1. What is uroscopy?

Uroscopy comes from the word “uroscopia,” meaning scientific examination of the urine. It is derived from the Greek words ouron meaning urine and skopeo meaning to behold, contemplate, examine, or inspect. Such analysis of the urine was historically termed uroscopy until the 17th century and is now called “urinalysis.”

2. What is the proper way of collecting and handling a urine specimen for analysis?

When collecting the urine specimen, the first 200 mL of early-morning voided urine should be discarded. In men a simple midstream urine collection should suffice, whereas in women the external genitalia should be cleaned first, to avoid contamination with secretions, before collecting a midstream specimen. The specimen should be collected in a clean but not necessarily sterile container.

Urine should be analyzed within 30 to 60 minutes of voiding; the initially uncentrifuged specimen is analyzed for color, pH, specific gravity, blood, protein, and glucose. Subsequently, it is centrifuged at 3000 rpm for 3 to 5 minutes. The sediment is then examined under the microscope for various elements, such as red blood cells (RBCs) and RBC casts, white blood cells (WBCs) and WBC casts, squamous epithelial cells, hyaline, and granular casts.

3. What are the typical elements in a urine dipstick?

See Table 2.1 .

  • Color: Normal urine color can vary from pale or light yellow to deep amber or dark yellow. The color is due the pigment urochrome. Two important characteristics influence the color: its chemical composition and concentration. In an individual who is volume depleted, urine concentration tends to be elevated, giving rise to a darker-yellow urine. However, in a patient with diabetes insipidus, the urinary concentrating ability is impaired, making the urine dilute and lighter yellow in color even in the presence of volume depletion. Certain medications and foods alter the urine color ( Table 2.2 ). In porphyria cutanea tarda, the urine turns the color of port wine.

    Table 2.2.

    Possible Causes of Altered Urine Color

    Red Foods: beets, blackberries, rhubarb
    Medications: laxatives, antipsychotics (chlorpromazine, thioridazine), anesthetics (propofol)
    Toxins: lead, mercury
    Conditions: urinary tract infections, nephrolithiasis, porphyria, hemoglobinuria (rhabdomyolysis); see Question 10
    Orange Foods: vitamin C, carrots
    Medications: rifampin, phenazopyridine
    Green Foods: asparagus
    Medications: vitamin B, propofol
    Conditions: Pseudomonas urinary tract infection
    Blue Medications: amitriptyline, indomethacin, IV cimetidine, IV promethazine, triamterene, methylene blue
    Conditions: blue diaper syndrome (see Question 6)
    Brown Foods: fava beans
    Medications: antimalarials (chloroquine, primaquine), antimicrobials (metronidazole, nitrofurantoin), laxatives (senna), methocarbamol, levodopa
    Conditions: hepatobiliary diseases (obstructive and nonobstructive), tyrosinemia, Gilbert syndrome
    Pink Uric acid crystals
    Purple See Question 5
    Black Conditions: malignant melanoma, porphyria, alkaptonuria (ochronosis)

    IV, Intravenous.

  • Clarity or turbidity: The degree of turbidity or cloudiness is usually influenced by excess amounts of cellular debris and casts but can also be secondary to proteinuria, crystals, or contamination with vaginal discharge.

  • pH: Normal urine pH ranges between 4.5 and 8.0. In a normal individual on a Western diet, the average endogenous acid production is 1 mEq/kg per day, and the kidneys are responsible for clearing this acid load, so urine is typically quite acidic, usually with a pH of 5 to 6. Urine pH is useful in differentiating the different types of renal tubular acidosis (RTA), which is characterized by the inability to acidify urine to a pH less than 5.5 (i.e., despite an overnight fast and acid loading). Similarly, it also provides a clue to the causes and therapies for certain disease states (e.g., nephrolithiasis). Alkaline urine is usually seen in urinary tract infection (UTI) caused by urea-splitting organisms (Proteus mirabilis) , associated with magnesium–ammonium phosphate crystals and staghorn calculi, whereas acidic urine is seen with uric acid calculi. Urine may also be deliberately (but cautiously, to avoid metabolic alkalemia and hypocalcemia) alkalinized, to a pH greater than 6.5, for therapeutic reasons, to aid in elimination of certain toxins (e.g., salicylates, barbiturates) or to prevent the crystallization of uric acid in tumor lysis syndrome or myoglobin in rhabdomyolysis.

