I. URINE ANALYSIS
URINE ANALYSIS PROCEDURE
For a routine urine analysis, a fresh (<1 hr old), clean-catch urine sample is acceptable. If the analysis cannot be performed immediately, refrigerate the sample. (When urine stands at room temperature for a long period, casts and red blood cells undergo lysis, and the urine becomes alkalinized with precipitation of salts.)
1. Pour 5–10 mL of well-mixed urine into a centrifuge tube.
2. Check for appearance (color, turbidity, odor). If a urine sample looks grossly cloudy, it is sometimes advisable to examine an unspun sample. If an unspun sample is used, make note that you have done so. In general, for routine urine analysis, a spun sample is more desirable.
3. Spin a capped sample at 3,000 rpm for 3–5 min.
4. While the sample is in the centrifuge, use the dipstick (Chemstrip, etc.) to perform the dipstick evaluation on the remaining sample. Read the results according to the color chart on the bottle. Allow the correct amount of time before reading the test (usually 1–2 min) to avoid false results. Chemstrip 10 provides 10 tests (specific gravity, pH, leukocytes, nitrite, protein, glucose, ketone, urobilinogen, bilirubin, and blood). Other strips may provide less. Agents that color the urine (eg, phenazopyridine [Pyridium]) may interfere with the reading. Dipstick specific gravity is also available on some assay strips.
5. Decant and discard the supernatant. Mix the remaining sediment by flicking it with your finger and pouring or pipetting 1 or 2 drops onto a microscope slide. Cover with a coverslip.
6. Examine 10 low-power fields (LPFs; 10× objective) for epithelial cells, casts, crystals, and mucus. Casts are usually reported as number per low-power field and tend to collect around the periphery of the coverslip.
7. Examine several high-power fields (HPFs; 40× objective) for epithelial cells, crystals, RBCs, WBCs, bacteria, and parasites (trichomonads). RBCs, WBCs, and bacteria are usually reported as number per high-power field.
Normal Urine Analysis Values
• Appearance: Yellow, clear, or straw-colored
• Specific gravity:
– Neonate: 1.012
– Infant: 1.002–1.006
– Child and adult: 1.001–1.035 (with normal fluid intake 1.016–1.022)
• pH:
– Newborn/neonate: 5–7
– Child and adult: 4.6–8.0
• Negative for bilirubin, blood, acetone, glucose, protein, nitrite, leukocyte esterase, reducing substances
• Trace: Urobilinogen
• RBC: The exact definition of microscopic hematuria is debated, but is generally defined as >3 RBC/HPF (40×).
• WBC: 0–4/HPF
• Epithelial cells: Occasional
• Hyaline casts: Occasional
• Bacteria: None
• Crystals: Some limited crystals, based on urine pH (see below)
Differential Diagnosis for Routine Urine Analysis
• Appearance (see Section II Urine, Abnormal Color; Section II Urine, Foaming; Urine, Odor; and Urine, Particles in)
• pH:
– Acidic: High-protein (meat) diet, ammonium chloride, mandelic acid and other medications, acidosis (due to ketoacidosis [starvation, diabetes], chronic obstructive pulmonary disease [COPD])
– Basic: Urinary tract infections (UTIs), renal tubular acidosis, diet (high-vegetable, milk, immediately after meals), sodium bicarbonate therapy, vomiting, metabolic alkalosis, diuretic therapy
• Specific gravity:
– Usually corresponds to osmolarity, except with osmotic diuresis. A value >1.023 indicates normal renal concentrating ability:
Increased: Volume depletion, congestive heart failure (CHF), adrenal insufficiency, diabetes mellitus, inappropriate antidiuretic hormone (ADH), increased proteins (nephrosis); if markedly increased (1.040–1.050), suspect artifact or excretion of radiographic contrast media.
Decreased: Diabetes insipidus, pyelonephritis, glomerulonephritis, water load with normal renal function
• Bilirubin:
– Positive: Obstructive jaundice (intrahepatic and extrahepatic), hepatitis (Note: False positive with stool contamination)
• Blood:
– Positive: See Section I Hematuria, gross and microscopic, adult and Hematuria, gross and microscopic, pediatric
– Note: If the dipstick is positive for blood, but no RBCs are seen, free hemoglobin may be present from trauma, from a transfusion reaction, or from lysis of RBCs (RBCs will lyse if the pH is <5 or >8), or there may be myoglobin present because of a crush injury, burn, or tissue ischemia.
