Chapter 4 - Physical Examination of Urine

Color

Normal Urine Color

• Under a good light source while looking down through the container against a white background, the color of normal urine should be pale yellow, yellow, dark yellow, and amber.
• The yellow color of urine is caused by the presence of the pigment urochrome, a product of endogenous metabolism the body produces at a constant rate.
  • It is usually increased in patients with thyroid conditions and fasting conditions, and when the specimen is left at room temperature.
• Two additional pigments, uroerythrin and urobilin, are also present in the urine in much smaller quantities and contribute little to the color.
  • Uroerythrin, a pink pigment, attaches to urates and appears when they precipitate during refrigeration.
  • Urobilin, an oxidation product of the normal urinary constituent urobilinogen, imparts an orange-brown color to urine that is not fresh.

Dark Yellow/Amber/Orange Color

• This indicates the possible presence of the abnormal pigment bilirubin.
  • This is further supported by yellow foam when the specimen is shaken.
  • A urine specimen with bilirubin may also contain the hepatitis virus.
  • When exposed to light, it photo-oxidizes and imparts a yellow-green color.
• It may also be due to the photo-oxidation of urobilinogen to urobilin, producing a yellow-orange specimen.
  • Unlike bilirubin, yellow foam does not appear when shaken.
• The administration of phenazopyridine (Pyridium) or azo-gantrisin compounds to persons with urinary tract infections can also cause a yellow-orange pigment in urine specimens.
  • It produces a yellow foam like bilirubin, so care must be taken not to confuse such cases.
  • It can interfere with chemical tests that are based on color reactions.

Red/Pink/Brown

• Red is the usual color that blood produces in urine, but the color may range from pink to brown, depending on the amount of blood, the pH of the urine, and the length of contact.
  • The specimen is red and cloudy.
  • RBCs remaining in an acidic urine for several hours produce a brown urine due to the oxidation of hemoglobin to methemoglobin.
  • A fresh brown urine containing blood may also indicate glomerular bleeding resulting from the conversion of hemoglobin to methemoglobin.
• Hemoglobin and myoglobin also produce a red urine and result in a positive chemical test result for blood.
  • Hemoglobinuria is usually caused by the in vivo breakdown of RBCs, while myoglobinuria is caused by the breakdown of skeletal muscle.
  • The specimen is red and clear; however, myoglobin frequently exhibits a browner color than hemoglobin.
  • Hemoglobinuria is usually accompanied by red plasma while myoglobin is more rapidly cleared from the plasma, therefore not affecting the color of the plasma.
• Urine specimens containing porphyrins resulting from the oxidation of porphobilinogen to porphyrins appear a shade of red similar to port wine.
• Nonpathogenic causes of red urine include menstrual contamination, ingestion of highly pigmented foods, and medications.
  • Fresh beets causes a red color in alkaline urine while blackberries can produce a red color in acidic urine.
  • Examples of medications include rifampin, phenolphthalein, phenindione, and phenothiazines.

Brown/Black

• Additional testing is recommended for urine specimens that turn brown or black on standing and have negative chemical test results for blood, as they may contain melanin or homogentisic acid.
  • Melanin is an oxidation product of the colorless pigment, melanogen, produced in excess when a malignant melanoma is present.
  • Homogentisic acid, a metabolite of phenylalanine, imparts a black color to alkaline urine from persons with the inborn-error of metabolism, called alkaptonuria.
• Medications producing brown/black urines include levodopa, methyldopa, phenol derivatives, and metronidazole (Flagyl).

Blue/Green

• This may be caused by bacterial infections, including urinary tract infection by Pseudomonas species and intestinal tract infections resulting in increased urinary indican metabolites.
  • Purple staining indicates the presence of Klebsiella or Providencia species.
• Breath deodorizers (Clorets) and phenol derivatives in intravenous medications can result in a green urine color while the medications methocarbamol (Robaxin), methylene blue, and amitriptyline (Elavil) may cause blue urine.

Clarity

• It is a a general term that refers to the transparency/turbidity of a urine specimen, an it is examined by holding the specimen in front of a light source.
• Common terminology used to report clarity includes clear, hazy, cloudy, turbid, and milky.

