Meghan Jobson, MD PhD
Lowell Lo, M.D.
Initial Approach
- Although H&P is helpful, urinalysis and examination of urine sediment are crucial in evaluating renal pathology & providing a window into systemic disease
- UA is the primary noninvasive diagnostic tool, but there is not always correlation between severity of disease & UA findings
- How to collect a UA: examine urine within 30-60 minutes of voiding, mid-stream urine specimen (except first-void for urine STD screening) and check dipstick. For microscopy, pour 10mL urine into graduated centrifuge tube and centrifuge at 3000 rpm for 3-5 mins. Decant 9mL supernatant and resuspend remaining 1mL urine with pipet. Place one drop of this resuspended urine on slide with coverslip. Examine under microscope.
Components of the Routine Urinalysis & Microscopy
Color
- Clear to dark yellow based on concentration
- Red (see Hematuria) – blood, beeturia, achlorhydria due to pernicious anemia, or coingestion of oxalate-containing foods (e.g., spinach & rhubarb)
- If lots of “blood” but no RBCs & heme-positive consider hemoglobinuria or myoglobinuria
- For true hematuria, initially check if transient or persistent, microscopic or gross
- Black: alkaptonuria/ochronosis, sometimes hemoglobinuria, myoglobinuria
- Purple: purple urine bag syndrome, porphyria
- Green: bilirubinuria, propofol, amitriptyline, methylene blue
- Orange/Brown: jaundice, rhabdomyolysis, Gilbert’s, porphyria, rifampin, pyridium
- Blue: methylene blue
- Pink (see Hematuria): uric acid crystals with high dose propofol
- White: pyuria, phosphate crystals, chyluria, propofol
Urine pH
- Usually between 4.6 – 6
- High urine pH suggests an alkali load or inability to acidify urine appropriately
- High urine pH seen with Proteus UTI, systemic alkalosis, proximal or distal RTA, carbonic anhydrase inhibitors
Protein (see chapter on Proteinuria for further details)
- Dipstick detects albumin (e.g., not light chains) – specific but insensitive, so you have to order urine microalbumin : creatinine ratio specifically to assess for early albuminuria and quantification
- If proteinuria seen on dipstick, check spot protein: creatinine ratio
- Mild transient proteinuria can be seen with infection, fever, heavy exercise and benign orthostatic proteinuria
- Nephrotic range proteinuria > 3.5 g/day
- Iodinated contrast gives a false positive for urine protein for at least 24 hours
Urine osmolality
- Unlike serum osmolality, vast variation, and you have to correlate with volume status – useful to check if patients with hypo/hypernatremia, AKI, polyuria
- UOsm < 100 mosm/kg in a hyponatremic patient often indicates appropriate ADH suppression and can indicate primary polydipsia
- UOsm > 300mosm/kg in hyponatremic patient often indicates low effective circulating volume (e.g., hypovolemia, cirrhosis, CHF) or SIADH
- In AKI, the tubule loses its ability to concentrate so UOsm is often < 350mosm/kg whereas UOsm > 500 suggests pre-renal state if patient is oliguric
Specific gravity
- Specific gravity determined by number & size of particles in the urine (water = 1.0 as a reference)
- Urine s.g. < 1.003 shows maximally dilute urine whereas > 1.035 is very concentrated
- Repeated urine s.g. = 1.010 represents isosthenuria which suggests loss of tubular concentrating and diluting capacity – seen clinically with significant renal diseases
Glucose/Ketones
- Usually glucosuria occurs if plasma glucose > 180mg/dL, not sensitive/specific
- Think of Fanconi’s syndrome (associated w/ multiple myeloma, tenofovir or ifosfamide use, heavy metal exposure) if glucosuria seen with hypophosphatemia, hypouricemia, RTA (i.e. proximal tubule dysfunction)
- Ketonuria seen with DKA, fasting, starvation, alcoholic ketoacidosis, isopropyl alcohol intoxication (with osmolar gap)
Crystals
- Uric acid, calcium phosphate, calcium oxalate (stones, various)
- Magnesium ammonium phosphate crystals (struvite stones)
- Calcium oxalate crystals (ethylene glycol ingestion)
- Uric acid crystals (nonspecific finding – can be seen in hypovolemia or severe problem like tumor lysis syndrome)
- Healthy people with concentrated urine from decreased fluid intake can have crystals in urine
Bacteria
- Leukocyte esterase represents pyuria (detects lysed WBCs)
- Nitrite represents Enterobacteraciae species
- Combination of +LE/+nitrite has PPV of 74% for UTI if both are positive, NPV of >97% if both are negative
Pyuria
- Infection is most common cause, but sterile pyuria can be seen in urinary TB, prostatitis, analgesic nephropathy, and interstitial nephritis
- Urine eosinophilia suggestive of AIN
- Correlate pyuria with number of epithelial cells seen to assess for contamination
Casts
- Hyaline casts - hypovolemia or diuretic use, not pathogenic
- RBC casts -specific for glomerulonephritis
- WBC casts -non-specific, pyelonephritis, tubulointerstitial disease, glomerular disorders
- Fatty casts -nephrotic syndrome, “Maltese cross” appearance
- Muddy brown and granular casts –ATN-
Suspicious Constellations on UA and Disease Associations
- Hematuria, RBC casts, dysmorphic RBCs, proteinuria - Glomerular disease
- Pyuria with WBC and granular or waxy casts but no proteinuria - Suggests tubular or interstitial disease or urinary tract obstruction
- Hematuria and pyuria but no casts - Seen in AIN, glomerular disease, vasculitis, obstruction, renal infarction
- Relatively “normal” UA does not rule out AKI. Cannot exclude pre-renal, obstructive or intrinsic renal diseases
References
Lin J, Denker BM. Azotemia and Urinary Abnormalities. Harrison’s Online.
Wald.. Urinalysis in the diagnosis of kidney disease. UptoDate, 2016.