Urine Studies
Urine Studies - Interpretation of urine dipstick, microscopy, quantitative urine tests and fractional excretions.
Practical orientation: Urine tests are most useful when interpreted with the clinical question: volume status, tubular function, protein type, electrolyte handling or sediment activity.
Urine dipstick
| Field | Interpretation |
|---|---|
| Specific gravity | Relative density; can approximate osmolality but is distorted by glucose, protein and contrast. |
| pH | Low with acid load; high with urease organisms, bicarbonaturia or selected tubular disorders. |
| Protein | Mainly detects albumin; misses LMW proteins and many light chains. |
| Blood | Detects hemoglobin/myoglobin activity; confirm RBCs on microscopy. |
| Leukocyte esterase | WBCs, but false positives from contamination. |
| Nitrite | Enterobacteriaceae; negative does not exclude UTI. |
| Glucose | Hyperglycemia, proximal tubule dysfunction or SGLT2 inhibitor. |
| Ketones | Acetoacetate/acetone; not beta-hydroxybutyrate. |
| Bilirubin/urobilinogen | Hepatobiliary and hemolysis clues. |
Specific gravity
| SG | Interpretation |
|---|---|
| 1.008-1.012 | Isosthenuria; possible concentration defect in ATN/CKD. |
| <1.008 | Dilute urine; low ADH effect, DI or primary polydipsia. |
| >1.012 | ADH activity; dehydration or volume depletion. |
Urine microscopy
| Finding | Significance |
|---|---|
| Dysmorphic RBCs/acanthocytes | Glomerular hematuria. |
| Isomorphic RBCs | Extraglomerular bleeding. |
| RBC casts | GN or vasculitis. |
| WBC casts | AIN, pyelonephritis or GN. |
| Fatty casts/oval fat bodies | Nephrotic syndrome. |
| Tubular epithelial cell casts | Tubular injury/ATN. |
| Muddy brown granular casts | ATN. |
| Waxy casts | Chronic advanced kidney disease or slow tubular flow. |
| Hyaline casts | Nonspecific; can be benign. |
Crystals
| Crystal | Clues |
|---|---|
| Calcium oxalate | Envelope/needle; ethylene glycol, hyperoxaluria, stones. |
| Calcium phosphate | Alkaline urine, phosphate nephropathy, stones. |
| Uric acid | Acidic urine, gout, tumour lysis, stones. |
| Cystine | Hexagonal; cystinuria. |
| Struvite | Coffin-lid; urease bacteria and infected stones. |
| Drug crystals | Acyclovir, indinavir, sulfa drugs, methotrexate and others. |
Quantitative urine tests
| Test | Use |
|---|---|
| Spot urine creatinine | Corrects spot assays for urine concentration. |
| UPCR/UACR | Estimates daily protein/albumin excretion. |
| 24-hour urine | Gold standard for many measures but collection error is common. |
| Uosm | Reflects solute concentration and ADH effect. |
| UNa | Sodium avidity; useful but confounded by diuretics, tubular injury and CKD. |
| FENa | Helps assess sodium handling in AKI, with limitations. |
| FEUrea | Sometimes useful on diuretics, also limited. |
| UAG/UOG | Estimate ammonium excretion in non-anion gap metabolic acidosis. |
| Electrolyte-free water clearance | Useful in complex sodium/water disorders. |