagnosis | action |
|---|---|---|
| RBC casts | glomerulonephritis / vasculitis | check creatinine + quantify proteinuria; send ANCA, anti-GBM, C3/C4, ANA; urgent nephrology referral |
| WBC casts | AIN or pyelonephritis | drug history; urine culture; consider biopsy if no recovery |
| muddy brown granular casts | ATN | supportive care; remove cause |
| oval fat bodies / fatty casts (maltese cross on polarised light) | nephrotic syndrome | quantify proteinuria; nephrotic workup |
| hyaline casts | non-specific | concentrated urine, exercise, CKD — no specific diagnosis |
| waxy / broad casts | advanced CKD (dilated, atrophic tubules) | marker of chronicity |
Examine within 1–2 h of collection; alkaline urine and delayed processing dissolve casts, so a “bland” stale sample is unreliable.
cellular elements
| finding | significance |
|---|---|
| dysmorphic RBCs | suggest glomerular origin but variable sensitivity; general “dysmorphic” thresholds (>25%) are inconsistent across studies |
| acanthocytes ≥5% | highly specific (~90–100% PPV) for glomerular haematuria — see approach to proteinuria and haematuria |
| isomorphic RBCs | suggests non-glomerular source — proceed with urological workup (imaging ± cystoscopy per risk) |
| WBCs (sterile pyuria) | AIN, TB, atheroemboli, tubulointerstitial disease, recently treated UTI, chlamydial urethritis |
| eosinophiluria | poor test characteristics (sensitivity ~31%, specificity ~68% at 1% Hansel cutoff); does not distinguish AIN from ATN — do not rely on it |
| squamous epithelial cells | contamination — resend a clean-catch sample |
| renal tubular epithelial cells | tubular injury (ATN, AIN) |
dipstick-microscopy discordance
| dipstick result | microscopy | interpretation |
|---|---|---|
| haem-positive | 0 RBCs | pigment nephropathy — myoglobinuria (rhabdomyolysis) or haemoglobinuria (intravascular haemolysis). Check CK, haptoglobin, LDH. |
| protein-negative | high quantitative protein on UPCR | overflow proteinuria — light chains ([[haematologic renal disease |
| leucocyte esterase-positive | 0 WBCs on microscopy | false positive (vaginal contamination, concentrated sample) or lysed WBCs |
A negative dipstick does not exclude significant proteinuria. Light chain overflow proteinuria (myeloma) and low-molecular-weight tubular proteinuria are missed. Always quantify with UPCR or UPEP when paraprotein-related disease is suspected.
traps
- Stale urine → false bland sediment — casts dissolve in alkaline or delayed samples. A “bland” result on a sample sitting for hours is meaningless.
- Eosinophiluria does not confirm AIN — likelihood ratios near 1; it does not shift pretest probability. Urinary CXCL9 is an emerging biomarker (AUC ~0.94) but not yet routine.
- Dipstick haem-positive with 0 RBCs — pigment (myoglobin or haemoglobin), not haematuria. Check CK if rhabdomyolysis suspected.