Possible causes can include urinary tract infection (UTI), irritation of the meatus or perineum, trauma, nephrolithiasis (including hypercalciuria without stones), sickle cell disease/trait, post-infectious glomerulonephritis, or IgA nephropathy, amongst other diseases.
There has clearly been a pretty thorough workup done already that would have evaluated most of these causes. If negative, it is less likely that a urological problem is present.
You may want to move up to the kidneys, since many of these disorders can also cause blood in the urine. A referral to a kidney specialist (nephrologist) can be considered for another opinion.
Followup with your personal physician is essential.
This answer is not intended as and does not substitute for medical advice - the information presented is for patient education only. Please see your personal physician for further evaluation of your individual case.
Kevin, M.D.
Medical Weblog:
kevinmd_b