Urine Output and Oliguria: When Reduced Output Matters

Urine output is more than a number on a fluid balance chart. A sustained fall can be an early sign that a patient is becoming unwell, sometimes before a blood result makes the problem obvious. It may reflect reduced kidney perfusion, acute kidney injury, urinary obstruction or a problem with how the output has been measured.

The important skill is not simply noticing a small volume. It is recognising the pattern, checking whether the measurement is reliable, assessing the whole patient and escalating appropriately.

What is oliguria?

Oliguria means abnormally low urine output. NICE identifies urine output below 0.5 mL/kg/hour as an important marker of possible acute kidney injury in adults.

For AKI detection, duration matters. NICE uses a fall in urine output to less than 0.5 mL/kg/hour for more than 6 hours as one of the diagnostic criteria for acute kidney injury in adults.

That means one small void does not automatically equal AKI. The concern is a sustained reduction, particularly when it is accompanied by other signs of deterioration.

How do you calculate weight-based urine output?

Use:

Urine output in mL ÷ body weight in kg ÷ number of hours = mL/kg/hour

Example: A 70 kg patient passes 180 mL over 6 hours.

180 ÷ 70 ÷ 6 = 0.43 mL/kg/hour.

This is below 0.5 mL/kg/hour and therefore requires clinical attention when the pattern is sustained and reliable.

Weight-based calculations are useful, but they are not perfect. Estimated body weight, incomplete urine collection and inaccurate charting can all affect the result.

Oliguria is not the same as diagnosed AKI

This distinction matters. Reduced urine output can be one criterion for AKI, but AKI can also be detected through changes in serum creatinine.

NICE advises detecting AKI in adults using any of the following:

• a rise in serum creatinine of 26 micromol/L or more within 48 hours
• a rise in serum creatinine of 50% or more, known or presumed to have occurred within the previous 7 days
• urine output of less than 0.5 mL/kg/hour for more than 6 hours

A patient may therefore have clinically important oliguria before creatinine has risen, while another patient may develop AKI without obvious oliguria. Use urine output as part of the clinical picture, not as a stand-alone diagnosis.

Why might urine output fall?

A useful way to organise the causes is to think about problems before the kidney, within the kidney and after the kidney.

Before the kidney: reduced perfusion

The kidneys need adequate blood flow to produce urine. Output may fall when circulating volume or cardiac output falls.

• dehydration or poor oral intake
• vomiting or diarrhoea
• blood loss
• sepsis
• hypotension
• heart failure or reduced cardiac output

Within the kidney: intrinsic kidney injury

Damage to kidney tissue can impair filtration and urine production.

• ischaemic injury
• inflammation, including nephritis
• severe infection
• toxins
• medicines with nephrotoxic potential

NICE highlights medicines such as non-steroidal anti-inflammatory drugs, aminoglycosides, angiotensin-converting enzyme inhibitors, angiotensin receptor blockers and diuretics as factors that can increase vulnerability to AKI in some clinical situations, especially when a patient is hypovolaemic. Students should recognise these as possible contributors and escalate concerns rather than independently stopping prescribed medicines.

After the kidney: obstruction

Urine may be produced normally but unable to drain.

• urinary retention
• stones
• prostatic enlargement
• pelvic or abdominal pathology
• blocked or kinked urinary catheter tubing

Patients can have more than one cause at the same time. An older adult with sepsis, poor intake and hypotension may have reduced renal perfusion while also taking medicines that increase their susceptibility to AKI.

Check that the measurement is real

Before interpreting a low number, make sure the output has actually been measured accurately.

Ask:

• When did the patient last pass urine?
• Has all urine been measured and recorded?
• Has the patient used the toilet without the volume being captured?
• Has a bottle, bedpan or drainage bag been emptied without being charted?
• Is the fluid balance chart complete?
• Is the recorded time period correct?

If a urinary catheter is already in place, check the system without unnecessarily breaking the closed drainage circuit.

• Is the tubing kinked, compressed or trapped?
• Is the drainage bag below bladder level?
• Is urine visible in the tubing?
• Is the bag overfull?
• Has the patient moved into a position that obstructs drainage?

Do not disconnect, irrigate or manipulate the catheter unless this is clinically indicated, authorised and within your competence and local policy.

A urinary catheter should not be inserted simply because hourly measurements would be convenient. The indication, infection risk and overall clinical need should be considered by the responsible team.

Could this be urinary retention?

Low measured output does not always mean the kidneys are producing very little urine. The bladder may be filling but failing to empty.

Features that may suggest retention include:

• suprapubic discomfort or fullness
• restlessness or new agitation
• frequent small voids
• overflow incontinence
• a history of voiding difficulty
• little or no drainage despite suspected bladder filling

A bladder scan can help when retention is suspected, but it should be performed by someone trained and interpreted alongside the clinical picture. Results can be affected by factors such as ascites, body habitus and pelvic structures.

Assess the whole patient, not just the urine bag

Reduced urine output is most useful when it is interpreted alongside the patient's observations, fluid status and recent clinical course.

Review:

• respiratory rate, oxygen saturation and oxygen requirement
• heart rate and rhythm
• blood pressure and trends
• temperature
• consciousness or new confusion
• capillary refill and peripheral perfusion
• oral and intravenous intake
• vomiting, diarrhoea, drains or other fluid losses
• oedema, breathlessness or signs of fluid overload
• recent weight where available

If the patient is deteriorating, use a structured ABCDE assessment and escalate early rather than waiting for a complete urine-output calculation.

What blood results matter?

