Low urine output
Check the catheter first. Then decide: is the patient dry, septic, bleeding, or are the kidneys failing?
"Bed 12 has passed only 40 mL in 4 hours." He is 70 kg, day 1 after a laparotomy, with a urinary catheter. BP 104/62, pulse 108, cool hands, capillary refill 3 seconds. His IV fluids were stopped this morning "because he's drinking". He had two doses of diclofenac for pain. Creatinine pre-op 1.0 mg/dL (88 µmol/L).
What do you check before thinking about furosemide, and what do you give?
- Check the catheterBlocked, kinked, bag full, clamped? Flush or change it. No catheter: bladder scan.
- Assess fluid statusPulse, BP, capillary refill, JVP, chest, oedema, mucous membranes, 24-hour fluid balance, weight.
- Fluid challenge if dry250–500 mL crystalloid over 15 minutes, then reassess.
- Look for sepsis and bleedingTemperature, wounds, drains, Hb, lactate.
- Bloods and gasCreatinine compared with baseline, potassium, urea, pH.
- Stop nephrotoxic drugsNSAIDs, ACE inhibitors, ARBs, diuretics, metformin, gentamicin, contrast where possible.
- Don't give furosemide to "make urine"Only for clear fluid overload.
- Potassium 6.0 or more, or ECG changes
- No urine at all
- Fluid overload with low urine
- Low BP not responding to fluid
- Rising creatinine with acidosis
- Post-op bleeding
- Blocked catheter or urinary retention
- Dehydration
- Sepsis
- Bleeding
- AKI from drugs or contrast
- Heart failure
- Obstructed kidneys
- Abdominal compartment syndrome after major surgery
| Oliguria | Below 0.5 mL/kg/hour for 6 hours or more (KDIGO), for 70 kg below 35 mL/hour |
| Fluid challenge | 250–500 mL crystalloid over 15 minutes, reassess |
| Potassium | Above 6.0 mmol/L needs urgent treatment (see high potassium) |
| AKI | Creatinine up by 0.3 mg/dL (26.5 µmol/L) in 48 hours, or 1.5 times baseline within 7 days |
| Anuria | No urine at all: blocked catheter or obstruction until proven otherwise |
| Target urine output | About 0.5 mL/kg/hour or more |
Check every dose against a current drug reference and your hospital protocol.
On this page
Why it matters
A urine output under 0.5 mL/kg/hour (about 35 mL an hour for a 70 kg adult) is an early warning. The commonest cause at night is a blocked catheter, which is easy to fix. Next come dehydration, sepsis and bleeding, then AKI from drugs or poor perfusion. Furosemide is almost never the answer.
On the phone
- Catheterised? Blocked or kinked? Has a bladder scan been done?
- Observations and fluid balance: is the patient dry, septic or bleeding?
- Latest creatinine and potassium?
- Nephrotoxic drugs (diclofenac and ketorolac are common after surgery)?
- Ask the nurse to flush the catheter (or bladder scan), and to measure urine hourly.
Step by step
- Is urine being made?Catheter blocked or bag full? Bladder scan without a catheter.
- Is the patient dry?Fluid challenge and reassess.
- Septic or bleeding?Treat the cause.
- Is the kidney being harmed?Stop nephrotoxic drugs, review contrast and doses.
- Is it obstruction?Ultrasound kidneys if no cause found or anuric.
- Is the patient overloaded?Then, and only then, consider furosemide with senior advice.
Pre-renal, renal or post-renal?
| Type | Common causes | Clues |
|---|---|---|
| Pre-renal | Dehydration, bleeding, sepsis, heart failure, drugs affecting kidney blood flow (NSAIDs, ACE inhibitors) | Low BP, fast pulse, dry; responds to fluid |
| Renal (intrinsic) | Prolonged low BP (ATN), nephrotoxic drugs, contrast, rhabdomyolysis, glomerulonephritis | Not responding to fluid; urine dip blood and protein in glomerulonephritis |
| Post-renal | Retention, blocked catheter, stones, prostate, pelvic tumour | Bladder scan, hydronephrosis on ultrasound |
Fluid status at the bedside
Fluid challenge 250–500 mL, reassess
- No fluid
- furosemide with senior
- consider dialysis if severe
- Look for kidney causes
- stop nephrotoxins
- monitor
Differential diagnosis
| Cause | Clues |
|---|---|
| Blocked catheter or retention | Palpable bladder, catheter not draining |
| Dehydration | Negative balance, fast pulse, dry mouth |
| Sepsis | Fever, low BP, high lactate |
| Bleeding | Post-op, drains filling, falling Hb |
| Drug or contrast AKI | Recent NSAID, gentamicin, ACE inhibitor, contrast scan |
| Heart failure | Overloaded, raised JVP |
| Rhabdomyolysis | Long lie, crush, very high CK, dark urine |
| Abdominal compartment syndrome | After major abdominal surgery, tense distension, high airway pressures |
| Normal response after surgery | Mild, transient, otherwise well |
Investigations
| Test | Why |
|---|---|
| Urea, creatinine, electrolytes | Compare with baseline; stage AKI |
| Blood gas | Potassium, pH, lactate |
| CBC | Bleeding, infection |
| Urine dipstick | Blood and protein (glomerulonephritis), infection |
| Urine culture | Infection |
| CK | Rhabdomyolysis |
| Ultrasound kidneys and bladder | Obstruction, especially if anuric |
Treatment
- Treat the cause: flush or change the catheter, fluids for dehydration, sepsis pathway, blood for bleeding.
- Stop nephrotoxic drugs and adjust doses for kidney function.
- Hourly urine output, daily weight, fluid balance.
- See AKI and high potassium if creatinine or potassium are up.
