Prerenal Azotaemia and Urinary Diagnostic Indices
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Direct answer
Prerenal azotaemia is a rise in urea and creatinine caused by reduced renal perfusion, with intact tubules that retain sodium and water. The most useful discriminator is fractional excretion of sodium (FENa), usually below 1 per cent, with fractional excretion of urea below about 35 per cent as the more reliable check. Urine osmolality above 500 mOsm/kg and a BUN:creatinine ratio above 20:1 further support a prerenal cause.
What you must remember
- Prerenal physiology: reduced perfusion with intact tubules, so sodium and water are reabsorbed strongly and the urine is concentrated.
- FENa: below 1 per cent suggests prerenal azotaemia, while above 2 per cent suggests tubular injury.
- FEUrea: below about 35 per cent suggests prerenal azotaemia, and it remains useful when diuretics raise FENa.
- Urine osmolality: above 500 mOsm/kg suggests prerenal physiology, whereas values near plasma osmolality suggest tubular injury.
- Urine sodium: below 20 mmol/L suggests a prerenal cause, while values above 40 mmol/L point to intrinsic injury.
- BUN:creatinine ratio: above 20:1 suggests prerenal azotaemia, because urea is reabsorbed with water when perfusion falls.
- Urine sediment: hyaline casts favour prerenal states, while muddy brown granular casts suggest acute tubular necrosis.
- Treatment principle: restore volume and stop drugs that alter renal haemodynamics, such as NSAIDs, ACE inhibitors, ARBs and diuretics.
A man with vomiting, diarrhoea and a rising creatinine
A 45-year-old man has had three days of vomiting and diarrhoea, with a dry mouth and reduced urine output. He takes an ACE inhibitor and an NSAID for back pain, and his creatinine is raised above his baseline.
- Confirm the rise: Creatinine is compared with the baseline value, and urine output is measured, since acute kidney injury is defined by a rise in creatinine or a fall in urine output.
- Assess the volume: Blood pressure, heart rate, skin turgor and mucosal moisture show whether hypovolaemia is present, and fluid balance is recorded.
- Stop the renal insults: NSAIDs, ACE inhibitors, ARBs and diuretics are held, because they reduce glomerular perfusion pressure.
- Collect paired samples: Urine and plasma sodium and creatinine are measured together, so that FENa and FEUrea can be calculated.
- Read the indices together: FENa below 1 per cent, FEUrea below about 35 per cent and urine osmolality above 500 mOsm/kg support prerenal azotaemia.
- Test the response: A fluid challenge is given, and creatinine that improves over the following day or two supports a volume-responsive prerenal cause.
- Examine the sediment: Hyaline casts favour a prerenal state, while muddy brown granular casts suggest acute tubular necrosis.
The indices are read together rather than one at a time, because a single value can mislead. Diuretics and upper gastrointestinal bleeding both distort individual markers, so the pattern across FENa, FEUrea, osmolality and sediment is what the examiner expects.
Where the examiner tests the index
Most errors come from reading one index in isolation or from using it in the wrong patient.
- Trap: using FENa alone in a patient on furosemide Correct: diuretics raise sodium excretion, so FEUrea is the more reliable index in that setting.
- Trap: calling a raised BUN:creatinine ratio diagnostic on its own Correct: upper gastrointestinal bleeding and high protein intake also raise the ratio, so it is only supportive.
- Trap: assuming every acute kidney injury responds to fluid Correct: a fluid challenge helps only in volume-responsive states, and excess fluid worsens outcome in heart failure.
Frequently asked questions
What cut-off of fractional excretion of sodium suggests prerenal azotaemia?
A fractional excretion of sodium below about 1 per cent suggests prerenal azotaemia, because the kidney is avidly reabsorbing sodium in response to low perfusion. A value above 2 per cent suggests tubular injury such as acute tubular necrosis.
Why is fractional excretion of urea preferred in patients on diuretics?
Diuretics increase sodium excretion regardless of renal perfusion, which makes FENa unreliable in prerenal states. Fractional excretion of urea is less affected by diuretics, so a value below about 35 per cent still supports a prerenal cause.
What does a BUN:creatinine ratio above 20:1 indicate?
A blood urea nitrogen to creatinine ratio above 20:1 suggests prerenal azotaemia, because reduced perfusion increases proximal reabsorption of urea along with water. It is supportive only, since upper gastrointestinal bleeding and high protein intake also raise the ratio.
Which urine sediment finding favours acute tubular necrosis?
Muddy brown granular casts, which are formed from degenerating tubular epithelial cells, favour acute tubular necrosis over a prerenal cause. Hyaline casts are typical of prerenal states, so sediment is an important bedside clue.
Which drugs should be stopped in prerenal azotaemia?
NSAIDs, ACE inhibitors, ARBs and diuretics should be stopped in prerenal azotaemia because they reduce glomerular perfusion pressure or impair renal haemodynamics. Stopping them early, together with restoration of volume, is the cornerstone of management.