What can cause consistently (several months, possibly a year or more) high leukocytes in urine with no nitrites and no symptoms of a uti or other infection?

Consistently high leukocytes in urine with no nitrites and no symptoms of a UTI or other infection may be indicative of an underlying medical condition. Some possible causes include kidney inflammation, kidney stones, interstitial cystitis, bladder cancer, prostate inflammation, and sexually transmitted infections such as chlamydia or gonorrhea. Also AID such as lupus or multiple sclerosis

Any mild or chronic condition of the urinary system can cause this. You need to be checked by a Urologist. Also have a clean catch mid stream or catheter obtained urinalysis for accuracy