Discolored urine: What does it mean?
Interpretation of color is subjective, and therefore varies from person to person. The most reliable results are obtained when a standardized method is consistently used. Urine color should be evaluated by placing a standardized volume of urine in a standardized clear plastic or glass container and viewing the sample against a white background with the aid of a good light source.
It is important to differentiate color from transparency. Freshly voided urine should be transparent. Regardless of color, if a freshly collected sample is turbid or cloudy, further evaluation is indicated.
The degree of clarity (transparency) or cloudiness (turbidity) is determined at the time of assessment of urine color using the same standardized methodology.
The transparency or turbidity of urine is commonly estimated by reading newspaper print through a clear container containing the urine sample.
The color of urine is a composite of a combination of all of the colored substances it contains. The intensity of urine color may be affected by several variables including: 1) the quantity of the colored substance in urine, 2) urine pH, and 3) the biochemical structure of the substance which can change in vivo and in vitro. Because the intensity of colors is dependent on the quantity of water in which associated pigments are excreted, the significance of color should be interpreted in light (no pun intended) of urine specific gravity.
Urine color may be altered by in vivo changes associated with: 1) urine concentration and dilution, 2) a variety of diseases, 3) some pharmacologic agents, and 4) some ingested substances
Most foods and drugs lose their colors during digestion and metabolism and therefore do not have any recognizable effect on urine color. The color, turbidity, and odor will change in vitro in urine allowed to stand at room temperature.
When performing routine urinalyses, proper assessment and recording urine color is important because abnormal color may discolor the reagent strip test pads and thereby interfere with evaluation of test results ( glucose, bilirubin, protein).
Caution: Do not over interpret the significance of urine color. Why? Because significant disease may exist when urine is normal in color (glucosuria, proteinuria), and unusual colors are not always indicative of disease.
Normal urine color
Normal urine is typically transparent, light yellow, yellow, or amber. The intensity of the yellow color in normal urine varies with the degree of urine concentration or dilution.
The yellow color is primarily associated with renal excretion of plasma urochrome. Urochrome is a yellow lipid soluble sulfur containing oxidation product of a colorless urochromogen. Because the 24-hour urinary excretion of urochrome is relatively constant, urine color provides a crude index of the degree of urine concentration and dilution. Highly concentrated urine will be amber in color, while dilute urine may be almost colorless or light yellow. Small quantities of urobilin, a normal orange-brown degradation product of the colorless urobilinogen, may contribute to the yellow color of urine.
Because urochrome excretion is proportional to metabolic rate, increased quantities of urochrome may be excreted as a result of fever or starvation. The quantity of urochrome may also increase in urine kept at room temperature. Urochrome may darken when exposed to light.
Abnormal urine color
Generalities: Detection of abnormal urine color should prompt questions related to the patient's diet, recent medication history and living environment (Table 1, p. 36).
It is often helpful to determine the duration of the problem and to ask when, during the course of micturition, the abnormal color was observed. When applicable, it may be helpful to identify the source of the container used by clients to collect abnormally colored urine.
The same type of discolored urine may be associated with several different endogenous or exogenous chromogens (Table 1, p. 36). Although abnormal urine color indicates that an abnormality is present, further information is usually required to localize its cause(s). Therefore, the underlying causes of abnormal colors should be investigated by complete urinalysis and appropriate laboratory tests. (For further details, refer to Osborne CA, Stevens JB: Urinalysis: A Clinical Guide To Compassionate Patient Care. Bayer Corp., Animal Health Division. Shawnee Mission, Kansas 66201, 1999.)
Hematuria: The color associated with hematuria may vary from red (Figure 3) to black (Figure 4) depending on the quantity of blood in the urine, the degree of urine acidity and the time interval that blood has been in contact with urine. In recently formed and freshly collected acidic urine, hematuria may be associated with urine color that is normal yellow, pink, or red. As red cells disintegrate, they release hemoglobin, which in an acid environment, may oxidize to methemoglobin and result in a brown or black color (Figure 4). Black urine viewed with the aid of bright light or in a thin layer usually appears brown or deep reddish-brown.
If gross hematuria has been observed, determining when during the process of micturition its intensity is most severe may be helpful in localizing the source of hematuria.
Hemoglobinuria and myoglobinuria: Hemoglobinuria resulting from hemoglobinemia may also cause freshly voided urine to appear brown or black in color if hemoglobin has been oxidized to methemoglobin. Myoglobinuria may also cause urine to appear brown. Thus, freshly collected urine that is brown or black in color may be associated with hematuria, hemoglobinuria or myoglobinuria. Because all three of these abnormalities may produce a positive result for occult blood detected by reagent strips, additional investigation is required to differentiate them. A negative reagent strip test for blood in red, black or brown urine suggests the presence of a chromogen other than hemoglobin or myoglobin.
Differentiation of hematuria from hemoglobinuria and myoglobinuria is of obvious importance. Centrifugation of an aliquot of a visibly discolored sample and comparison of the supernatant to an uncentrifuged aliquot of the sample is often of value. The supernatant of samples with significant hemoglobinuria or myoglobinuria will remain equally discolored (Figure 1). The supernatant of samples with significant hematuria will be normal in color or less discolored, and contain significant RBC in the sediment.
Observation of plasma may aid in differentition of myoglobinuria from hemoglobinuria. Clear plasma in a patient with red, brown, or black urine suggests myoglobinuria or hematuria, whereas pink plasma suggests hemoglobinuria. The ammonium sulfate solubility test may also be used to help differentiate hemoglobinuria from myoglobinuria.
Bilirubinuria: Bilirubin or its degradation products may result in a yellow-brown (amber) color that is darker than normal (Figure 6). It may produce yellow foam if the urine sample is shaken. Caution: If urine containing bilirubin glucuroide (so-called conjugated bilirubin) is exposed to light and/or is not analyzed for several hours following collection, bilirubin glucuronide may hydrolize to form free bilirubin and oxidize to form biliverdin (verde is the Latin term for green). If sufficient quantities of biliverdin form, the urine may become green (Figure 7, p. 35). Biliverdin and free bilirubin are much less reactive to colorimetric diazo tests characteristic of reagent strips, resulting in false negative results. Harrison's spot test may be used to detect biliverdin.
Abnormal quantities of bilirubinuria may be associated with a variety of disorders including hepatocellular disorders, post-hepatic obstruction and abnormal intravascular hemolysis.
Dr. Osborne, a diplomate of the American College of Veterinary Internal Medicine, is professor of medicine in the Department of Small Animal Clinical Sciences, College of Veterinary Medicine, University of Minnesota.