Dehydration: This is the most common cause of elevated Blood Urea Nitrogen (BUN) without a corresponding increase in creatinine. Dehydration leads to a decrease in blood volume, which in turn reduces renal blood flow and glomerular filtration rate, causing the kidneys to reabsorb more urea, thus elevating BUN levels.

High protein diet: Consuming a diet high in protein can increase urea production, leading to elevated BUN levels without affecting creatinine.
Gastrointestinal bleeding: Bleeding in the gastrointestinal tract can lead to an increase in protein absorption, which the body breaks down into urea, elevating BUN levels.
Heart failure: Reduced cardiac output can lead to decreased renal perfusion, resulting in elevated BUN levels due to increased urea reabsorption.

Prerenal azotemia due to renal artery stenosis: Although less common, this condition can lead to reduced blood flow to the kidneys, causing elevated BUN without a significant increase in creatinine. Missing this diagnosis could lead to progressive renal damage.
Early post-renal obstruction: Obstruction of the urinary tract can cause a build-up of urea, leading to elevated BUN levels. Early detection is crucial to prevent long-term kidney damage.

Rhabdomyolysis with minimal creatinine elevation: Although creatinine is typically elevated in rhabdomyolysis, there can be cases where the elevation is minimal or delayed, making BUN an important marker.
Certain medications (e.g., steroids, tetracyclines): Some medications can increase BUN levels without affecting creatinine, though this is less common and usually seen with specific drug classes or in certain contexts.