What could contaminate urine and what would be the next step after learning this information?

Patti asks:My mom is 71 years old and had a partial hysterectomy many years ago. She has had bleeding that includes small blood clots and her urine is dark. She had a lab test, but the lab said her urine specimen was contaminated and referred her to her physician. She is concerned and wondering what could contaminate urine and what should her next step be?Lily Arya, MD, responds:I would first