blood in urine

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my mum 80 years of age had a urine test just for check up. everything is ok only it says blood in the urine, RBC 5-6, WBC 3-4 but no bacteria and all other things negative. the culture also showes no growth. the appearance is clear. What can this be. is there any cause for concern? She had symptoms of cold a week before this testing. can this be a result of inflamation in the bladder due to cold.