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Acute azotaemia without serum cystatin C elevation

A 44-year-old woman presented with abdominal pain and oliguria 4 days after undergoing total abdominal hysterectomy and right oophorectomy due to uterine myoma and ovarian chocolate cysts. Laboratory findings showed that blood urea nitrogen (BUN) and serum creatinine (Scr) were elevated to 38.7 and 3.75 mg/dL, respectively, whereas preoperative Scr had been within the normal range (0.44 mg/ dL). The patient had no medical history and was not currently on medication. Her physical findings were blood pressure 109/88 mmHg, heart rate 118 beats/min and temperature 36.7 C. Her lower abdomen was slightly hard with rebound tenderness. Laboratory findings (Table 1) showed that serum cystatin C was 0.63 mg/L (normal), in the setting of elevated BUN and Scr. Abdominal computed tomography showed moderate ascites and free air in the abdominal cavity. Levels of urea nitrogen and creatinine in ascites after paracentesis were 92.3 and 13.7 mg/dL, respectively. Both values were higher than those of BUN and Scr (38.7 and 3.75 mg/dL, respectively). Therefore, we diagnosed intraperitoneal urine leakage, because an ascites-Scr ratio >1.0 indicates creatinine reabsorption across the peritoneal membrane. Cystoscopy revealed micro perforation at the posterior bladder wall. Bladder catheterization to reduce intrabladder pressure and obtain urine output reduced BUN and Scr to normal values of 7.0 and 0.52 mg/dL, respectively. Intraperitoneal urine leakage resulted in BUN and Scr elevation through urea nitrogen and creatinine reabsorption across the peritoneal membrane. This unique mechanism is referred to as ‘pseudo-azotaemia’, since glomerular filtration is preserved. Our patient notably had a normal serum cystatin C value, indicating that glomerular filtration was indeed preserved. Although the mechanism of the discrepancy between BUN and Scr and serum cystatin C in pseudoazotaemia remains unclear, it might be associated with molecular weight. The molecular weight of cystatin C is quite large (13 200 Da) and similar to that of β2microglobulin (11 800 Da), indicating that it cannot be easily reabsorbed across the peritoneal membrane. Consequently, serum cystatin C is not elevated in pseudoazotaemia, unlike BUN and Scr. Measuring serum cystatin C is less invasive than measuring urea nitrogen and creatinine by paracentesis. Clinicians should measure serum cystatin C when intraperitoneal urine leakage is suspected.