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Urinalysis and Urine Microscopy
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The relatively simple chemical tests performed during routine urinalysis rapidly provide important information about a number of primary kidney and systemic disorders. The microscopic examination of the urine sediment is an indispensable part of the evaluation of patients with reduced glomerular filtration, proteinuria, hematuria, urinary tract infection, or nephrolithiasis, and the urine sediment provides valuable clues about the kidney parenchyma.
Urine dipstick tests can be readily automated, and most high-throughput clinical laboratories rely on computerized optical scanning or flow cytometry with automated instruments to perform microscopic urinalyses. Red blood cells (RBCs) and white blood cells (WBCs) can be counted with precision, and reasonable results are obtainable for squamous epithelial cells. However, these methods cannot reliably identify important elements such as renal tubular epithelial cells, oval fat bodies, crystals, or casts. Their accuracy for detection even of RBCs and WBCs decreases with specimen aging, and sensitivity is reduced within as little as 2 hours after voiding. The interval between collection of a “routine” urine specimen, delivery to the lab, and processing may vary considerably.
When a primary kidney disorder is suspected, the automated urinalysis should be regarded only as a screening test. It does not supplant careful examination under the microscope of a specimen picked up promptly at the bedside, spun down, and examined at once. This task of careful review of the urine under the microscope must not be delegated; it should be performed personally by specialists experienced in examining the urine. Studies show both that a urinalysis performed by a nephrologist is more likely to aid in reaching a correct diagnosis than a urinalysis reported by a clinical chemistry laboratory and that urinalyses performed by physicians without special training are more often inaccurate. The features of a complete urinalysis are listed in Box 4.1 .
Box 4.1
Routine Urinalysis
Appearance and Odor
Specific Gravity
Chemical Tests (Dipstick)
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pH
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Protein
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Glucose
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Ketones
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Blood
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Urobilinogen
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Bilirubin
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Nitrites
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Leukocyte esterase
Microscopic Examination (Formed Elements)
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Crystals: urate; calcium phosphate, oxalate, or carbonate; triple phosphate; cystine; drugs
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Cells: leukocytes, erythrocytes, renal tubular cells, oval fat bodies, transitional epithelium, squamous cells
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Casts: hyaline, granular, red blood cell, white blood cell, tubular cell, degenerating cellular, broad, waxy, lipid laden
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Infecting organisms: bacteria, yeast, Trichomonas, nematodes
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Miscellaneous: spermatozoa, mucous threads, fibers, starch, hair, and other contaminants
Specimen Collection and Handling
Urine should be collected with a minimum of contamination. A clean-catch midstream sample is preferred. If this is not feasible, bladder catheterization is appropriate in adults; the risk of inducing a urinary tract infection with a single in-and-out catheterization is negligible. Suprapubic aspiration is used in infants. In the uncooperative male patient, a clean, freshly applied condom catheter and urinary collection bag may be used. Urine in the collection bag of a patient with an indwelling bladder catheter is subject to stasis, but a sample suitable for examination may be collected by withdrawing urine from above a clamp placed on the tube that connects the catheter to the drainage bag.
The chemical composition of the urine changes with standing, and the formed elements within a urine sample degenerate over time. The urine is best examined when fresh. A brief period of refrigeration is acceptable but risks precipitation of crystals. Because bacteria multiply at room temperature, bacterial counts from unrefrigerated urine are unreliable. High urine osmolality and low pH favor cellular preservation, and these two characteristics of the first-voided morning urine give it particular value in cases of suspected glomerulonephritis. Some experts favor use of the second morning urine to avoid effects of overnight bladder stasis. However, the most important goal is examination without delay, irrespective of what specimen is used.
Physical and Chemical Properties of the Urine
Appearance and Odor
Normal urine is clear with a faint yellow tinge due to the presence of urochrome. As the urine becomes more concentrated, its color deepens. Bilirubin, other pathologic metabolites, and a variety of drugs may discolor the urine or change its smell. Suspended erythrocytes, leukocytes, or crystals may render the urine turbid. Conditions associated with a change in the appearance or odor of the urine are listed in Table 4.1 .
Table 4.1
Selected Substances That May Alter the Physical Appearance or Odor of the Urine
| Color Change | Substances |
| White | Chyle, pus, calcium phosphate crystals, triple phosphate (struvite) crystals, propofol |
| Pink/red/brown | Erythrocytes, hemoglobin, myoglobin, porphyrins, beets, blackberries, senna, cascara, levodopa, methyldopa, deferoxamine, phenolphthalein and congeners, food colorings, metronidazole, phenacetin, anthraquinones, doxorubicin, phenothiazines, propofol, triple phosphate (struvite) crystals (salmon colored) |
| Yellow/orange/brown | Bilirubin, urobilin, phenazopyridine urinary analgesics, senna, cascara, mepacrine, iron compounds, nitrofurantoin, riboflavin, rhubarb, sulfasalazine, rifampin, fluorescein, phenytoin, metronidazole |
| Brown/black | Methemoglobin, homogentisic acid (alcaptonuria), melanin (melanoma), levodopa, methyldopa |
| Blue or green, green/brown | Biliverdin, Pseudomonas infection, dyes (methylene blue and indigo carmine), triamterene, vitamin B complex, methocarbamol, indican, phenol, chlorophyll, propofol, amitriptyline, triamterene |
| Purple staining of indwelling plastic urine collection devices | Infection with Escherichia coli, Pseudomonas, Enterococcus, others |
| Odor | Substance or Condition |
| Sweet or fruity | Ketones |
| Ammoniac | Urea-splitting bacterial infection |
| Fetid, pungent | Asparagus (sulfurous breakdown products) |
| Maple syrup | Maple syrup urine disease |
| Musty or mousy | Phenylketonuria |
| “Sweaty feet” | Isovaleric or glutaric acidemia, or excess butyric or hexanoic acid |
| Rancid | Hypermethioninemia, tyrosinemia |
Specific Gravity
The specific gravity of any fluid is the ratio of that fluid’s weight to the weight of an equal volume of distilled water. The urine specific gravity is a conveniently determined but inaccurate surrogate for osmolality. Specific gravities of 1.001 to 1.035 correspond to an osmolality range of 50 to 1000 mOsm/kg. A specific gravity near 1.010 connotes isosthenuria, with a urine osmolality matching that of plasma. Relative to osmolality, the specific gravity is elevated when dense solutes, such as protein, glucose, or radiographic contrast agents, are present.
