Clots

I recently graduated and I am currently working in a fairly busy ER. I imagine like most of the country, we draw all of our labs out of saline locks if we have one. However, it seems like some people don't flush, and when I come back later to start fluids or push meds, there is tons of blood sitting in the lock. Or sometimes, I can tell that it was flushed but there is still some blood that has been sitting in the lock. I have this fear that a clot will have formed in the lock or the actual catheter and when flushing I will throw a clot and give them a stroke or PE or something. Is this even possible? Am I just worrying to much? THanks.