When a fistula
clots, which they unfortunately do, it can not be used for dialysis
since there is no flow.
We are pretty good at clearing up the clot ( "declotting" the fistula) so it can be used again. Sometimes the fistulas are still working but lined with clot. As long as the access needles can be inserted and used this should be no problem. Typically clot forms when there is a blockage.