Urine Sediment of the Month: Urine’s True Colors

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The golden days of urine are here to stay, and “liquid biopsy” is not just for NephMadness. Discolorations of urine are common and never fail to startle both patients and professionals.

When analyzing urine coloration, it is helpful to distinguish between the “apparent” and the “true” color:

Figure 1. The “apparent” yellow-white coloration of the urine on the left in a patient with UTI is removed by centrifugation on the right. The “true” urine color is unremarkable.

True color is defined as the color of urine after suspended particulate matter or turbidity has been removed by centrifugation.

Apparent color is what we actually see initially, resulting from the combined effect of true color and turbidity.

Figure 2. The “apparent” red coloration of the urine on the left in a patient with hematuria is removed by centrifugation on the right. The “true” urine color is unremarkable.

The visible sediment dealt with common causes and colors of turbidity, here we take a short look at the true colors of urine:

The yellowish to brownish coloration of most urine samples is caused by the so called urochromes, a group of rather ill-defined substances. The most commonly cited members include uroerythrin, urobilin, porphyrins and bilirubin.

Abnormal urine colors result from the excretion of unusual amounts or types of endogenous and exogenous pigments (Table 1). The exogenous chromogens typically originate from food, medication or bacterial metabolism.

Table 1. True colors of urine with exemplary causes.
Figure 3. Orange colored urine in a patient treated with rifampin.
(Courtesy of Krishna Penmatsa )

When assessing urine color, one should always bear in mind that

1. It is highly dependent on the concentration of the urine

Figure 4. Green urine in a patient treated with methylene blue.
(Courtesy of Keia Sanderson )

2. The perceived color commonly results from the combined absorption of light by multiple pigments (Fig. 5)

Figure 5. Purple urine bag syndrome. The purple color is thought to result from the combination of the red pigment indirubin and the blue pigment indigo.
(Courtesy of YJ Anupama )

3. Although usually negligible- it is also the type of light illuminating the urine that determines its color (Fig. 6).

Figure 6. Two urine samples under light from a regular ceiling lamp (left) and illuminated with a Wood’s lamp (right). With regular light both samples show a yellow coloration, under Wood’s light sample A appears colorless. Sample B, collected from a patient after fluorescein angiography, exhibits a greenish glow.

Post by Florian Buchkremer

Reviewed by Anna Gaddy


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