Basic investigations for AKI patients in the Emergency Department

1. Bloods

Urea and electrolytes, Creatinine, eGFR, arterial or venous blood gas, full blood count, liver function tests and bone profile is a good place to start with patients with AKI. Other things that are useful to have to hand when discussing with the renal team will be (if known) a medication list, baseline eGFR, urine output since arrival in ED and an assessment of their fluid balance including haemodynamic parameters.

It should also be noted that although the most useful diagnostic test for AKI remains the serum creatinine concentration, this test has significant limitations – it is an insensitive test (a rise in creatinine only occurs once 50% of kidney function has been lost) and therefore a delayed marker of renal injury. A fall in the urine output will be the earliest sign of an evolving AKI.

Learning bite

Serum Creatinine only starts to rise once 50% of kidney function is lost, therefore patients presenting to ED considered to be at risk of AKI should always have careful fluid balance monitoring commenced.

2. Urine dip

All patients presenting with AKI should have a urine dipstick checked (unless completely anuric). Patients with a pre-renal AKI will usually have a negative urinalysis as the pathology lies outside of the kidneys.

The finding of blood and/or protein on urinalysis is a hallmark feature of many of the intrinsic (or ‘renal’) causes of AKI. Intrinsic renal disease is a very uncommon cause of AKI (<5%) and causes include:

• glomerulonephritis
• haematological disorders (eg haemolytic uraemic syndrome, thrombotic thrombocytopaenic purpura, myeloma)
• rhabdomyolysis
• interstitial nephritis (usually due to medications)
• vasculitis

It is important not to miss the diagnosis of a possible intrinsic renal disease as these conditions often require specialist diagnostic tests and management.

Obstructive causes account for about 10% of cases of AKI. The hallmark features of obstructive AKI include pain, visible (macroscopic) haematuria and/or complete anuria.

Learning bite

Urine dipstick is a cheap and easy test that is vital to establish the cause of an AKI (and therefore its investigation and management). Always document the result in the patients’ notes.

3. Renal tract ultrasound

Renal ultrasonography is only indicated in the following situations:

• if an obstructive cause of AKI is strongly suspected
• if pyelonephritis is considered (to exclude an abscess)
• if there is no obvious explanation for the AKI
• if a presumed pre-renal AKI fails to respond to appropriate treatment