Dark urine is a frequent finding. Under certain circumstances, it may be normal, but sometimes it is associated with diseases of the urinary tract or other organs.
Dark colored urine usually accompanies low fluid intake and dehydration. In this case, kidneys secrete waste products with a minimum amount of water, urine is concentrated and dark. People with adequate fluid intake have light diluted urine.
Diseases of liver and biliary tract
In liver tissue a substance called bilirubin is processed. Bilirubin is a pigment that arises during red blood cells breakdown. Liver cells “conjugate” bilirubin so it becomes soluble in water (i.e. in bile, or in urine). The conjugated bilirubin is then excreted into bile and through biliary tract it gets into intestine. If bile flow is disrupted, bilirubin starts to accumulate in the body and causes jaundice. Conjugated bilirubin, that does not leave the body with bile, travels in blood vessels until it reaches kidneys and there it is filtered into urine. Urine gets dark color because of bilirubin presence. There are many diseases causing problems with bilirubin excretion. Let’s just name liver inflammations (hepatitis), liver cirrhosis and obstruction if bile ducts by gallstones or nearby tumor (gallbladder tumor, pancreas tumor, biliary tract tumors).
Medication and food influence
Some drug substances can cause dark coloration of urine, for example phenazopyridine. Food as blueberries or red beets can cause black color of stool and also darker urine presence.
Myoglobin is a substance located within muscles. If there is massive muscle injury and muscle breakdown (rhabdomyolysis), myoglobin is released into bloodstream and transported to kidneys. It damages kidney tissue and changes urine color to dark brown. An excessive amount of myoglobin can even cause acute renal failure.
This is a very rare inherited genetic disorder affecting metabolism of certain amino acids that causes dark coloration of urine when exposed to air.
Dehydration is most common cause of dark urine and therefore it is advisable to look for other symptoms of dehydration. Urinalysis shall be always performed. Blood samples allow evaluation of bilirubin and liver tests concentration. Diseases of liver and biliary tract (particularly cancers in these locations) can be detected by imaging methods – ultrasound or computed tomography of the abdomen.