• Glucose:
– Positive: Diabetes mellitus, pancreatitis, pancreatic carcinoma, pheochromocytoma, Cushing syndrome, shock, burns, pain, steroids, hyperthyroidism, renal tubular disease, iatrogenic causes
(Note: The glucose oxidase technique in many kits is specific for glucose and will not react with lactose, fructose, or galactose.)
• Ketones:
– Detects primarily acetone and acetoacetic acid and not β-hydroxybutyric acid:
– Positive: Starvation, high-fat/low-carbohydrate diet, diabetic ketoacidosis, vomiting, diarrhea, hyperthyroidism, pregnancy, febrile states (especially in children)
• Nitrite:
– Many bacteria will convert nitrates to nitrite. (See also the section on Leukocyte Esterase, below.)
Positive: Infection (A negative test does not rule out infection, because some organisms, such as Streptococcus faecalis and other gram-positive cocci, will not produce nitrite, and the urine must also be retained in the bladder for several hours to allow the reaction to take place.)
• Protein:
– Indication by dipstick of persistent proteinuria should be quantified by 24-hr urine studies:
Positive: Pyelonephritis, glomerulonephritis, Kimmelstiel-Wilson syndrome (diabetes), nephrotic syndrome, myeloma, postural causes, preeclampsia, inflammation, and malignancies of the lower tract, functional causes (fever, stress, heavy exercise), malignant hypertension, congestive heart failure
• Leukocyte esterase (see Section I Pyuria.):
– This test detects ≥5 WBCs/HPF or lysed WBCs. When combined with the nitrite test, it has a predictive value for UTI of 74% if both tests are positive, and >97% if both tests are negative:
Positive: Infection (false-positive with vaginal contamination)
• Reducing substance:
– Positive: Glucose, fructose, galactose, false-positives (vitamin C, salicylates, antibiotics, etc.)
• Urobilinogen:
– Positive: Cirrhosis, CHF with hepatic congestion, hepatitis, hyperthyroidism, suppression of gut flora with antibiotics (Note: With obstructive jaundice, urobilinogen is usually normal, but bilirubin is elevated.)
Urine Sediment
Many labs no longer do microscopic examinations unless specifically requested or if the dipstick test shows evidence of an abnormal finding (such as positive leukocyte esterase):
• RBCs: Trauma, pyelonephritis, genitourinary tuberculosis (TB), cystitis, prostatitis, stones, tumors (malignant and benign), coagulopathy, and any cause of blood on dipstick test (see above on hemoglobin)
• WBCs: Infection anywhere in the urinary tract, TB, renal tumors, acute glomerulonephritis, radiation, interstitial nephritis (analgesic abuse)
• Epithelial cells: Acute tubular necrosis (ATN), necrotizing papillitis (most epithelial cells are from an otherwise unremarkable urethra)
• Parasites: Trichomonas vaginalis, Schistosoma haematobium infections
• Yeast: Candida albicans infection (especially in diabetics, immunosuppressed patients, or if a vaginal yeast infection is present)
• Spermatozoa: Normal in males immediately after intercourse or nocturnal emission
• Crystals: Note that urine should be examined fresh and warm because clouding due to phosphate precipitation may be observed when urine cools:
– Abnormal: Cystine, sulfonamide, leucine, tyrosine, cholesterol
– Normal in acidic urine: Oxalate (small square crystals with a central cross), uric acid
– Normal in alkaline urine: Calcium carbonate, triple phosphate (resemble coffin lids)
• Contaminants: Cotton threads, hair, wood fibers, amorphous substances (all usually unimportant)
• Mucus: Large amounts suggest urethral disease (normal from ileal conduit or other forms of urinary diversion).
• Glitter cells: WBCs are lysed in hypotonic solution.
• Casts: The presence of casts in a urine sample localizes some or all of the disease process to the kidney itself:
– Hyaline casts (occasionally acceptable, unless they are numerous), benign hypertension, nephrotic syndrome, after exercise
– RBC casts: Acute glomerulonephritis, lupus nephritis, subacute bacterial endocarditis (SBE), Goodpasture disease, after a streptococcal infection, vasculitis, malignant hypertension
– WBC casts: Pyelonephritis
– Epithelial (tubular) casts: Tubular damage, nephrotoxin, virus
– Granular casts: Breakdown of cellular casts leads to waxy casts; dirty brown granular casts typical for ATN
– Waxy casts (end stage of granular casts): Severe, chronic renal disease; amyloidosis
– Fatty casts: Nephrotic syndrome, diabetes mellitus, damaged renal tubular epithelial cells
– Broad casts: Chronic renal disease
II. SPOT OR RANDOM URINE STUDIES
The so-called spot urine is often ordered to aid in diagnosing various conditions. It relies on only a small sample (10–20 mL) of urine:
• Spot urine for β2 microglobulin (<0.3 mg/L):
– A marker for renal tubular injury:
Increased: Diseases of the proximal tubule (ATN, interstitial nephritis, pyelonephritis), drug-induced nephropathy (aminoglycosides), diabetes, trauma, sepsis
• Spot urine for electrolytes:
– The usefulness of this assay is limited because of large variations in daily fluid and salt intake, and the results are usually indeterminate if a diuretic has been given. (See also Section I Anuria and oliguria, adult and Anuria and oliguria, pediatric.)
