Blood Culture question for ICP's

I work in the IC office part time, mostly doing surveillance, though I'm starting to take on other responsibilities. We have an awesome ICP and I love working with her. But, I have a question about blood cultures. We draw aerobic and anaerobic cx's from two different sites, half an hour apart. So, if we have one set that comes back positive and then next set is negative, how do you interpret this? Most of the pts seem to have resp issues, and if they're growing the same bug, I go with the pos. But, we have IVDA's with no resp issues. I look at temp, WBC's, urine, etc...I guess I'm wondering what other ICP's use as a guideline as to what's a true bacteremia vs poor technique and a contaminant. Our ICP says it may be peak and trough of the ATB that's given, etc and to consider them pos if aerobic and anaerobic grow out. But I just can't wrap my head around it. What are your thoughts? Any easy answers or tricks of the trade? Thanks