Recently began having painful burning urination. Urine always appeared clear, no discharge, no smell. 54 year old female. Went to Doctor, urine test showed negative for blood, and no infection. Sent for culture, came back with a slight elevation. Antibiotics (Macribid). Completed antibiotics, and for three days all fine. Day four, burning when urinating again. Went back to doctor urine test showed microscopic blood in urine +2. Doctor wants to send out for a culture again. Then decide what to do. Is this serious?