Yellow urine

The presence of urochrome in urine imparts its characteristic normal yellow hue. Deviation from this color is produced by either increasing or decreasing the concentration of urochrome or by the presence of additional pigments.

Dark or bold yellow urine: The term “dark” may be applied to urine that is a bolder than normal shade of yellow, brown, orange, or red. For purposes of discussion here, dark urine refers to bold yellow; other hues will be considered separately. In the postoperative patient, dark urine most commonly indicates abnormally concentrated urine with relatively increased urochrome. Concentrated urine may be confirmed on urinalysis by increased specific gravity and osmolality, but laboratory confirmation is seldom necessary. Dark urine and low urine output are typically associated with intravascular volume depletion. Intravascular hypovolemia is common after surgery and may be due to inadequate volume replacement, inappropriate vasodilatation, ongoing hemorrhage, or fluid loss into the third space. Improvement of urine output with an intravascular fluid challenge with 500 to 1000 ml of crystalloid for the typical adult patient is both diagnostic and therapeutic. Fluid challenge is an appropriate treatment for the majority of postoperative patients after intravascular volume overload has been excluded by physical examination.

Dark urine may occur in the euvolemic or hypervolemic patient owing to the syndrome of inappropriate antidiuretic hormone secretion (SIADH). SIADH is more typically recognized in the clinical setting by mild hyponatremia, but may be first suspected by dark urine from the postoperative patient without laboratory testing. Unlike dark urine caused by hypovolemia, SIADH will not respond appropriately to a fluid challenge with increased urine output or urinary dilution, and inappropriate fluid challenge also may exacerbate the severity of hyponatremia.^[1]

Red urine

The differential diagnosis of red to reddish-brown urine is broad. Red discoloration of the urine may indicate hematuria. Hematuria may be due to tissue trauma which might be expected after urological surgery or may occur unexpectedly after other procedures of the abdomen or pelvis. Traumatic and infectious complications of bladder catheterization or urolithiasis may also present with hematuria. It is difficult to gauge the severity of bleeding by visual examination of urine color, with insignificant amounts of blood sometimes causing profound color alteration. As little as 1 milliliter of blood per liter of urine may cause visible reddening of the urine. Serial blood tests are necessary to monitor blood loss from hematuria. Contamination of urine from menstrual or hemorrhoidal bleeding may be mistaken for true hematuria.

Reddish or red to brown urine may also be due to hemoglobinuria caused by intravascular hemolysis. Hemolysis may occur postoperatively because of transfusion reaction in patients receiving blood products during or after surgery, in patients with glucose-6-phosphate deficiency (G6PD), or patients with sickle cell anemia, thalassemia, idiopathic thrombocytopenic purpura, or thrombotic thrombocytopenic purpura. Patients of African or Mediterranean descent are at increased risk for G6PD and may experience hemolysis when exposed to many of the medications used perioperatively.^[2] Hereditary spherocytosis also results in hemolysis, with crisis being precipitated typically by systemic infection, but these patients may commonly present for splenectomy or cholecystectomy and hemolysis may be seen postoperatively. Discolored urine due to hemoglobinuria will test positive for the presence of heme.

The presence of myoglobin in the urine may cause similar reddish or red to brown discoloration. Myoglobinuria requires immediate treatment owing to the risk of nephrotoxicity. Rhabdomyolsis causing myoglobinuria may occur in the surgical patient for a variety of reasons; trauma with significant crush or electrical injury, as a complication of intraoperative positioning in long procedures, malignant hyperthermia, propofol infusion syndrome, or rarely as a complication of depolarizing neuromuscular blockers. Discolored urine due to myoglobinuria will also test positive for heme. Serum creatine kinase is usually elevated.

Additionally, red urine may also indicate the presence of porphyrin. Acute porphyria may occur in the postoperative patient through exposure to triggering medications. Porphyria is actually a group of diseases which may be caused by a number of enzymatic defects of heme synthesis and occurs in the general population at an incidence of approximately 1 in 25,000.^[3] Abnormality of urine color is unlikely to be the sole presentation of illness, as most patients experience abdominal pain, symptoms of sympathetic excess including tachycardia and sweating, cutaneous manifestations, and/or psychosis. In the acute postoperative setting, some of these symptoms may be masked or the diagnosis otherwise complicated by the confounding effects of opioids, sedatives, and perioperative stress.

A variety of medications and foods have been associated with reddish urine, but none would be commonly encountered in the postoperative patient. Red or reddish brown urine has been described in propofol infusion syndrome, a rare but potentially fatal syndrome characterized by bradycardia, hypotension, lactic acidosis, rhabdomyolysis, serum hyperkalemia, elevated serum creatine kinase, and/or hepatomegaly.^[4] Propofol may interfere with mitochondrial function, causing cellular death. Although initially described after prolonged infusion at high doses, it is now recognized that propofol infusion syndrome may be triggered after as little as 5 hours, so it may be seen during or after surgeries where propofol is used during maintenance of anesthesia. Of note, propofol infusion has also been reported to cause pinkish urine^[5] and later examination of this urine by microscopy revealed apparent uric acid crystals as well as elevated uric acid levels in the urine,^[6] but this abnormality is unrelated to propofol infusion syndrome.