this is called "occult blood" or hidden blood. just a few blood cells in the urine can cause the chemical dipstick test to be positive so usually the presence of blood is confirmed microscopically on a urine specimen that is centrifuged. if no blood is found than this could just be a false positive test. if blood is found in small amounts a cause will need to be determined. traces of blood can come simply from irritation of the urethra so this does not necessarily mean something is wrong. infections or other types of inflammation kidneys stones etc. all can cause hematuria (medical term for blood in the urine).
i was told i had blood in my urine and to see a doctor. why? i didn't see any blood.