What ratio of BUN to creatinine typically indicates acute kidney injury (AKI)?

A ratio of 20:1 between blood urea nitrogen (BUN) and creatinine is typically indicative of acute kidney injury (AKI). This ratio helps healthcare providers differentiate between prerenal causes of AKI, such as dehydration or decreased blood flow to the kidneys, and intrinsic renal causes, where the kidney's own structures are damaged.

In prerenal azotemia, the reabsorption of urea is increased due to factors that lead to reduced perfusion of the kidneys, often resulting in an elevated BUN relative to creatinine. A ratio of around 20:1 suggests that urea is being reabsorbed while creatinine is being excreted normally, consistent with prerenal conditions.

The other ratios (10:1, 15:1, and 25:1) are less commonly associated with AKI. A lower ratio, such as 10:1, typically reflects a state of more severe intrinsic renal damage where both BUN and creatinine levels rise but the increase in creatinine may be proportionately greater. A ratio of 25:1 may indicate more significant prerenal azotemia, but it could also be suggestive of other conditions, such as gastrointestinal bleeding.