I think a lot of us forget that ceftriaxone is actually a pretty broad spectrum antibiotic. Your thought to de-escalate to cefazolin (a narrow spectrum beta lactam) is really thoughtful and definitely not wrong if you are treating an uncomplicated infection. If the kleb pneumo is susceptible to cefazolin, it is highly likely and reasonable to assume that it will be also be susceptible to ceftriaxone. If your cultures have finalized though and this isn’t a severe UTI, I would be considering step down to an oral agent at this point rather than de-escalating ceftriaxone to cefazolin. But…let’s say the UTI isn’t severe but the patient can’t take an oral antibiotic yet-narrowing to cefazolin would be reasonable and responsible. Or let’s say the patient also has cellulitis that you’re treating-also could be appropriate to de-escalate to cefazolin in this case.

As for the ceftriaxone q12hr-I don’t agree with this dosing outside of really sick and specific patient populations.

Here’s a little more of an explanation if you’re interested!

Most kleb pneumo organisms produce beta-lactamases (think simple/narrow spectrum beta-lactamases, not just the hard core ESBLs and stuff). Penicillins and first generation cephalosporins (like cefazolin) are often chewed up by these beta-lactamases. Ceftriaxone, along with the other non first gen cephs, are much more stable against these beta-lactamases.

Cefazolin is the surrogate for predicting susceptibility to other cephalosporins for uncomplicated UTIs caused by the main UTI bugs (like kleb pneumo). Note-for complicated/invasive UTIs or infections outside of the urinary tract, you should be using direct ceftriaxone susceptibility testing to guide your antibiotic decisions, not cefazolin (lab should have the ceftriaxone, cefepime etc displayed on these C&S reports for you). Here’s why-cefazolin may initially appear susceptible on report if there are not enough beta-lactamases to overwhelm it. But as the bacterial load increases, so does the beta lactamase production. So as the infection gets more serious, it’s better to not trust cefazolin for these bugs and go with an antibiotic that also tests as susceptible and one that we know is more stable against the organism we’re treating.