Compared with biopsy of other sites, native renal biopsy has the greatest risk of post-procedure hemorrhage (1.2%). However, the incidence of major bleeding and death due to bleeding is very low (0.3 and 0.02% respectively).
Post-biopsy bleeding can occur into the perinephric space or the collecting system or in the form of a subcapsular hematoma.
On a sonogram, fresh hematomas appear anechoic and organizing ones are of mixed echogenicity due to clotting. There will be little to no Doppler flow because the blood is not flowing.
Note that urinoma can appear similar to fresh hematoma (urine is anechoic as well) and should be included in the differentials depending on the context (e.g. recent kidney transplant).
Patent track sign: a color Doppler signal along the course of a biopsy, indicating that the needle track has not collapsed and that there is still active bleeding from the procedure. The track usually extends out of the biopsied organ into a surrounding hematoma. Case courtesy of Dr Matt A. Morgan, Radiopaedia.org, rID: 50201