Hi Sophie

Thank you for your questions. My colleagues might have other thoughts to add, but here are some. Before addressing your questions, thought it'd be helpful to draft a quick summary on Dakin's solution:

For management of infected and/or non-healing wounds, Dakin’s solution, modified at a concentration of ten-fold dilution or less (NaOCl 0.025% or less) has been shown to maintain its bactericidal and fungicidal properties without significantly adversely affecting the normal healing process. [1]

The original Dakin's solution was a 4% hypochlorite solution, which caused visible tissue irritation. In current practice though, a full strength Dakin’s solution has 0.5% of sodium hypochlorite whereas half and quarter strength solutions are more diluted preparations. These diluted preparations maintain similar antiseptic effects as the original preparation, with less associated tissue irritation. In vitro studies performed per Heggers et al and Lineaweaver et al showed that the 10-fold dilution of sodium hypochlorite (that is, NaOCl 0.025%) retained its bactericidal effects without significantly impeding wound healing or injuring fibroblasts in tissue culture.[1] These authors concluded that in the wound environment there should be a mechanism that either protects and/or neutralizes the toxicity of NaOCl, promoting the normal healing process. Other in vitro studies however, concluded that in order to be safe to fibroblasts, keratinocytes and osteoclasts, the Dakin's solution concentration would need to be at least 0.00025%.[2]

Nevertheless, although less cytotoxic commercial antimicrobial cleansers exist (e.g. hypochlorous acid-based such as NeutroPhase, and sodium hypochlorite + hypochlorous acid-based such as Puracyn) [3,4], Dakin's solution (at 0.025% or less) continues to be widely used by several surgical services worldwide for treatment of infected and non-healing wounds.[1,5]

Now, to your questions:
1) Would it be appropriate to use Dakins in a surgical incision on post op day 4?
For surgical incisions, guidelines for prevention and treatment of surgical site infection recommend NICE [6]:
- Use sterile saline for wound cleansing up to 48 hours after surgery.
- Advising patients that they may shower safely 48 hours after surgery.
- Use of tap water for wound cleansing after 48 hours if the surgical wound has separated or has been surgically opened to drain pus.
As described above, Dakin's solution has been primarily indicated for the management of infected and/or non-healing wounds.

2) Would Dakins break down fascial sutures?
Surgical wound dehiscence is a complication that when present, is often noticed between 4 to 14 days post-operatively.[7,8] Risk factors include continuing wound exudate past 48 hours post-op, an surgical site infection, poor glycemic control, malnutrition, obesity, mechanical stress on the wound bed from heavy lifting, coughing, vomiting, sneezing and straining.[7,8]

Given that Dakin's solution (even at 0.5%) when applied on a wound does not penetrate deeply into tissue and that cells/tissue below the surface in contact with the solution remain fully viable [2], and assuming that the solution is not actively injected into the tissue (e.g. mechanical pressure), it seems more likely that a breakdown of fascial sutures would be primarily caused by the elements mentioned above.

3) and if there were to be any degree of fascial dehiscence would the solution cause peritonitis?
Use of Dakin's solution (0.25%) as an antimicrobial solution for irrigation in appendicitis to lower surgical site infection rates has been reported, so peritonitis as a direct consequence of contact with Dakin's solution at clinically used concentrations seems unlikely. [9]

Hope this helps!

[1] UENO https://pubmed.ncbi.nlm.nih.gov/30173720/
[2] Barsoumian https://journals.lww.com/jorthotrauma/Abstract/2013/08000/In_Vitro_Toxicity_and_Activity_of_Dakin_s.2.aspx
[3] RANI https://pubmed.ncbi.nlm.nih.gov/24440863/
[4] armstrong https://pubmed.ncbi.nlm.nih.gov/28692424/
[5] OTTESEN https://pubmed.ncbi.nlm.nih.gov/32490051/
[6] https://www.nice.org.uk/guidance/ng125/chapter/Recommendations#postoperative-phase
[7] World Union of Wound Healing Societies https://www.woundsinternational.com/resources/details/consensus-document-surgical-wound-dehiscence-improving-prevention-and-outcomes
[8] https://woundreference.com/app/topic?id=surgical-wound-complications&find=surgical+incision+cleanser#risks-for-wound-dehiscence
[9] https://pubmed.ncbi.nlm.nih.gov/19969145/