Discolored urine: What does it mean?

Methodology

Interpretation of color is subjective, and therefore varies from personto person. The most reliable results are obtained when a standardized methodis consistently used. Urine color should be evaluated by placing a standardizedvolume of urine in a standardized clear plastic or glass container and viewingthe sample against a white background with the aid of a good light source.

It is important to differentiate color from transparency. Freshly voidedurine should be transparent. Regardless of color, if a freshly collectedsample is turbid or cloudy, further evaluation is indicated.

The degree of clarity (transparency) or cloudiness (turbidity) is determinedat the time of assessment of urine color using the same standardized methodology.

The transparency or turbidity of urine is commonly estimated by readingnewspaper print through a clear container containing the urine sample.

Interpretation

The color of urine is a composite of a combination of all of the coloredsubstances it contains. The intensity of urine color may be affected byseveral variables including: 1) the quantity of the colored substance inurine, 2) urine pH, and 3) the biochemical structure of the substance whichcan change in vivo and in vitro. Because the intensity of colors is dependenton the quantity of water in which associated pigments are excreted, thesignificance of color should be interpreted in light (no pun intended) ofurine specific gravity.

Urine color may be altered by in vivo changes associated with: 1) urineconcentration and dilution, 2) a variety of diseases, 3) some pharmacologicagents, and 4) some ingested substances

Most foods and drugs lose their colors during digestion and metabolismand 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 roomtemperature.

When performing routine urinalyses, proper assessment and recording urinecolor is important because abnormal color may discolor the reagent striptest 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 indicativeof 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 degreeof urine concentration or dilution.

The yellow color is primarily associated with renal excretion of plasmaurochrome. Urochrome is a yellow lipid soluble sulfur containing oxidationproduct of a colorless urochromogen. Because the 24-hour urinary excretionof urochrome is relatively constant, urine color provides a crude indexof the degree of urine concentration and dilution. Highly concentrated urinewill be amber in color, while dilute urine may be almost colorless or lightyellow. Small quantities of urobilin, a normal orange-brown degradationproduct of the colorless urobilinogen, may contribute to the yellow colorof urine.

Because urochrome excretion is proportional to metabolic rate, increasedquantities 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 promptquestions related to the patient's diet, recent medication history and livingenvironment (Table 1, p. 36).

It is often helpful to determine the duration of the problem and to askwhen, during the course of micturition, the abnormal color was observed.When applicable, it may be helpful to identify the source of the containerused by clients to collect abnormally colored urine.

The same type of discolored urine may be associated with several differentendogenous or exogenous chromogens (Table 1, p. 36). Although abnormal urinecolor indicates that an abnormality is present, further information is usuallyrequired to localize its cause(s). Therefore, the underlying causes of abnormalcolors should be investigated by complete urinalysis and appropriate laboratorytests. (For further details, refer to Osborne CA, Stevens JB: Urinalysis:A Clinical Guide To Compassionate Patient Care. Bayer Corp., Animal HealthDivision. 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 theurine, the degree of urine acidity and the time interval that blood hasbeen in contact with urine. In recently formed and freshly collected acidicurine, hematuria may be associated with urine color that is normal yellow,pink, or red. As red cells disintegrate, they release hemoglobin, whichin an acid environment, may oxidize to methemoglobin and result in a brownor black color (Figure 4). Black urine viewed with the aid of bright lightor in a thin layer usually appears brown or deep reddish-brown.

If gross hematuria has been observed, determining when during the processof micturition its intensity is most severe may be helpful in localizingthe source of hematuria.

Hemoglobinuria and myoglobinuria: Hemoglobinuria resulting fromhemoglobinemia may also cause freshly voided urine to appear brown or blackin color if hemoglobin has been oxidized to methemoglobin. Myoglobinuriamay also cause urine to appear brown. Thus, freshly collected urine thatis brown or black in color may be associated with hematuria, hemoglobinuriaor myoglobinuria. Because all three of these abnormalities may produce apositive result for occult blood detected by reagent strips, additionalinvestigation is required to differentiate them. A negative reagent striptest for blood in red, black or brown urine suggests the presence of a chromogenother than hemoglobin or myoglobin.

Differentiation of hematuria from hemoglobinuria and myoglobinuria isof obvious importance. Centrifugation of an aliquot of a visibly discoloredsample and comparison of the supernatant to an uncentrifuged aliquot ofthe sample is often of value. The supernatant of samples with significanthemoglobinuria or myoglobinuria will remain equally discolored (Figure 1).The supernatant of samples with significant hematuria will be normal incolor or less discolored, and contain significant RBC in the sediment.

Observation of plasma may aid in differentition of myoglobinuria fromhemoglobinuria. Clear plasma in a patient with red, brown, or black urinesuggests myoglobinuria or hematuria, whereas pink plasma suggests hemoglobinuria.The ammonium sulfate solubility test may also be used to help differentiatehemoglobinuria from myoglobinuria.

Bilirubinuria: Bilirubin or its degradation products may resultin a yellow-brown (amber) color that is darker than normal (Figure 6). Itmay produce yellow foam if the urine sample is shaken. Caution: If urinecontaining bilirubin glucuroide (so-called conjugated bilirubin) is exposedto light and/or is not analyzed for several hours following collection,bilirubin glucuronide may hydrolize to form free bilirubin and oxidize toform biliverdin (verde is the Latin term for green). If sufficient quantitiesof biliverdin form, the urine may become green (Figure 7, p. 35). Biliverdinand free bilirubin are much less reactive to colorimetric diazo tests characteristicof reagent strips, resulting in false negative results. Harrison's spottest may be used to detect biliverdin.

Abnormal quantities of bilirubinuria may be associated with a varietyof disorders including hepatocellular disorders, post-hepatic obstructionand abnormal intravascular hemolysis.

Dr. Osborne, a diplomate of the American College ofVeterinary Internal Medicine, is professor of medicine in the Departmentof Small Animal Clinical Sciences, College of Veterinary Medicine, Universityof Minnesota.