10. Low Urine Output

Definition

Normal urine output is typically > 0.5 cc/kg/hr. Oliguria can be a symptom of many potentially life threatening conditions (acute renal failure, sepsis, heart failure, etc.), and should be evaluated during the night.

  • Oliguria: defined as urine output < 400-500 cc/day
  • Anuria: defined as < 100 cc/day.

Differential Diagnosis

Consider a broad DDx, including:

  • Pre-renal (e.g., hypovolemia, sepsis)
  • Intra-renal (e.g., ATN, AIN)
  • Post-renal (e.g., bladder outlet obstruction, neurogenic bladder)
  • Poor forward flow (e.g., CHF exacerbation, obstructive shock)
  • Inaccurate recording/measurement of urine output
  • Abdominal compartment syndrome in the setting of tense ascites

Evaluation

  • Is the patient stable? Always ask for a full set of vital signs.
  • Examine the patient
    • Vital Signs: BP, HR can help with estimating volume status. Infection/sepsis: Fever, hypotension, tachycardia, tachypnea.
    • HEENT: Jugular venous pressure to estimate CVP, look for moist mucous membranes.
    • Heart: Tachycardia, S3, rub (uremia).
    • Chest: Crackles suggest volume overload, absent breath sounds suggesting effusions.
    • Extremities: peripheral edema suggests volume overload.
    • Consider measuring bladder pressure to evaluate for abdominal compartment syndrome if the patient has tense ascites.
  • Is urine output accurate?
    • How has the urine been collected (foley vs. urinal)? Ask the patient and nursing staff if they are accurately collecting the urine. Review daily Ins and Outs, daily weights.
    • Flush foley and see if urine output improves.
  • Is there lower urinary tract obstruction? 
    • Ask RN to do a bladder scan. If unavailable or if >500 cc, then check post-void residual by inserting Foley catheter after patient voids. If the volume is >300 cc, leave the catheter in; this indicates significant residual bladder volume/urinary retention.
    • Some reasons for urinary retention include prostatic hypertrophy and anticholinergic side effects of medications (opioid, diphenhydramine, etc.).
  • Labs:
    • If low urine output is not quickly responsive to fluids, then evaluate for intrarenal cause: start with STAT urinalysis with microscopy, urine lytes, and basic metabolic panel.
    • Additional labs (BNP, blood cultures, urine culture, etc.) based on suspected cause. See Renal: Acute Renal Failure for interpretation of urine lytes/FeNa.

Management

  • If hypovolemia on exam: Fluid bolus starting with 500 ml – 1L (if known history of CHF, can start with 250 ml).
  • If hypervolemia on exam: Initiate diuresis. Limit additional volume including IV medications. If not responding to diuresis, evaluate for cardiogenic or obstructive shock. Otherwise, may be going into renal failure.
  • If concerned for renal failure: The patient may require dialysis. See Renal: Acute Renal Failure for more details.

Key points

  • Post-void bladder scan is a quick and easy way to evaluate for lower urinary obstruction (much more comfortable for the patient than straight-cath or foley placement).
  • Always evaluate volume status in an oliguric/anuric patient, as this will determine your overall strategy (e.g., fluid boluses for hypovolemia, diuresis for hypervolemia).

Klahr S, Miller SB. Acute oliguria. N Engl J Med 1998;338:671-675.

Kellum, J. Acute kidney injury. Critical Care Medicine 2008; 36: S141-S145