Review recent results when available, especially:

• creatinine and the patient's previous baseline
• urea
• potassium
• sodium
• bicarbonate where reported
• estimated glomerular filtration rate, while remembering that eGFR is less reliable during rapidly changing kidney function

Creatinine may lag behind the clinical event, so a normal or unchanged result does not automatically make sustained oliguria reassuring.

What about urinalysis?

NICE recommends urine dipstick testing for blood, protein, leucocytes, nitrites and glucose when AKI is suspected or detected, with abnormal findings acted on appropriately.

For students, the important point is not to diagnose the cause from a dipstick alone. Record the result accurately, consider it alongside symptoms and other findings, and escalate abnormalities to the registered nurse or medical team.

When does reduced urine output need urgent escalation?

Escalate promptly when reduced urine output is sustained, unexpected or accompanied by clinical deterioration.

Particularly concerning features include:

• hypotension or signs of shock
• sepsis or suspected severe infection
• active bleeding
• severe abdominal, loin or suprapubic pain
• a distended bladder or suspected obstruction
• new confusion or reduced consciousness
• hyperkalaemia
• pulmonary oedema or worsening breathlessness
• rapidly rising creatinine
• complete absence of urine

Anuria means essentially no urine production or passage and is more concerning than mild oliguria. Treat complete absence of urine as a finding that needs urgent review, particularly if it is new.

Do not assume the answer is more fluid

One of the most important clinical lessons is that low urine output does not automatically mean a patient needs a fluid bolus.

Fluids may be appropriate when hypovolaemia is contributing, but they can worsen pulmonary oedema or fluid overload in patients with heart failure, renal impairment or other conditions.

The decision to give intravenous fluid requires assessment and an authorised clinical plan. A student nurse should recognise the problem, gather relevant information and escalate rather than independently deciding on fluid therapy.

Three placement examples

Example 1: the patient with poor intake

A 68 kg patient has passed 150 mL over 6 hours.

150 ÷ 68 ÷ 6 = 0.37 mL/kg/hour.

They have had poor oral intake and diarrhoea, their heart rate has risen and their blood pressure is lower than usual. The low output is not just a charting issue. The pattern suggests possible reduced circulating volume and needs prompt assessment and escalation.

Example 2: the catheter is not draining

A catheterised patient has no urine recorded for 2 hours but is otherwise stable. You notice the drainage tubing is trapped beneath the patient's leg. Once the tubing is repositioned according to local practice, urine begins to drain.

The lesson is to check the measurement system before assuming the kidneys have suddenly stopped producing urine.

Example 3: low output with fluid overload

A patient with heart failure has falling urine output, increasing ankle oedema and worsening breathlessness.

This is exactly why low urine output should not automatically trigger the assumption that the patient needs more fluid. The patient needs clinical review and a wider assessment of circulation, congestion and kidney function.

How to escalate clearly

A useful SBAR gives the pattern, not just the phrase "low urine output".

Situation: "I'm concerned that Mr Patel's urine output has fallen."

Background: "He was admitted with sepsis and has a urinary catheter in place."

Assessment: "He has passed 120 mL in the last 6 hours. He weighs 72 kg, so that is around 0.28 mL/kg/hour. The tubing is patent, his blood pressure has fallen from 128 systolic to 98 and his heart rate is now 108."

Recommendation: "Could you review him now? I'm concerned about deterioration and possible AKI."

This is much more useful than reporting the urine volume without the time period, body weight or associated observations.

Your role as a student nurse

Your role depends on your stage of training, competence, supervision and local policy, but you can contribute significantly to early recognition.

• measure and document urine output accurately
• calculate mL/kg/hour with appropriate supervision
• review fluid balance and trends
• check an existing drainage system for obvious problems
• recognise signs of deterioration
• complete observations and structured assessment within your competence
• report concerns promptly using SBAR
• check that escalation has led to review
• document what you observed, reported and were asked to do

You are not expected to independently diagnose AKI, prescribe fluids, stop medicines or manage obstruction. Your value is in recognising change early, gathering accurate information and making sure the concern reaches the right clinician.

The bit worth remembering

Low urine output is a clinical sign, not a diagnosis.

When you notice it:

1. check that the measurement is accurate
2. calculate the rate if appropriate
3. look for obstruction or drainage problems
4. assess the whole patient
5. review trends and relevant blood results
6. escalate sustained or concerning oliguria

A fluid balance chart can tell you that something has changed. Your assessment helps the team work out why.

Continue with Fluid Balance Charts and Urea and Electrolytes Explained.

References

National Institute for Health and Care Excellence (NICE) (2024) Acute kidney injury: prevention, detection and management. NICE guideline NG148. Available at: https://www.nice.org.uk/guidance/ng148.

National Institute for Health and Care Excellence (NICE) (2023) Acute kidney injury in adults. Quality standard QS76. Available at: https://www.nice.org.uk/guidance/qs76.

UK Kidney Association (2019) Clinical Practice Guideline: Acute Kidney Injury. Available at: https://www.ukkidney.org/health-professionals/guidelines/acute-kidney-injury-aki.

Think Kidneys (2016) The use of oliguria to detect acute kidney injury. Archived position statement. Available at: https://www.thinkkidneys.nhs.uk/aki/wp-content/uploads/sites/2/2016/04/Position-Statement-on-Oliguria-final.pdf.

Nursing and Midwifery Council (NMC) (2018) The Code: Professional standards of practice and behaviour for nurses, midwives and nursing associates. Available at: https://www.nmc.org.uk/standards/code/read-the-code-online/.

Educational disclaimer: This article is for student learning and placement preparation. Always follow the escalation pathway, catheter care guidance, fluid management policy and supervision requirements used in your placement area.