  • Specific gravity: The urine specific gravity (1.005 to 1.025) reflects the ability of the kidneys to concentrate urine. In patients with impaired urinary-concentrating ability (acute tubular necrosis, sickle cell nephropathy, diabetes insipidus), the specific gravity tends to be low. Although imprecise at times, it can reflect a person’s volume status.

  • Blood: The urinary dipstick test for blood detects the peroxidase activity of RBCs; however, myoglobin and hemoglobin also catalyze this reaction, so a positive dip stick may indicate hematuria, hemoglobinuria (paroxysmal nocturnal hemoglobinuria, transfusion-related reactions, infection with Plasmodium falciparum, infection with Clostridium welchii ), or myoglobinuria (rhabdomyolysis). In the presence of positive blood on dipstick analysis, the presence of RBCs on urine microscopy confirms the diagnosis of hematuria (see Question 4 for more on this topic).

  • Protein: Normal urinary protein excretion should not exceed 150 mg/day. Among these urinary proteins are filtered albumin and the tubular Tamm-Horsfall mucoproteins (also called uromodulin). Urinary dipsticks detect only the presence of albumin, and hence they are notorious for being poor indicators of the presence of nonalbumin proteins, especially Bence Jones proteins (immunoglobulin [Ig] light chains, commonly seen in multiple myeloma). It is also important to recognize that the dipstick measurement of urine protein is dependent on the concentration of the urine specimen so that a patient with concentrated urine may test 2+ for protein, but when a 24-hour urine collection is obtained, the actual amount of protein excreted could be modest. Conversely, a patient with dilute urine may test only trace positive for protein but may have a larger amount of protein on a 24-hour collection. Thus it is important to quantify the amount of proteinuria found on dipstick testing. This can be done either with a 24-hour collection or with a random protein-to-creatinine ratio (PCR). A more reliable test for the presence of overall (including nonalbumin) proteins is called the sulfosalicylic acid test. It detects the presence of both albumin and nonalbumin proteins (including light chains) in the urine, even in low amounts. See Questions 16 to 19 for more on proteinuria.

  • Glucose: Normal urinary glucose should not exceed 130 mg/day. Glucosuria (also termed glycosuria) commonly indicates the presence of diabetes mellitus. Other causes include the Fanconi syndrome (i.e., renal glycosuria accompanying proximal RTA) and the use of sodium-glucose cotransporter inhibitors. Urinary dipsticks (which use the glucose oxidase reaction) detect only the presence of glucose. For pediatric patients with suspected inborn errors of metabolism, other semiquantitative tests are used, such as Clinitest and Benedict test.

  • Ketones: The presence of ketones in the urine is abnormal. There are three ketones found in diabetic ketoacidosis (acetone, acetoacetic acid, and bb-hydroxybutyric acid), but the urine dipstick detects only acetoacetate. Conditions whereby ketonuria may be present include poorly controlled diabetes, pregnancy, and starvation.

  • Nitrite: Urinary nitrates are converted into nitrite by certain, commonly pathogenic bacterial species (Escherichia coli, Klebsiella, Proteus, Pseudomonas, Enterobacter, Citrobacter) . Therefore a positive nitrite is an indication of the presence of such bacteria. Other bacteria (Haemophilus, Staphylococcus, Streptococcus) do not have the ability to convert nitrate to nitrite. Therefore this test is considered specific but not very sensitive, whereas a positive nitrite may be suggestive of an active UTI; a negative result does not necessarily rule it out.

  • Leukocyte esterase: When WBCs in the urine undergo lysis, esterases are released. Therefore a positive leukocyte esterase test implies pyuria and a UTI. If the urine culture is negative (i.e., sterile pyuria), this could indicate an infection that is not easily cultured (e.g., bacterial infections such as Chlamydia , Ureaplasma urealyticum , Mycobacterium tuberculosis , viral, fungal, or parasitic infections) or noninfectious causes of pyuria, such as urinary tract stones, interstitial nephritis, or acute glomerulonephritis.

