First, a word on nomenclature:

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You’re not doing a ‘IV to oral stepdown’, because IV isn’t superior to oral (except by way of dosing). You are switching therapy from IV to oral, hence: IV to oral switch.

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What are the pros and cons of IV and oral antibiotics?

E.g. up to 2h e.g. for Co-trimoxazole, compared to IV. | ~5 mins | | Adherence | Depends on patient | Depends on nurse | | Dosing | Limited by PO absorption / GI tolerability | Higher doses can be given IV (e.g. amoxicillin; up to 20g/d). | | Rx options | Limited by what’s available orally | Many | | Dosing | 1 – 4x daily | 1 – 6x daily | | IV line complications | N/A | Present | | ADRs | Fewer | More | | Expense | £ | £££ | | Carbon / plastic cost | £ | £££ |

Let’s talk about a couple of these:

• Pharmacokinetics

  • Absorption: See our page on Bioavailability, but for several drugs this is near equivalent to IV. It’s important to know, however, when it isn’t:

    

    Also: Some antibiotics have absorption limitations (e.g. amoxicillin & cefalexin have maximal absorptions per oral dose (750mg for Amoxicillin, ~1g for Cefalexin).

  • Distribution: Dose equivalent IV and PO doses will result in different Cmax, but for most antibiotics that shouldn’t matter: For beta-lactams it’s T>MIC that matters, Aminoglycosides/Daptomycin it’s Cmax, and for most other antibiotics it’s AUC/MIC, which is a function of Cmax and half-life (I’m simplifying a bit). The differential time to Cmax administering IV compared to PO is unlikely to result in reduced attainment of PD target.

• Cost

  • Financial: IV generally more expensive than PO (Costs from BNF):

    Drug \ Daily cost	PO	IV
    Amoxicillin 500mg TID	£0.21	£2.88
    Co-trimoxazole 960mg BD	£0.46	£18.86
    Ciprofloxacin 500mg BD
    (or 400mg BD for IV)	£0.19	£43.52

  • Plastic/carbon: See the section on metronidazole oral switching from our episode on the BSAC spring conference for an example; many others are published.

  • Nursing time: This study compared prep and admin time for antibiotics:

    • Oral: 2.42 min
    • IV Bolus: 5.53 min
    • IV infusion: 10.27 min

So oral switching is a good idea: when to do it?

When to oral switch

• The UK HSA has guidance available here; image below also:

  

• My issues with this is that it’s way too conservative:

  • Infections requiring ‘Special Consideration’:

    • BSI, BJI, Endocarditis - all of whom have excellent oral ABx data
    • Empyema, Undrained abscess: These need source control, not IV antibiotics per se.

    (admittedly they just say for those infections to have a clearly documented plan or specialist advice, i.e. they want an ID doc involved in these cases - not unreasonable)

  • 1. Enteral route: I’d say that vomiting 23.9h prior to oral switch isn’t actually a contraindication, but rather that you have to make sure you’re on top of the vomiting. Similarly, the risk of the patient not adhering to treatment has to be balanced against the risk of them absconding.
  • 2. Signs improving: Again, this is seems to be based on the erroneous assumption that IV is superior to oral. Past the first dose, why wouldn’t you consider oral switching as long as the oral regimen offered equivalent cover?
  • 3. Infection markers: This is the worst of all:
    • T 36-38 for past 24h: too restrictive. Is using fever as a surrogate for non-control of the infection. But there are certain conditions (e.g. pyelonephritis) where the patient will clinically improve but remain febrile for several days; are you going to keep all those people on IVs?
    • (N)EWS(2) decreasing: fair enough
    • WCC normalising: Nice to see but by no means essential
    • CRP decreasing: I’ll let this article speak to that.

• This checklist was written by consensus, and incorporated the prejudices of that consensus, i.e. that IV is superior to oral, and that you need to make sure the patient is improving before you move them to an inferior treatment (oral therapy). That has never been true

• In defence of this checklist, it does advocate oral switching to be considered “from first dose of IV antimicrobial with formal review completed within 48 hours and daily thereafter, unless clearly documented exemptions.”

For those of you wanting to update IVOS criteria in your own department, my draft IVOS criteria is here:

IVOS.docx