Sodium <10 mEq/L (mmol/L): Volume depletion, hyponatremic states, prerenal azotemia (CHF, shock, etc.), hepatorenal syndrome, glucocorticoid excess
Sodium >20 mEq/L (mmol/L): Syndrome of inappropriate antidiuretic hormone (SIADH), ATN (usually >40 mEq/L), postobstructive diuresis, high salt intake, Addison disease, hypothyroidism, interstitial nephritis
Chloride <10 mEq/L (mmol/L): Chloride-sensitive metabolic alkalosis (vomiting, excessive diuretic use), volume depletion
Potassium <10 mEq/L (mmol/L): Hypokalemia, potassium depletion, extrarenal loss
• Spot urine for protein (normal: <10 mg/dL [0.1 g/L] or <20 mg/dL [0.2 g/L] for a sample taken in the early morning)
– See Section I Proteinuria for the differential diagnosis of protein in the urine.
• Spot urine for eosinophils (present with Hansel/Wright staining and white light microscopy):
– Associated with acute interstitial nephritis (especially nephritis associated with drug hypersensitivity) or acute cystitis
– Present with interstitial nephritis; absent with tubular disorders (ATN).
• Spot urine for erythrocyte morphology:
– The morphology of RBCs in a sample of urine that tests positive for blood may give some indication of the nature of the hematuria. Eumorphic red cells are typically seen in cases of postrenal, nonglomerular bleeding. Dysmorphic red cells are more likely associated with glomerular causes of bleeding. Each reference lab has standards, but as a general rule, the presence of >90% dysmorphic erythrocytes in patients with asymptomatic hematuria indicates a renal glomerular source of bleeding, especially if associated with proteinuria and/or casts (ie, IgA nephropathy, poststreptococcal GN, sickle cell disease or trait, etc.). If <90% eumorphic erythrocytes or even mixed results (10–90% eumorphic erythrocytes), this indicates a post-renal cause of hematuria, requiring a complete urologic evaluation (ie, hypercalcuria, urolithiasis, cystitis, trauma, tumors, hemangioma, exercise-induced, benign prostatic hypertrophy [BPH], etc.).
• Spot urine for osmolality (ranges from 40–1,400 mOsm/kg water [mmol/kg]; varies with water intake):
– Patients with normal renal function should concentrate >400–800 mOsmol/kg (mmol/kg) after a 14-hr fluid restriction; <200–400 mOsmol/kg (mmol/kg) is a sign of renal impairment:
Increased: Dehydration, CHF, hypercalcemia, SIADH, adrenal insufficiency, glycosuria, high-protein diet
Decreased: Excessive fluid intake, diabetes insipidus, acute renal failure, medications (acetohexamide, glyburide, lithium)
• Spot urine for myoglobin (qualitative negative):
– Positive: Skelet al muscle conditions (crush injury, electrical burns, carbon monoxide poisoning, delirium tremens, surgical procedures, malignant hyperthermia), polymyositis
III. CREATININE CLEARANCE AND GLOMERULAR FILTRATION RATE
CREATININE CLEARANCE
• Normal:
– Adult male: Total creatinine 1–2 g/24 h (8.8–17.7 mmol/d); clearance 85–125 mL/min/1.73 m2
– Adult female: Total creatinine 0.8–1.8 g/24 h (7.1–15.9 mmol/d); clearance 75–115 mL/min 1.73 m2 (1.25–1.92 mL/s/1.73 m2)
– Child: Total creatinine (>3 yr) 12–30 mg/kg/ 24 h; clearance 70–140 mL/min/1.73 m2 (1.17–2.33 mL/s/1.73 m2)
– Decreased: A decreased creatinine clearance results in an increase in serum creatinine, usually secondary to renal insufficiency. See Section I Renal Failure, Acute, and Renal Failure, Chronic for the differential diagnosis of increased serum creatinine.
