Acute kidney injury is detected when serum creatinine rises by 26 micromol/L or more within 48 hours, rises by 50% or more within 7 days, or urine output falls below 0.5 ml/kg/hour for more than 6 hours. NICE NG148, last updated 16 October 2024, sets those criteria and the referral rules that follow. At the CT1 anaesthetics interview the scenario is a post-operative patient who has stopped passing urine, and the marks are for a structured response with the guideline named.
Key takeaways
- Detection: creatinine up 26 micromol/L or more in 48 hours, up 50% or more in 7 days, or urine output under 0.5 ml/kg/hour for over 6 hours in adults (NICE NG148)
- The risk drugs are NSAIDs, aminoglycosides, ACE inhibitors, angiotensin receptor blockers and diuretics, particularly in a hypovolaemic patient
- Urgent ultrasound within 24 hours if there is no identified cause or a risk of obstruction; within 6 hours if pyonephrosis is suspected
- Discuss with nephrology within 24 hours for stage 3 AKI, no clear cause, inadequate response, complications, a transplant, or CKD stage 4 to 5
- Refer immediately for renal replacement if hyperkalaemia, acidosis, uraemic complications, fluid overload or pulmonary oedema are not responding to treatment
- NG148 says the decision to start renal replacement rests on the patient as a whole, not an isolated urea, creatinine or potassium
How is acute kidney injury defined?
Acute kidney injury is defined by NICE NG148 using any one of three criteria taken from the KDIGO, AKIN and RIFLE systems. A creatinine rise of 26 micromol/L or more within 48 hours, a rise of 50% or more known or presumed within 7 days, or urine output below 0.5 ml/kg/hour for more than 6 hours in an adult.
Creatinine lags. That said, urine output does not, which is why the hourly chart on a catheterised post-operative patient is the earliest signal you have.
NG148 asks for creatinine to be monitored in everyone with or at risk of AKI, typically daily in hospital. The staging behind stage 3, which triggers a nephrology discussion, follows the NHS England algorithm that NG148 cites.
What do you do first?
The first step in a ward AKI is an ABCDE assessment with the fluid status decided at the bedside, because most post-operative AKI is pre-renal. Then the drug chart, the catheter and the potassium.
- Airway and breathing: pulmonary oedema is both a cause of hypoxia and an indication for renal replacement
- Circulation: pulse, blood pressure, capillary refill, jugular venous pressure and a fluid balance chart; resuscitate if hypovolaemic and reassess
- Potassium: a venous gas now, and an ECG if it is raised
- Catheter: flush or replace it before calling anuria a renal problem
- Drug chart: stop what is harming the kidney, as below
In practice the anaesthetic CT1 is asked because the patient is post-operative. Say that you would review the operation note and the anaesthetic chart for blood loss, hypotension and the drugs given.
Who is at risk, and which drugs do you stop?
Patients at risk of AKI, according to NG148, include those with chronic kidney disease and an eGFR below 60, diabetes, heart failure, liver disease, age 65 or over, and those given drugs that damage the kidney around surgery, in particular NSAIDs after the operation.
The drugs NG148 names as able to cause or worsen AKI are NSAIDs, aminoglycosides, ACE inhibitors, angiotensin receptor blockers and diuretics, especially within the past week and in a hypovolaemic patient.
NG148 says to consider temporarily stopping ACE inhibitors and angiotensin receptor blockers in patients with diarrhoea, vomiting or sepsis until they have improved and stabilised. It also asks for the risk of AKI to be part of the routine consent discussion before surgery.
What investigations do you send?
The AKI investigations that change the decision are a venous gas for potassium and acid-base, urea and electrolytes, a full blood count, a urine dipstick, and an ECG. NG148 gives the dipstick a specific job.
If the dipstick shows haematuria and proteinuria in a patient with no obvious cause, without urinary infection or catheter trauma, NG148 says to think about acute nephritis and refer to nephrology. That single line separates the candidate who understands intrinsic renal disease from the one who does not.
Bladder scan or catheter residual, a fluid balance review and a medication review complete the picture. Weight twice daily is the NG148 way of tracking fluid balance in the days that follow.
When is an ultrasound needed?
