If you've had an abnormal urine speciman, why would they want a renal ultrasound?

To look for cause. Renal us is an easy noninvasive way to image the kidneys and bladder to look for obstruction, stones, or tumors. All can cause abnormal renal function studies.
Kidney stones. Also, tumors or cysts or shrunken kidney may cause abnormal findings and become readily diagnosed by ultrasound.