– Increased: Early diabetes mellitus, pregnancy
DETERMINATION OF CREATININE CLEARANCE
Creatinine clearance (CrCl) is a sensitive indicator of early renal insufficiency and is a measure of glomerular filtration rate (GFR); however, the GFR does not provide information on the etiology of the renal disease. CrCl decreases with age, with a CrCl of 10–20 mL/min indicating severe renal failure, and usually the need for dialysis. The National Kidney Disease Education Program (NKDEP) recommends using an estimation of GFR (eGFR) from serum creatinine in adults (>18yr) with chronic kidney disease (CKD) and those at risk for CKD (diabetes, hypertension, and family history of kidney failure).
Methods
1. Formal 24-hr Urinary Collection for Creatinine Clearance
Order a concurrent serum creatinine (SCr) and a 24-hr urine creatinine. A shorter time interval can be used (eg, 12 hr), but the formula must be corrected for this change; a 24-hr sample is less prone to collection error.
Where time = 1,440 min if 24-hr collection is used
Example: The following are calculations of (a) CrCl from a 24-hr urine sample with a volume of 1,000 mL, (b) a urine creatinine of 108 mg/100 mL, and (c) a SCr of 1 mg/100 mL (1 mg/dL), where time = 1,440 min if 24-hr collection is used.
To determine if there is a valid, full 24-hr collection, the sample should contain 18–25 mg/kg/24 h of creatinine for adult males or 12–20 mg/kg/24 h for adult females. If the patient is an adult (150 lb = body surface area of 1.73 m2), adjustment of the clearance for body size is not routinely done. Adjustment for pediatric patients is a necessity.
If the values in the previous example were for a 10-yr-old boy who weighed 70 lb (1.1 m2), the clearance would be:
2. Estimated Creatinine Clearance (eGFR)
Estimated glomerular filtration rate (eGFR) is based on SCr combined with other factors such as age, sex, and race and has generally replaced 24-hr urinary CrCl determinations. Online calculators for adults and children are found at: http://nkdep.nih.gov/lab-evaluation/gfr-calculators.shtml (Accessed April 19, 2014)
Adult:
A. Modification of Diet in Renal Disease (MDRD) equation (Ann Intern Med. 1999;130, 137–147): Although more cumbersome than Cockcroft-Gault, the MDRD equation is believed to be more accurate. The equation does not require weight; results are normalized to 1.732 body surface area (BSA), an accepted adult average BSA:
B. Cockcroft–Gault equation:
CrCl estimate
Children:
Use the Schwartz equation:
Where:
• k = Constant (0.33, premature infant; 0.45, term infants to 1 yr; 0.55, children to 13 yr; 0.65, adolescent males; 0.55, adolescent females)
• Height in cm, and SCr in mg/dL
IV. 24-HR URINE STUDIES
• Calcium, urine: See also Section II Hypercalcuria (Absorptive, Renal and Resorptive) and Metabolic Stone Evaluation (24 hr Urine Studies)
– Normally ordered as part of a urolithiasis metabolic evaluation:
Normal: Calcium-free diet <150 mg/24 h (3.7 mmol/d); average calcium diet (600–800 mg/24 h) 100–250 mg/24 h (2.5–6.2 mmol/d)
Increased: Hyperparathyroidism, hyperthyroidism, hypervitaminosis D, distal renal tubular acidosis (type I), sarcoidosis, immobilization, osteolytic lesions (bony metastasis, multiple myeloma), Paget disease, glucocorticoid excess
Decreased: Medications (thiazide diuretics, estrogens, oral contraceptives), hypothyroidism, renal failure, steatorrhea, rickets, osteomalacia, vitamin D deficiency
• Catecholamines, fractionated (norepinephrine, epinephrine, and dopamine):
– Used to evaluate pheochromocytoma and paraganglioma. Avoid drugs that can interfere with the test, leading to falsely high catecholamines: Tricyclic antidepressants, labet alol, levodopa, methyldopa, sotalol, benzodiazepines, amphetamines, decongestants, and most psychoactive agents. All these drugs should be discontinued 2 wk prior to testing:
Normal: Values are variable and depend on the assay method used. Norepinephrine 15–80 mg/24 h (89–473 nmol/24 h), epinephrine 0–20 mg/24 h (SI: 0–118 nmol/24 h), dopamine 65–400 mg/24 h (SI: 384–2,364 nmol/24 h)
Increased: Pheochromocytoma (levels are > twice the upper normal value), paraganglioma, epinephrine administration, presence of drugs (see above)
