Contamination: This is often the most common reason for an isolated elevated leukocyte esterase in an otherwise normal UA, especially in men. The presence of leukocyte esterase can be detected due to contamination from the skin during the urine collection process.

Prostatitis: Although the UA is otherwise normal, chronic prostatitis can sometimes present with minimal or no other urinary findings except for an elevated leukocyte esterase due to the inflammation of the prostate gland.
Urethritis: Similar to prostatitis, urethritis (inflammation of the urethra) can cause an elevation in leukocyte esterase without significant other findings on the UA, especially if the inflammation is mild or chronic.
Recent Urinary Tract Instrumentation: Procedures like catheterization can introduce bacteria or cause irritation, leading to an elevated leukocyte esterase.

Kidney Stone with Obstruction: Although the UA might be otherwise normal, a kidney stone causing obstruction can lead to infection or inflammation that might only be hinted at by an elevated leukocyte esterase. Missing this diagnosis could lead to severe consequences, including sepsis or kidney damage.
Early Pyelonephritis or Upper Urinary Tract Infection: In the early stages, these infections might not have produced enough bacteria or leukocytes to be detected by other UA parameters, making an elevated leukocyte esterase a critical early sign.

Interstitial Nephritis: This condition involves inflammation of the spaces between the renal tubules and can be caused by drugs, infections, or other diseases. It might present with minimal findings on UA except for an elevated leukocyte esterase in some cases.
Renal Tubular Acidosis (RTA): Certain types of RTA can lead to chronic inflammation or infection, potentially causing an isolated elevation in leukocyte esterase, although this would be quite rare and usually accompanied by other clinical or laboratory findings.