Dipstick Interpretation

Protein: dipstick detects albumin (not light chains). Trace = normal. +1 to 4+ suggests glomerular disease (nephrotic/nephritic). 24-hour urine protein >3.5g/day = nephrotic range; >300mg/day in pregnancy = preeclampsia. Microalbuminuria (30-300mg/day): early diabetic nephropathy. Blood: positive dipstick blood with no RBCs on microscopy = hemoglobin (hemolysis) or myoglobin (rhabdomyolysis). Both give pink/red urine with positive dipstick but no RBCs. Leukocyte esterase: from WBCs; suggests UTI, but can be contamination. Nitrites: gram-negative bacteria (E. coli, Klebsiella, Proteus) reduce nitrate → nitrite; gram-positive organisms (Staphylococcus, Enterococcus) do NOT produce nitrites. Glucose: appears when blood glucose >~180 mg/dL (saturates renal threshold) OR in Fanconi syndrome (proximal tubule dysfunction → glycosuria at normal glucose). pH: alkaline urine (pH >7): Proteus UTI (urease splits urea → ammonia), RTA type 1 (cannot acidify urine), post-meal alkaline tide. Acidic urine: high protein diet, uric acid stones.

Urine Casts: The High-Yield List

Casts form in the tubular lumen when Tamm-Horsfall protein (uromodulin) precipitates around cells or debris. Hyaline casts: acellular, transparent; normal in small numbers; increased in dehydration and diuretic use — nonpathologic. RBC casts: the most pathognomonic finding in nephrology — virtually diagnostic of glomerulonephritis (GN). The RBCs must enter the tubule from glomerular injury. Seen in: IgA nephropathy (Berger's disease, most common GN worldwide), post-streptococcal GN, RPGN (rapidly progressive GN — crescentic), lupus nephritis, Goodpasture syndrome, ANCA-associated vasculitis. WBC casts: pyelonephritis (#1) or acute interstitial nephritis (AIN). In AIN, also look for eosinophiluria (Wright stain). Granular casts ('muddy brown' casts): casts with cellular debris; pathognomonic of acute tubular necrosis (ATN). Fatty casts / oval fat bodies: lipid in urine → polarized light shows 'Maltese cross' pattern; pathognomonic of nephrotic syndrome. Waxy casts / broad casts: wide tubular casts of waxy material; indicate advanced chronic kidney disease (wide casts = collecting duct dilation from chronic nephron loss).

Specific Disease Patterns

Nephrotic syndrome: heavy proteinuria (>3.5g/day) + hypoalbuminemia + edema + hyperlipidemia + lipiduria. UA: 3-4+ protein, fatty casts, oval fat bodies, no RBC casts. Causes by age: children = minimal change disease (#1 — responds to steroids, no findings on LM); adults = FSGS (#1 in African Americans, HIV), membranous nephropathy (anti-PLA2R antibodies, hepatitis B, cancer, medications), diabetic nephropathy. Nephritic syndrome: hematuria (dysmorphic RBCs) + RBC casts + proteinuria + hypertension + variable renal function decline. Acute Tubular Necrosis (ATN): most common cause of AKI in hospitalized patients. Causes: ischemia (hypotension, sepsis, major surgery) or nephrotoxins (aminoglycosides, contrast, myoglobin/hemoglobin). UA: muddy brown granular casts + tubular epithelial cell casts + renal tubular epithelial (RTE) cells. BUN:Cr ratio typically <20 (tubular cells cannot concentrate urine). Acute Interstitial Nephritis (AIN): drug hypersensitivity (NSAIDs, PCN/cephalosporins, PPIs, allopurinol). Classic triad: fever + rash + eosinophilia (only 10% have all three). UA: WBC casts + eosinophiluria + mild proteinuria + hematuria.