From the Guidelines
The urine studies for Acute Kidney Injury (AKI) include urine output and urine biomarkers, with urine output being a foundational measure for AKI diagnosis, and biomarkers potentially useful for staging and classification, but with limited clinical applicability at present 1.
Key Urine Studies for AKI
- Urine Output: measured as < 0.5 ml/kg/h, which is a criterion for AKI diagnosis, and its evaluation should consider factors such as fluid status and body composition 1
- Urine Biomarkers: although not yet widely used in clinical practice, biomarkers such as those indicating cell cycle arrest may have moderate predictive power for AKI outcomes, but their clinical applicability and cost-effectiveness need further evaluation 1
Clinical Considerations
- Fluid Status: should be considered when evaluating urine output, as fluid overload can impact AKI diagnosis and management 1
- Body Composition: differences in body composition, such as overweight or fluid overload, may affect urine output and should be considered when interpreting AKI criteria 1
- Timing of Insult: the timing of renal insult(s) can impact the performance of biomarkers, and their use should be interpreted with caution in cases where the timing is unknown 1
From the Research
Urine Studies for Acute Kidney Injury (AKI)
- Urinary indices are classically believed to allow differentiation of transient (or pre-renal) acute kidney injury (AKI) from persistent (or acute tubular necrosis) AKI, however, the data validating urinalysis in critically ill patients are weak 2
- Urinary AKI markers may be classified as enzymes released from damaged tubular cells, low-molecular-weight proteins, and proteins specifically produced in the kidney and associated with the development of AKI 3
- Some of the urinary biomarkers for AKI include:
- Enzymes: alkaline phosphatase, gamma-glutamyl transpeptidase, alanine aminopeptidase, isoenzymes of glutathione transferase, N-acetyl-beta-D-glucosaminidase
- Low-molecular-weight proteins: alpha(1)-microglobulin, beta(2)-microglobulin, retinol-binding protein, cystatin C
- Proteins specifically produced in the kidney: cysteine-rich protein 61, neutrophil gelatinase-associated lipocalin, kidney injury molecule 1, cytokines and chemokines (Gro-alpha, IL-18)
- Urine output is an early and strong predictor of acute kidney injury and associated mortality, and its measurement is underutilized for diagnosis of AKI in clinical practice 4
- The use of urine output criteria is more often associated with increased incidence of AKI, and provides substantial diagnostic and prognostic value to the detection of AKI 4
- Other biomarkers, such as tissue inhibitor of metalloproteinases-2 (TIMP-2) and insulinlike growth factor-binding protein 7 (IGFBP7), have obtained regulatory approval in many countries and have begun to establish clinical utility 5