Three methods are available for specific gravity measurement. The hydrometer is the reference standard but requires both a sufficient volume of urine to allow flotation of the hydrometer and equilibration of the specimen to the calibrated temperature. The second method is based on the well-characterized relationship between urine specific gravity and refractive index. Refractometers calibrated in specific gravity units are commercially available and require only a drop of urine. Finally, the specific gravity may also be estimated by dipstick.
The specific gravity is used to determine whether the urine is concentrated. During a solute diuresis accompanying hyperglycemia, diuretic therapy, or relief of obstruction, the urine is isosthenuric. In contrast, with a water diuresis caused by overhydration or diabetes insipidus, the specific gravity is typically 1.004 or lower. In the absence of proteinuria, glycosuria, or iodinated contrast administration, a specific gravity of more than 1.018 implies preserved concentrating ability. Iodinated radiographic contrast is very dense, and if the specific gravity is supraphysiologic (i.e., >1.035), one should suspect that contrast is responsible. Measurement of specific gravity is useful in differentiating between prerenal azotemia and acute tubular necrosis (ATN) and in assessing the significance of proteinuria observed in a random voided urine sample. Because the protein indicator strip responds to the concentration of protein, the significance of a borderline reading depends on the overall urine concentration.
Routine Dipstick Methodology
The urine dipstick is a plastic strip to which absorbent tabs impregnated with chemical reagents have been affixed. The reagents in each tab are chromogenic. After timed development, the color is compared with a chart or read by a colorimetric instrument. Some reactions are highly specific. Others are affected by the presence of interfering substances or extremes of pH. Discoloration of the urine with bilirubin or blood may obscure the color changes.
pH
Test pads for pH use indicator dyes that change color with pH. The physiologic urine pH ranges from 4.5 to 8. The determination is most accurate if performed promptly because growth of urea-splitting bacteria and loss of dissolved carbon dioxide raise the pH. In addition, bacterial metabolism of glucose may produce organic acids that lower pH. These strips are not sufficiently accurate to be used for the diagnosis of renal tubular acidosis. A specimen collected anaerobically under mineral oil should be promptly assayed with a pH electrode when precision is required.
Protein
Protein measurement uses the protein-error-of-indicators principle. The pH at which some indicators change color varies with the protein concentration of the bathing solution. Protein indicator strips are buffered at an acid pH near their color change point. Wetting them with a protein-containing specimen induces a color change. The protein reaction may be scored from trace to 4+ or by protein concentration. Their equivalence is approximately as follows:
| Trace | 5–20 mg/dL |
| 1+ | 30 mg/dL |
| 2+ | 100 mg/dL |
| 3+ | 300 mg/dL |
| 4+ | >2000 mg/dL |
Highly alkaline urine, especially after contamination with quaternary ammonium skin cleansers or from patients who abuse sodium bicarbonate, may produce false-positive reactions by overwhelming the pH buffer of the chromogenic tab.
Protein strips are highly sensitive to albumin but less so to globulins, hemoglobin, or light chains. If light chain proteinuria is suspected, assays that are more sensitive should be used. With acid precipitation tests, an acid that denatures protein (i.e., sulfosalicylic acid) is added to the urine specimen, and the density of the precipitate is related to the protein concentration. Urine that is negative by dipstick but positive by sulfosalicylic acid precipitation is highly suspicious for the presence of light chains. Tolbutamide, high-dose penicillin, sulfonamides, and radiographic contrast agents may yield false-positive turbidimetric reactions. More sensitive and specific tests for light chains, such as immunoelectrophoresis or immunonephelometry, are preferred and necessary for confirmation and more definitive diagnosis.
If the urine is very concentrated, the presence of a modest protein reaction is less likely to correspond to significant proteinuria in a 24-hour collection or when assessed by spot urine protein-to-creatinine ratio. Even so, it is unlikely that a 3+ or 4+ reaction would be seen solely because of a high urine concentration or, conversely, that the urine would be dilute enough to yield a negative reaction despite significant proteinuria. The protein indicator used for routine dipstick analysis is neither sufficiently sensitive nor specific for albuminuria in the moderately increased (30 to 299 mg/g) or high normal range (10 to 29 mg/g).
Blood
Reagent strips for blood rely on the peroxidase activity of hemoglobin to catalyze an organic peroxide with subsequent oxidation of an indicator dye. Free hemoglobin produces a homogeneous color. Intact red cells cause punctate staining if present only in a small quantity. False-positive reactions occur if the urine is contaminated with other oxidants, such as povidone-iodine, hypochlorite, or bacterial peroxidase. Ascorbate yields false-negative results. Myoglobin is also detected because it has intrinsic peroxidase activity. A urine sample that is positive for blood by dipstick analysis but shows no red cells on microscopic examination is suspect for myoglobinuria or hemoglobinuria. A specific assay for urine myoglobin can be used to confirm the diagnosis, if required.
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