Decreased urine volume, often called oliguria, means a noticeable drop in how much urine a person passes. It is a symptom, not a diagnosis — numerous reversible and more serious conditions can cause it. Below is a concise guide to what it may indicate, how clinicians evaluate it, and which warning signs need prompt attention.
Clinically, oliguria is often defined by a marked reduction in urine volume: for example, less than about 400–500 mL in 24 hours or a sustained urine flow under roughly 0.5 mL/kg per hour. Short-lived decreases (for example after low fluid intake) can be benign, while rapid onset or prolonged low output warrants more thorough assessment.
Low urine output can result from low fluid intake or dehydration, reduced blood flow to the kidneys (as in heart failure or shock), intrinsic kidney injury, urinary tract obstruction (enlarged prostate, stones, tumors), severe infections or sepsis, and effects of certain medications. The underlying mechanism—pre-renal (low perfusion), renal (kidney tissue problem), or post-renal (obstruction)—guides further tests.
Initial evaluation focuses on medical history (fluid intake, recent illness, medications), a physical exam and measuring urine output. Common tests include urinalysis (and urine culture if infection is suspected), blood creatinine and BUN, serum electrolytes, and bedside bladder scanning or renal ultrasound to check for obstruction. Hourly urine monitoring and specialist referral (nephrology or urology) may follow depending on findings.
Seek prompt medical care if urine output falls suddenly or you stop passing urine, or if the change is accompanied by high fever, severe abdominal or back pain, fainting or lightheadedness, confusion, shortness of breath, or new widespread swelling. These signs may indicate a condition needing urgent treatment and assessment.
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