Evaluation of simple diagnostic parameters in acute kidney injury in hospitalized patients—diagnostic recommendations for non-nephrologists

Acute kidney injury (AKI) is very common in hospitalized patients, affecting patient’s mortality and morbidity. Major causes are prerenal AKI and acute tubular necrosis (ATN). Even though a variety of parameters/indices exist, their reliability and practicability are controversial: in fact, there is a need for a simple diagnostic approach for AKI in in-patients with parameters easily obtained in any hospital. The objective of the study was: (1) to assess reliability of simple laboratory parameters/indices to differentiate pre-/intrarenal AKI; (2) to evaluate the most reliable and feasible parameters/indices; and (3) to identify the possible impact of confounding factors. Retrospectively, in-patients with AKI hospitalized in 2020 in a university nephrology department were included. Spot urine and 24-h collection urine was analyzed with urine sodium (U Na ), urine specific gravity (U SG ), fractional excretion of sodium (FE Na ), fractional excretion of urea (FE Urea ), urine osmolality (U Osm ), urine to plasma creatinine ratio (U Cr /P Cr ) and renal failure index (RFI). Overall, 431 patients were included. U Na , U Osm , U SG and RFI showed high specificity > 85% for prerenal AKI, U Na and RFI provided good specificity for ATN. Loop diuretics, ACE inhibitors/AT1 blockers or pre-existing chronic kidney disease had no impact. In patients with AKI, U Na , U SG and RFI: (1) proved to be very specific for prerenal AKI and showed high sensitivity for ATN; (2) can be easily determined using serum and spot urine; and (3) are not confounded by medication or comorbidities. These parameters/indices are helpful to identify the aetiology of AKI and to guide therapy, thereby improving patients’ safety and outcome.