Contamination: The most common cause of an isolated elevated leukocyte esterase on a urinalysis (UA) is contamination, often from vaginal or skin flora during the collection process. This is particularly likely if the patient is asymptomatic and other UA parameters are normal.

Asymptomatic Bacteriuria: This condition, where bacteria are present in the urine without causing symptoms, can lead to an elevated leukocyte esterase. It's more common in certain populations, such as pregnant women or the elderly.
Dehydration: Concentrated urine can sometimes cause false positives for leukocyte esterase due to the concentration of substances that may interfere with the test.
Recent Urinary Tract Infection (UTI) Treatment: Patients who have recently completed treatment for a UTI might still have an elevated leukocyte esterase due to the body's ongoing response to the infection.

Early or Mild Urinary Tract Infection: Although other abnormalities might not be present, an elevated leukocyte esterase can be an early sign of a UTI. Missing this diagnosis could lead to the progression of the infection.
Interstitial Nephritis: This condition, often caused by an allergic reaction to medications, can present with minimal symptoms and an isolated elevation in leukocyte esterase.
Tubulointerstitial Disease: Certain diseases affecting the tubules and interstitium of the kidneys can cause an elevated leukocyte esterase without other significant UA findings.

Parasitic Infections: Certain parasitic infections, such as those caused by Schistosoma species, can lead to an elevated leukocyte esterase due to the body's immune response.
Malignancy: Although extremely rare, some urinary tract malignancies can cause an isolated elevation in leukocyte esterase, especially if they provoke an inflammatory response.
Foreign Body in the Urinary Tract: The presence of a foreign body, such as a retained urinary catheter fragment, can cause chronic inflammation and an elevated leukocyte esterase.