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:
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.
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.
When assessing urine color, one should always bear in mind that
1. It is highly dependent on the concentration of the urine
2. The perceived color commonly results from the combined absorption of light by multiple pigments (Fig. 5)
3. Although usually negligible- it is also the type of light illuminating the urine that determines its color (Fig. 6).
Post by Florian Buchkremer
Reviewed by Anna Gaddy