investigations

Last reviewed 09/2023

Investigations include:

• urine:
• microscopy, culture and sensitivity
• sodium, urea, creatinine and osmolality
• proteinuria
• haematuria and proteinuria suggest a renal cause. Red cell casts suggest glomerulonephritis; tubular cell casts, acute tubular necrosis
• blood:
• FBC, ESR, urea and electrolytes, creatinine, LFTs, CRP, CK
• arterial blood gas
• blood culture
• ECG - may reveal changes associated with hyperkalaemia
• renal imaging - assess renal size ?evidence of obstruction
• CXR - ?evidence of pulmonary oedema
• renal biopsy might be indicated
• daily weighing

Urinalysis can provide important clinical information to patients with AKI.

• positive protein values of 3+ and 4+ on reagent strip testing of the urine suggest intrinsic glomerular disease
• a reagent strip positive for blood suggests the presence of red blood cells (> 5/high power field)
  • although red cell morphology may not be particularly useful the observation of large numbers of red cells in the presence of proteinuria suggests a glomerular aetiology for AKI
    • suspicion is strengthened by the finding of red cell casts on a freshly collected sample of urine (this is rarely performed in the UK)
  • haematuria may also be found in cases of lower urinary tract obstruction often in association with tumours and less commonly associated with calculi, infection or severe renal ischaemia due to arterial or venous thrombosis
  • characteristically myoglobinuria will cause a positive reagent strip reaction for haematuria without evidence of red cells on urine microscopy
• increased numbers of white cells (> 5 per high power field) are non-specific but are found more commonly with acute interstitial nephritis, infection and glomerulonephritis
• eosinophiluria is not a very specific test for interstitial nephritis and has a very poor positive predictive value
  • however, the value of eosinophiluria in interstitial nephritis is in ruling out the disease, the negative predictive value for patients with AKI is greater than 90%

Urine microscopy can be informative in particular clinical scenarios such as suspected poisoning

• presence of crystalluria may provide vital information and in the case of ethylene glycol poisoning oxalate crystals may be visible
• tumour lysis syndrome can produce urate crystal deposition
• drugs may lead to AKI and crystalluria including sulphonamides, acyclovir, triamterene, indinavir and cathartics high in phosphates

Measurement of urinary electrolytes

• useful in the diagnosis of hepatorenal syndrome as the cause of AKI in patients with liver disease
• diagnostic criteria for hepatorenal failure include a urine sodium of less than 10 mmol/L (although not a major diagnostic criterion)

Ultrasound

• the gold standard test for diagnosis of upper tract obstruction through the finding of hydronephrosis and/or hydroureter
  • however upper urinary tract obstruction may not be initially detected by ultrasound in a patient who is volume depleted
    • therefore recommended to repeat the renal tract ultrasound if upper urinary tract obstruction is suspected once the patient is adequately fluid resuscitated
• there are other circumstances when ultrasound may not be diagnostic, such as in retroperitoneal fibrosis or early in the course of obstructive disease, in which case additional imaging studies may be considered such as dynamic nuclear medicine studies or CT

Dynamic nuclear medicine studies will be of little diagnostic use if the patient has oligo-anuric AKI

Reference:

• UK Renal Association (2011). Acute Kidney Injury.