• Cortisol, free:
– Used to evaluate adrenal cortical hyperfunction; screening test of choice for Cushing syndrome:
Normal: 10–55 μg/24 h (SI: 27–150 nmol)
Increased: Cushing syndrome (adrenal hyperfunction from a pituitary tumor secreting ACTH or ectopic secretion of ACTH by other tumors such as bronchial carcinoid or adrenal tumor secreting cortisol), stress during collection, pregnancy
• Cystine:
– Used to detect cystinuria, homocystinuria:
– Normal: <30–40 mg/d (0.13 mmol/d):
Increased: Homozygotic cystinuria: 400 mg/d (1.7 mmol/d); heterozygotes cystinuria and Fanconi syndrome: Up to 250 mg/d (1 mmol)
• Electrophoresis, protein (24-hr urine protein, 24-hr urine globulins):
– Used to evaluate overall renal function; screen for myeloma, macroglobulinemia, lymphoma, amyloidosis; can differentiate types of proteinuria (see table below)
• 5-Hydroxyindoleacetic acid (5-HIAA):
– 5-HIAA is a serotonin metabolite and is useful in diagnosing carcinoid syndrome:
Normal: 2–8 mg (SI: 10.4–41.6 mmol)/24-hr urine collection
Increased: Carcinoid tumors (except rectal), certain foods (banana, pineapple, tomato, walnuts, avocado), phenothiazine derivatives
• Heavy met als:
– Measures exposure to arsenic (total), arsenic (inorganic), cadmium, lead, and mercury, usually following occupational or environmental exposure:
Normal: Arsenic (total): 0–50 μg/24 h (<50 μg/L); arsenic (inorganic): <20 μg/L; cadmium: <3.0 μg/24 h (<2 μg/g creatinine); lead: <80 μg/24 h (< 50 μg/L); mercury: <20 μg/L
Increased: Indicative of exposure
• Metanephrines:
– These metabolic products of epinephrine and norepinephrine are a primary screening test for pheochromocytoma and paraganglioma (in conjunction with urinary catecholamines). Avoid drugs that can interfere with the test, leading to falsely high catecholamines: Tricyclics antidepressants, labet alol, levodopa, methyldopa (Aldomet), sotalol, benzodiazepines, amphetamines, decongestants, and most psychoactive agents. All these drugs should be discontinued 2 wk prior to testing:
Normal: <1.3 mg/24 h (7.1 mmol/L) for adults, but variable in children
Increased: Pheochromocytoma, paraganglioma, false positive with drugs (see above)
• Protein (see Section I Proteinuria):
– Normal: <150 mg/24 h (<0.15 g/d)
– Increased: Nephrotic syndrome is usually associated with >4 g/24 h.
• 17-Ketogenic steroids (17-KGS, corticosteroids):
– Overall adrenal function test, largely replaced by serum or urine cortisol levels:
Normal: Males: 5–24 mg/24 h (17–83 mmol/24 h); females: 4–15 mg/24 h (14–52 mmol/24 h)
Increased: Adrenal hyperplasia (Cushing syndrome), adrenogenital syndrome
Decreased: Panhypopituitarism, Addison disease, acute steroid withdrawal
• 17-Ketosteroids, total (17-KS):
– Measures dehydroepiandrosterone (DHEA), androstenedione (adrenal androgens); largely replaced by assay of individual elements in the blood (serum DHEA-S and serum androstenedione):
Normal: Adult males: 8–20 mg/24 h (28–69 mmol/L); adult females: 6–15 mg/dL (21–52 mmol/L). Note: Low values in prepubertal children
Increased: Adrenal cortex abnormalities (congenital adrenal hyperplasia, adrenal carcinoma, Cushing syndrome)
Decreased: Panhypopituitarism, Addison disease
• Urea nitrogen, urine (urine nitrogen, nitrogen balance, blood urea nitrogen (BUN):
– Measures urine nitrogen concentration:
Normal: 12,000–20,000 mg/24 h
Increased: Nitrogen wasting with hyperalimentation, bladder tap with amniocentesis
Decreased: Poor nitrogen balance with hyperalimentation
• Vanillylmandelic acid (VMA):
– VMA is the urinary product of both epinephrine and norepinephrine; the 24-hr urinary VMA excretion has poor diagnostic sensitivity and specificity compared to fractionated 24-hr urinary fractionated metanephrines. Can be affected by many foods. No longer recommended by most endocrinologists:
Normal: <7–9 mg/24 h (35–45 mmol/L)
Increased: Pheochromocytoma, paraganglioma, factitious (chocolate, coffee, tea, methyldopa)
REFERENCES
Simerville JA. Urinalysis: A comprehensive review. Am Fam Physician. 2005;71(6):1153–1162.
Gomella LG. Urine studies. In: Gomella LG, Haist SA, eds. Clinician’s Pocket Reference, 11th ed. New York, NY: McGraw-Hill, 2007.