  • Bacteria: In the appropriate clinical scenario, with positive tests for nitrite and leukocyte esterase, the presence of bacteria further strengthens the diagnosis of an underlying UTI. However, despite this, one has to also consider the presence of squamous epithelial cells, which if abundant (≥15 to 20/high power field [hpf]) indicate a contaminated specimen.

  • Bilirubin and urobilinogen: Normally, bilirubin in the urine should be undetectable. Conjugated bilirubin is secreted in bile and metabolized to urobilinogen, which is reabsorbed via the portal circulation, with a small amount being filtered by the glomerulus. The presence of significant amounts of water-soluble, conjugated bilirubin in the urine may be a clue to underlying liver disease or obstructive hepatobiliary conditions and gives the urine the dark characteristic color seen in jaundice. Increased levels of urobilinogen are seen in conditions characterized by excessive hemolysis and liver disease.

Table 2.1.

Urine Dipstick Testing

Specific gravity Expected value: 1.002–1.030
<1.005: inability to concentrate urine (e.g., diabetes insipidus, acute tubular necrosis)
>1.030: SIAD, adrenal insufficiency, hypovolemia
Increased values in the presence of protein >1 g/L, ketoacids, iodinated contrast media, dextran
Urine osmolality is a more exact measure of urinary solutes
pH Expected value: 5.5–6.5
Alkaline: Diet (e.g., vegetarian), renal tubular acidosis (type I or II), salicylate overdose, acetazolamide, alkali therapy
Acidic: diabetic ketoacidosis, diarrhea, furosemide, systemic acidosis
Increased values when urine left standing (increased ammonia synthesis)
Hemoglobin Expected value: Negative
Positive due to presence of erythrocytes, or free hemoglobin (e.g., with intravascular hemolysis)
False negative: Presence of ascorbic acid or formaldehyde, high nitrite concentration, delayed examination, high density of urine
False positive: Myoglobin, microbial peroxidases, oxidizing detergents, hydrochloric acid
Glucose Expected value: negative
Positive in presence of glucose, typical diabetes mellitus, or with reduced renal threshold for glucose (e.g., proximal RTA, pregnancy, use of sodium-glucose transporter inhibitors, i.e., gliflozins)
False negative: Presence of ascorbic acid, urinary tract infection
False positive: Presence of oxidizing detergents, hydrochloric acid
Albumin Expected value: Negative
Positive (+ or more): Proteinuria (glomerular disease)
False negative: Immunoglobulin light chains, hydrochloric acid, tubular proteins, other globulins, colored urine
False positive: Alkaline urine (pH > 7.5), quaternary ammonium detergents, chlorhexidine, polyvinylpyrrolidone
Leukocyte esterase Expected value: Negative
Positive indicates pyuria, which would indicate a urinary tract infection, or other causes of sterile pyuria (as discussed previously)
False negative: Presence of ascorbic acid, recent treatment with gentamicin, tetracycline, cephalosporins, nitrofurantoin; proteinuria, glycosuria
False positive: Oxidizing detergents, formaldehyde (0.4 g/L), sodium azide, colored urine from beet ingestion, or bilirubin
Nitrites Expected value: Negative
Positive in presence of bacteria that convert nitrates in urine to nitrites (typically gram-negative bacilli such as E. coli, Klebsiella )
False negative: Short bladder incubation time, presence of ascorbic acid, gram-positive bacteria
False positive: colored urine
Ketones Expected value: Negative
Positive with ketonuria (e.g., ketoacidosis due to diabetes, starvation; may also occur in pregnancy, low-carbohydrate diets)
False negative: Test does not detect presence of β-hydroxybutyrate
False positive: Free sulfhydryl groups (e.g., captopril), l -dopa, salicylates, phenothiazines, pigments in urine
Bilirubin Expected value: Negative
Positive in the presence of jaundice (hepatic or cholestatic disease)
Negative in the presence of hyperbilirubinemia due to hemolysis
False negative: Presence of ascorbic acid, delayed examination, rifampin, exposure of urine to ultraviolet (UV) light
False positive: Phenothiazines
Urobilinogen Expected value: Negative
Positive in presence of hyperbilirubinemia due to hepatic disease or hemolysis; negative in cholestatic disease

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Jul 23, 2019 | Posted by in NEPHROLOGY | Comments Off on Urinalysis
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