First of all you have to look at the system as a whole,and what I mean by that is you have to account for what kind of cap you are using and what kind of PICC you are dealing with. If you have a positive displacement valve or cap you do not need to flush as frequentlly.
ONS has a an entire document with their recommendations for all types of catheter care. With regards to flushing PICCs that are locked off and not being used between weekly visits,b/c you will always flush after use the recommendation is this
Flush locked lumens q week with Normal Saline ( usually this is 5-10ml) followed by 5 ml Heparin (100 units per ml. Notice the 100 units per ml!!!!!! Many hospitals will use lower concentrations of Heparin like 10 units per ml,since they are flushed frequently.
In our OP clinic we flush the locked lumens q week and prn and even with open-ended PICCs we have not had any significant problems or an increased number of occlusions. Generally speaking,1 out of CVCs will have an occlusion problem,even with great care. If you have negative displacement valves,you might want to consider changing to them. Also do not underestimate the value of a GOOD PULSATILE START STOP FLUSH TECHNIQUE. This will keep the occlusion risk down,PWO down and in theory decrease infection risk.