Urinalysis Interpretation
A Step 2 CK-focused walkthrough of urinalysis that pairs each dipstick and microscopy finding with its diagnosis and next best step, anchored on cast recognition and the nephritic/nephrotic/tubulointerstitial framework.
How the boards use urinalysis
Urinalysis is a three-part test: gross appearance, dipstick (chemical), and microscopy (sediment). On exams it is rarely the whole answer — it is the tie-breaker that localizes disease and dictates the next best step. Read it systematically by pairing each dipstick square with the sediment: leukocyte esterase/nitrites with pyuria and bacteria (infection), heme with RBCs vs. myoglobin, protein with casts (glomerular vs. tubular).
The single highest-yield principle: urinary casts localize pathology to the kidney. A cast is a Tamm-Horsfall protein mold of a tubule, so cells trapped inside it must have originated in the nephron — not the bladder or urethra. Master three axes and most vignettes solve themselves: infection vs. inflammation, glomerular (nephritic/nephrotic) vs. tubulointerstitial, and pre-renal vs. intrinsic injury.
- Leukocyte esterase: neutrophil enzyme = pyuria; sensitive screen for UTI
- Nitrites: Enterobacteriaceae (E. coli, Klebsiella, Proteus) reduce nitrate → nitrite; specific, NOT sensitive. Enterococcus, S. saprophyticus, and Pseudomonas are often nitrite-negative
- Blood (heme): dipstick-positive with no RBCs on microscopy → myoglobinuria (rhabdomyolysis) or hemoglobinuria (hemolysis)
- Protein: dipstick detects albumin and misses Bence-Jones light chains — order urine protein electrophoresis if myeloma suspected; nephrotic range >3.5 g/day
- Glucose: serum exceeds renal threshold ~180 mg/dL, SGLT2 inhibitors, or proximal (Fanconi) tubular defect
- Ketones: nitroprusside detects acetoacetate/acetone but NOT β-hydroxybutyrate — early DKA may read falsely low
- pH: alkaline + UTI → urease organism (Proteus) → struvite stone; persistently acidic → uric acid stone
- Specific gravity: fixed near 1.010 (isosthenuria) in ATN/CKD
Urinary casts → the disease they prove
| Cast | Classic association | Vignette clue |
|---|---|---|
| RBC cast | Glomerulonephritis / vasculitis | Cola-colored urine, HTN, dysmorphic RBCs |
| WBC cast | Pyelonephritis; acute interstitial nephritis | Flank pain + fever; or new drug + eosinophiluria |
| Muddy brown / granular | Acute tubular necrosis (ATN) | Post-hypotension/contrast/aminoglycoside; FENa >2% |
| Fatty cast / oval fat body | Nephrotic syndrome | "Maltese cross" under polarized light |
| Waxy / broad | Advanced CKD | Chronic, bland, wide casts |
| Hyaline | Nonspecific / normal | Dehydration, exercise, loop diuretics |
| Epithelial cell | ATN / AIN | Tubular injury |
Vignette: A 24-year-old man develops cola-colored urine, periorbital edema, and BP 158/96 two weeks after streptococcal pharyngitis. UA: 2+ protein, 3+ blood, dysmorphic RBCs and RBC casts.
Diagnosis: Nephritic syndrome — post-streptococcal glomerulonephritis (PSGN). RBC casts localize the bleeding to the glomerulus.
Next best step: Serum complement — low C3 (with normal C4) — plus anti-streptococcal titers (ASO, anti-DNase B). Management is supportive (salt/water restriction, treat HTN and edema); C3 normalizes within ~8 weeks. Prognosis is excellent in children, but adults more often have residual renal impairment.
Board contrast: If gross hematuria occurs concurrently with (not 2 weeks after) a URI in a young adult → IgA nephropathy (synpharyngitic), with normal complement.
Vignette: A 62-year-old woman started a PPI (or a β-lactam / NSAID) 10 days ago has rising creatinine, low-grade fever, and a maculopapular rash. UA: WBCs, WBC casts, and eosinophils; urine culture negative (sterile pyuria).
Diagnosis: Acute interstitial nephritis (AIN) — drug hypersensitivity. The classic triad (fever, rash, eosinophilia) is present in only a minority.
Next best step: Stop the offending drug — this is the primary treatment; renal function usually recovers. A short corticosteroid course is used if no improvement after withdrawal.
Sterile pyuria differential: AIN, chlamydia/gonococcal urethritis, TB, partially treated UTI, nephrolithiasis.
See the shape, name the disease — exactly how the boards test crystals and casts:
- Envelope / octahedron → calcium oxalate → most common stone; the needle / dumbbell (monohydrate) form points to ethylene glycol poisoning
- Coffin-lid → struvite (Mg-ammonium-phosphate) → urease organisms (Proteus), staghorn calculi, alkaline urine
- Hexagonal plate → cystine → cystinuria (positive cyanide-nitroprusside test)
- Rhomboid / needles → uric acid → gout, tumor lysis syndrome, acidic urine
- Maltese cross (polarized light) → fatty cast / oval fat body → nephrotic syndrome
- Muddy brown granular cast → ATN
Nephritic vs. nephrotic at a glance
| Feature | Nephritic | Nephrotic |
|---|---|---|
| Proteinuria | <3.5 g/day | >3.5 g/day |
| Hematuria | Dysmorphic RBCs, RBC casts | Bland / minimal |
| Casts | RBC casts | Fatty casts, oval fat bodies |
| Exam | HTN, periorbital edema | Generalized pitting edema |
| Hallmark labs | Low complement (some) | Hypoalbuminemia, hyperlipidemia, lipiduria |
| Classic causes | PSGN, IgA, anti-GBM, vasculitis | Minimal change, FSGS, membranous, diabetic |
- Blood on dipstick, 0 RBCs on micro → send serum CK / haptoglobin: myoglobinuria (rhabdo) or hemoglobinuria
- Positive nitrite is highly specific — treat; a negative nitrite does NOT exclude UTI (Enterococcus, S. saprophyticus, Pseudomonas)
- Asymptomatic bacteriuria: treat ONLY in pregnancy or before a urologic procedure with expected mucosal bleeding; otherwise do not treat
- Isolated microscopic hematuria (no protein/casts) in a smoker or age ≥35 → cystoscopy + CT urography to exclude urothelial cancer
- Nephrotic-range proteinuria + bland sediment in a long-standing diabetic → diabetic nephropathy; biopsy usually not needed
- Suspect myeloma but dipstick protein negative/low → dipstick misses light chains; order urine protein electrophoresis
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