If RBCs become swollen in dilute urine to the point that the cell membrane ruptures, the cell loses its hemoglobin so that only the membrane and free hemoglobin remain. These empty membranes are known as "ghost" cells.
Phase-contrast microscopy enhances the appearance of the red cell membrane on ghost cells. If the membrane can be visualized, the cell would be counted as an RBC. A positive blood result on the reagent dipstick would also help to confirm them as RBCs.