What does small traces of blood in urine without an infection mean?

Blood in urine: If only a trace amount of blood (without any protein in urine), the most likely possibilities include kidney stones or crystals in the urine, recent heavy exercise, thin basement membrane nephropathy, or IgA nephropathy. The latter 2 are detected with a kidney biopsy, but in the absence of protein in the urine most nephrologists would not pursue one. Also need to consider a kidney or bladder mass.