Nonpathologic Turbidity

• The presence of squamous epithelial cells and mucus, particularly in specimens from women, can result in a haziness.
• Improper preservation can cause the growth of bacteria and increase turbidity.
• Refrigerated specimens frequently develop a thick turbidity caused by the precipitation of amorphous phosphates, carbonates, and urates.
  • Amorphous phosphates and carbonates produce a white cloudy precipitate in urine with an alkaline pH, whereas amorphous urates produce a precipitate in acidic urine that resembles pink brick dust due to the presence of uroerythyrin.
• Additional nonpathologic causes of urine turbidity include semen, fecal contamination, radiographic contrast media, talcum powder, and vaginal creams.

Pathologic Turbidity

• The most commonly encountered pathologic causes of turbidity in a fresh specimen are RBCs, WBCs, and bacteria caused by infection or a systemic organ disorder.
• Other less frequently encountered causes of pathologic turbidity include abnormal amounts of nonsquamous epithelial cells, yeast, abnormal crystals, lymph fluid, and lipids.
• Questionable causes of urine turbidity can be confirmed by chemical tests.

Specific Gravity

• It is defined as the density of a solution compared with the density of a similar volume of distilled water at a similar temperature.
• It is influenced not only by the number of particles present but also by their size.

Urinometer

• It consists of a weighted float attached to a scale that has been calibrated in terms of urine specific gravity, and it displaces a volume of liquid equal to its weight and has been designed to sink to a level of 1.000 in distilled water.

  • The additional mass provided by the dissolved substances in urine causes the float to displace a volume of urine smaller than that of distilled water, and the level to which the urinometer sinks represents the specimen’s mass or specific gravity.
  • However, it is less accurate than other methods and requires an extensive volume of urine (10 to 15 mL) and a large container that does not allow the instrument to touch the sides and bottom.

• The calibration temperature is printed on the instrument, usually 20°C, and temperature changes must be accounted for.

  • 0.001 must be subtracted from the reading for every 3°C that the specimen temperature is below or above the urinometer calibration temperature.

• Corrections must also be accounted for if glucose or protein are present, as both glucose and protein are highmolecular-weight substances that have no relationship to renal concentrating ability.

  • For each gram of protein present, 0.003 must be subtracted from the specific gravity reading, and 0.004 must be subtracted for each gram of glucose present.

    

Refractometer

• It makes use of the refractive index, a comparison of the velocity of light in air with the velocity of light in a solution where the concentration of dissolved particles present in the solution determines the velocity and angle at which light passes through a solution.

• Calibration of the refractometer is performed using distilled water that should read 1.000, followed by 5% NaCl, which as shown in the refractometer conversion tables should read 1.022 ± 0.001 or 9% sucrose that should read 1.034 ± 0.001.

  • If necessary, the instrument contains a zero set screw to adjust calibration readings.
  • Urine control samples representing low, medium, and high concentrations should also be run.

• When using the refractometer, a drop of urine is placed on the prism, the instrument is focused at a good light source, and the reading is taken directly from the specific gravity scale. The prism and its cover should be cleaned after each specimen is tested.

  

• The advantages are that only a small volume of urine is needed (around 1-2 drops), and temperature corrections are not necessary within a 15°C to 38°C range.

• Corrections for glucose and protein are still calculated, although refractometer readings are less affected by particle density than are urinometer readings.

Harmonic Oscillation Densitometry

• It is based on the principle that the frequency of a sound wave entering a solution changes in proportion to the density of the solution.
• A portion of the urine sample enters a U-shaped glass tube with an electromagnetic coil at one end and a motion detector at the other end, and as an electric current is applied, a sound wave oscillates through the sample.
  • Its frequency is altered by the density of the specimen, and this change is recorded by a microprocessor at the other end of the tube and converted to specific gravity that closely correlates with gravimetric measurement.
  • All dissolved solutes are measured by this method, and there is no clarification of cloudy specimens nor correction for temperature variances required.
  • Results are linear up to a specific gravity of 1.080.

Odor