An ultrasound of the urinary tract is needed urgently, within 24 hours of assessment, when the cause of AKI is not identified or the patient is at risk of obstruction. NG148 makes it immediate, within 6 hours, when pyonephrosis, an infected obstructed kidney, is suspected.
However, NG148 says not to routinely scan when the cause has already been identified. A post-operative patient who was hypotensive for an hour and has responded to fluid does not need one.
Any upper tract obstruction is referred to a urologist, and NG148 lists the features that make that referral immediate, including pyonephrosis, an obstructed solitary kidney, bilateral obstruction and complications of AKI caused by the obstruction.
What is the management?
AKI management is to treat the cause, restore and then maintain volume, stop the offending drugs, relieve obstruction, and watch for the complications that need renal replacement. NG148 refers fluid decisions to the intravenous fluid guideline CG174.
Resuscitate a hypovolaemic patient with a crystalloid bolus and reassess before the next. Once volume is replete, further fluid will not improve the kidney and can produce the pulmonary oedema you are trying to avoid.
NG148 allows loop diuretics for fluid overload or oedema while the patient awaits renal replacement or is recovering without it. They are not a treatment for the AKI itself.
When does nephrology need to know?
Nephrology needs to know within 24 hours of detecting AKI, according to NG148, when any of seven features is present. A diagnosis that may need specialist treatment such as vasculitis, glomerulonephritis, interstitial nephritis or myeloma; no clear cause; inadequate response to treatment; complications; stage 3 AKI; a renal transplant; or CKD stage 4 or 5.
That said, NG148 also says not to refer when there is a clear cause and the AKI is responding promptly to treatment, unless the patient has a transplant. Referring everyone is not the safe answer.
Patients in intensive care are referred when the cause is uncertain or specialist kidney management might be needed. After recovery, NG148 suggests nephrology referral if the eGFR is 30 or less.
When is renal replacement therapy indicated?
Renal replacement therapy is indicated by NG148 when hyperkalaemia, metabolic acidosis, symptoms or complications of uraemia such as pericarditis or encephalopathy, fluid overload, or pulmonary oedema are not responding to medical management. Any potential indication is discussed with a nephrologist or critical care immediately so therapy starts as soon as it is needed.
NG148 is explicit that the decision rests on the condition of the patient as a whole and not on an isolated urea, creatinine or potassium value. Quote that sentence; it is the one candidates get wrong by naming a number.
Where the patient has significant comorbidity, NG148 asks the team and the patient to discuss whether renal replacement would offer benefit, following the shared decision making guideline.
Which traps catch candidates?
The AKI traps at interview are a creatinine threshold for dialysis, a fluid bolus that never stops, and an ultrasound ordered on everyone. Each one contradicts a line in NG148.
- Naming a creatinine or potassium value at which you would 'start dialysis': NG148 says the whole patient, not an isolated value
- Forgetting the catheter: a blocked catheter is anuria with a normal kidney
- Restarting the ACE inhibitor on discharge without a plan to recheck
- Missing the dipstick, and with it the nephritis that needed a nephrologist the same day
What is the interviewer listening for?
In an AKI scenario the assessors are listening for the NG148 definition, a bedside volume assessment, the drug chart reviewed by name, and a referral plan with a time on it.
- The three detection criteria with the numbers
- Hypovolaemia treated and reassessed, then fluid stopped
- NSAIDs, ACE inhibitors, ARBs, diuretics and aminoglycosides stopped or held
- Ultrasound within 24 hours if the cause is unclear, 6 hours if pyonephrosis is suspected
- The renal replacement indications, and the whole-patient sentence
How this comes up at the CT1 anaesthetics interview
AKI comes up in the clinical judgement station as the patient who has passed 20 ml in the last four hours after a laparotomy, and the question is what you do and who you tell. The station is one of two 15-minute stations, each marked out of 50, and scored on clinical judgement and decision making, reflective practice and working under pressure.
The follow-up usually adds a potassium of 6.6 or a chest that sounds wet after your bolus, so the escalation step is tested directly. anaestheticinterview's 340-question bank includes an AKI scenario with AI-marked spoken practice, which is where the NG148 numbers get rehearsed out loud.