Compare and contrast the utility of the following in the assessment of acute kidney injury in a critically ill patient:
- Creatinine clearance
- Serum creatinine
- Urine output measurements
- Novel biomarkers
Answer and interpretation
- Gives estimation of Glomerular Filtration Rate (GFR).
- Requires timed urine collection (usually 24 hours).
- Accuracy may be limited due to creatinine secretion, thus overestimating GFR, and incomplete urine collection.
- Assumes steady state in GFR, which may not be the case in acute renal failure.
- Determining exact GFR is rarely clinically necessary.
- Simple to measure and widely available.
- Specific for renal function.
- Indicator of GFR based upon constant production from muscle creatine and relatively constant renal excretion rate.
- Production may be increased by trauma, fever or immobilisation.
- Decreased in individuals with small stature, cachexia, reduced muscle mass (eg muscle disease, amputations)
- Decreased production may occur in liver disease because of decreased hepatic conversion of creatine to creatinine, decreased dietary protein intake, muscle wasting and increased renal tubular secretion of creatinine.
- May be influenced by volume of distribution changes in critically ill patients
- Simple to measure and widely available
- Not specific for renal function
- May be affected by liver disease, protein intake, catabolic state, volume status, upper gastrointestinal bleeding, and drug therapy – eg corticosteroids.
Urine output measurements
- Simple to measure and universally available.
- More sensitive to changes in renal function than biomarkers
- Non-specific – can have normal urine output despite severe acute renal failure
- Include a plasma panel (NGAL and cystatin C) and urine panel (NGAL, IL-8 and KIM-1)
- Represent sequential biomarkers and so have potential for timing the initial insult and assessing the duration of AKI and for predicting overall prognosis
- May also distinguish between various types and pathogeneses of AKI
- Potential for high sensitivity and specificity
- So far only tested in small studies and limited clinical situations and need further validation