Laboratory Medicine
Urinalysis
Diagnostic urine analysis — visual, dipstick, and microscopic clues to renal, metabolic, and infectious disease
Urinalysis is a cheap, rapid, high-yield test combining gross inspection, dipstick chemistry, and microscopy. Visual: color (dark yellow → dehydration; red → blood/myoglobin; orange → rifampin/bilirubin) and clarity (turbid → infection or crystals). Dipstick: pH, specific gravity, glucose, ketones, protein, blood, leukocyte esterase, nitrites, bilirubin, urobilinogen. Microscopy: red and white blood cells, bacteria, casts (hyaline, RBC, WBC, granular, waxy), crystals (calcium oxalate, uric acid, struvite, cystine), epithelial cells. Used to diagnose UTI, glomerulonephritis, nephrotic syndrome, diabetes, ketoacidosis, kidney stones, dehydration, and rhabdomyolysis. Cheap (cents per test), bedside, but interpretation requires clinical context.
- Normal pH4.5-8 (typically 5.5-6.5)
- Normal specific gravity1.005-1.030
- UTI screeningLeukocyte esterase + nitrites
- CastsForm in distal tubules; localize disease to kidney
- Proteinuria threshold>150 mg/day or >300 mg/g creatinine
- Hematuria threshold>3 RBCs per high-power field
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Why urinalysis matters
- UTI diagnosis. Cheap rapid screen; positive leukocyte esterase + nitrites supports empiric antibiotics.
- Glomerular disease. RBC casts, dysmorphic RBCs, and proteinuria point to nephritis warranting biopsy.
- Diabetes monitoring. Glycosuria, ketones, and microalbuminuria guide management and detect early nephropathy.
- Kidney stones. Crystal type, pH, hematuria narrow stone composition for prevention.
- Rhabdomyolysis. Dipstick blood positive without RBCs on micro suggests myoglobinuria; check CK.
- Pregnancy care. Routine screening for protein (preeclampsia), glucose (gestational DM), and infection (asymptomatic bacteriuria, treat to prevent pyelonephritis).
- Drug monitoring. Drugs of abuse panels, BMD treatment compliance, organ rejection markers all use urine.
Common misconceptions
- Positive dipstick blood means RBCs. Heme reagent reacts with myoglobin and free hemoglobin too; confirm with microscopy.
- Asymptomatic bacteriuria always needs treatment. Treat only in pregnancy and pre-urologic procedures; otherwise antibiotics cause harm.
- Color reflects hydration alone. Color also reflects bilirubin, blood, beets, rifampin, B-vitamins.
- Sterile pyuria is innocuous. Sterile pyuria points to TB, atypical organisms (Chlamydia, Mycoplasma), or interstitial nephritis.
- Negative nitrites rules out UTI. Gram-positive cocci and Pseudomonas don't reduce nitrate; short bladder dwell time also misses positives.
- Foamy urine is always proteinuria. Foam alone is nonspecific; quantify protein:creatinine ratio for confirmation.
Frequently asked questions
How does urinalysis diagnose UTI?
Dipstick leukocyte esterase indicates pyuria (white blood cells); nitrites indicate gram-negative bacteria (E. coli, Klebsiella) that reduce dietary nitrate. Both positive: high specificity for UTI. Either alone has lower specificity. Microscopy confirms WBCs (>5/hpf) and bacteria. Culture quantifies organism and sensitivity. Catheter or contaminated samples reduce specificity. In symptomatic women with pyuria, empiric treatment without culture is often acceptable. Fever or flank pain suggests pyelonephritis requiring blood and urine cultures.
What does proteinuria tell you?
Healthy kidneys excrete <150 mg protein/day. Glomerular proteinuria (filter damage) leaks albumin and larger proteins — nephrotic syndrome (>3.5 g/day) presents with edema, hypoalbuminemia, hyperlipidemia. Tubular proteinuria leaks low-MW proteins (β2-microglobulin) — interstitial nephritis, Fanconi syndrome. Overflow proteinuria from excess plasma protein — Bence Jones in multiple myeloma. Quantify with 24-hour collection or spot urine protein:creatinine ratio. Microalbuminuria (30-300 mg/day) is early diabetic nephropathy.
What are urinary casts?
Cylindrical molds formed in distal tubules from Tamm-Horsfall protein. Hyaline (acellular): nonspecific, dehydration, exercise. RBC casts: glomerulonephritis (post-streptococcal, IgA, lupus, Wegener). WBC casts: pyelonephritis or acute interstitial nephritis. Granular casts: acute tubular necrosis ("muddy brown" casts). Waxy casts: chronic kidney disease. Fatty casts ("Maltese cross" under polarized light): nephrotic syndrome. Casts are pathognomonic for renal-origin disease — distinguish kidney from lower urinary tract source.
What is hematuria and how is it evaluated?
Microscopic hematuria: >3 RBCs per high-power field; gross hematuria visible. Dipstick blood is sensitive but cross-reacts with myoglobin (rhabdomyolysis) and free hemoglobin (intravascular hemolysis) — confirm with microscopy. Glomerular hematuria: dysmorphic RBCs, RBC casts, proteinuria → nephrology workup. Non-glomerular: round RBCs, no casts → CT urography and cystoscopy for stones, tumors, trauma. Painless gross hematuria in adults is bladder cancer until proven otherwise.
What does glycosuria mean?
Glucose appears in urine when serum glucose exceeds renal threshold (~180 mg/dL). Diabetes is the most common cause; monitor poorly controlled DM. SGLT2 inhibitors (empagliflozin, dapagliflozin) cause glycosuria therapeutically — block proximal tubule glucose reabsorption, lower glucose, and improve cardiovascular and renal outcomes in diabetes and heart failure. Pregnant women may have benign glycosuria from increased GFR. Fanconi syndrome causes glycosuria with normal blood glucose due to proximal tubule dysfunction.
How does urinalysis detect kidney stones?
Crystals visible on microscopy can suggest stone composition: calcium oxalate (envelope-shaped), uric acid (rhomboid, low pH), struvite (coffin-lid, alkaline urine, infection-related Proteus/Klebsiella), cystine (hexagonal, autosomal recessive cystinuria). Hematuria is often present. Acidic urine favors uric acid stones; alkaline urine favors struvite and calcium phosphate. CT without contrast is gold-standard imaging. 24-hour urine collection identifies metabolic risk factors (hypercalciuria, hyperuricosuria, low citrate, low volume).
What about ketones and bilirubin?
Ketones (acetoacetate, β-hydroxybutyrate, acetone) appear during fasting, prolonged exercise, low-carbohydrate diet, alcohol use, and diabetic ketoacidosis. Dipstick detects acetoacetate; β-hydroxybutyrate (predominant in DKA) requires serum measurement. Bilirubin in urine indicates conjugated hyperbilirubinemia from biliary obstruction or hepatocellular injury (water-soluble conjugated bilirubin can be excreted; unconjugated cannot). Urobilinogen is normally trace; elevated in hemolysis or hepatic dysfunction; absent in complete biliary obstruction.