Don't give furosemide to "get the urine going" in a dry patient. It worsens kidney injury. Furosemide is only for fluid overload.
Common mistakes
- Not checking the catheter first.
- Giving furosemide to a dry patient.
- Repeated fluid boluses into an overloaded patient.
- Leaving diclofenac or ketorolac on the chart after surgery.
- Not checking potassium.
- Forgetting obstruction in an anuric patient.
Document
Urine output over the last hours, catheter checks, fluid status, fluids given and response, blood results, drugs stopped, plan and review time.
Frequently asked questions
What is a normal urine output?
At least 0.5 mL/kg/hour, about 35 mL an hour for a 70 kg adult.
Should furosemide be given for low urine output?
Only if the patient is fluid overloaded. In a dry patient it worsens kidney injury.
What is the first thing to check when urine output is low?
The catheter: is it blocked, kinked or clamped, or is the bag full? Without a catheter, do a bladder scan.
Which drugs should be stopped in low urine output?
NSAIDs, ACE inhibitors, ARBs, diuretics, metformin and nephrotoxic antibiotics such as gentamicin, unless clearly needed.
Test yourself
Exam-style questions on this guide, from our two question banks. Choose an answer to see why each option is right or wrong.
1. A catheterised patient suddenly has no urine for 3 hours. What is the first step?
Answer: Flush the catheter. Sudden anuria in a catheterised patient is a blocked catheter until proven otherwise.
- Restrict fluids: No.
- IV furosemide: Doesn't fix a blocked catheter.
- CT kidneys: Not first.
- Dialysis: Not first.
2. What is the threshold for oliguria in a 70 kg adult?
Answer: Below 35 mL per hour. Oliguria is less than 0.5 mL/kg/hour for 6 hours or more.
- Below 100 mL per hour: Too high.
- Below 500 mL per day only: Not the hourly definition.
- Below 10 mL per hour: Too low.
- Below 70 mL per hour: Too high.
3. A dry post-operative patient has low urine output. When is furosemide appropriate?
Answer: Only if fluid overloaded. Furosemide is for fluid overload, not to increase urine in a dehydrated patient.
- Always first: Worsens dry kidneys.
- With every fluid challenge: No.
- In all AKI: No.
- To make urine whatever the cause: Doesn't treat the cause.
4. On the evening after a laparotomy, a patient has passed 15 mL/h of urine for 3 hours. HR 110, BP 105/65, cool peripheries, catheter flushes well. What is the first step?
Answer: 500 mL fluid challenge. Post-operative oliguria (< 0.5 mL/kg/h): first exclude a blocked catheter (flush/replace), then assess volume status — most commonly hypovolaemia (bleeding, third-space losses, inadequate replacement). Give a fluid bolus (250–500 mL crystalloid) and reassess; look for bleeding. Consider sepsis, nephrotoxins, abdominal compartment syndrome, cardiac failure. Diuretics only once euvolaemic/overloaded.
- IV furosemide 40 mg: Furosemide worsens hypovolaemia.
- Low-dose dopamine: Dopamine gives no renal benefit.
- Wait until morning: Waiting risks AKI.
5. A 70 kg man has passed 60 mL of urine in the last 4 hours after a laparotomy. BP 104/62 mmHg, HR 106/min, his mouth is dry. The catheter flushes freely. What is the next step?
Answer: IV fluid bolus and reassess. Urine output below 0.5 mL/kg/hour (35 mL/hour for 70 kg) is oliguria. After confirming the catheter is not blocked, the commonest cause post-operatively is hypovolaemia. Give a fluid bolus and reassess HR, BP, urine output and lactate. Look for bleeding, stop nephrotoxic drugs and check renal function.
- IV furosemide 40 mg: Furosemide would worsen hypovolaemia; low urine output with tachycardia and dry mouth needs fluid.
- Restrict fluids overnight: Restricting fluid in a hypovolaemic patient risks acute kidney injury.
- Change the urinary catheter: It flushes freely — not blocked.
6. Which two mechanisms keep glomerular filtration rate steady when blood pressure changes?
Answer: Myogenic response and TGF. Autoregulation keeps GFR steady between mean pressures of about 80–180 mmHg. The afferent arteriole constricts when stretched (myogenic). The macula densa senses increased NaCl when GFR rises and triggers afferent constriction (tubuloglomerular feedback, via adenosine). Angiotensin II also constricts the efferent arteriole when pressure falls.
- ADH and aldosterone: Control water and sodium, not GFR autoregulation.
- ANP and BNP: Raise GFR and sodium excretion; not autoregulation.
- Sympathetic nerves and adrenaline: Reduce GFR in severe stress.
- Angiotensin II and prostaglandins alone: Help, but are not the two main mechanisms.
7. Which drug is a recognised cause of SIADH and hyponatraemia?
Answer: Carbamazepine. Drugs causing SIADH: carbamazepine, SSRIs, tricyclics, cyclophosphamide, vincristine, chlorpropamide, opioids and PPIs. Lithium and demeclocycline cause nephrogenic diabetes insipidus (demeclocycline is used to treat SIADH). Tolvaptan is a V2 antagonist used for hyponatraemia.
- Lithium: Nephrogenic DI.
- Demeclocycline: Treats SIADH.
- Furosemide by water retention: Not via SIADH.
- Tolvaptan: Treats hyponatraemia.
References and dose sources
- Ringer's lactate (Hartmann's solution) dosing: Drugs.com: Lactated Ringer's dosage (see also our ringer's lactate (hartmann's solution) dose page)
- Sodium chloride (normal saline, 3% saline) dosing: Drugs.com: Sodium chloride dosage (see also our sodium chloride (normal saline, 3% saline) dose page)
International guidance is linked where it reflects good practice. Where your hospital’s protocol differs, follow your hospital.