Table 5.

TDF domains, mapped against summary of barrier and example quotes

TDF domain	Example of barrier	Quote representing barrier	Quote representing intervention suggestions
Social influences	Feeling pressured into doing an X-ray first	'If my boss told me to do one it would be very difficult for me to, depending on which the boss was, generally you’d be like no but don’t you know that local guidelines are…they’d be like I said get a chest x-ray, you’d be like oh alright’ (Junior doctor, H1).	'Well you’ve got to bring the consultants on board…I think it needs a big cascade…we could have it as a screen saver (Junior doctor, H2).
			'If at one point during a couple of weeks all the screen savers had something about NG tubes, a load of posters and then there was sort of a couple of meetings or something…what you want to do its just to raise awareness and people will actually think about it a lot more and that’s what you can hope for’ (Consultant, H2).
		'They [nurses] always justified it with 'we’d rather get an x-ray, we’re told not to feed without an x-ray.’ I pushed a couple of times, when I was very confident, when it had gone down very easily it was very acidic…but quite frequently they’d still send for an x-ray or they’d get someone else to request the x-ray, you know, they were adamant they wanted the x-rays and wanted them reported’ (Junior doctor, H2).
Skills	Working with staff who lacked the correct skills or necessary training	'What I’ve identified……is that I get newly qualified staff nurses coming through who have never been taught this as a method of checking, don't know how to check it’ (Dietician, H3).	'The Trust should to do teachings about the use of ph paper vs x-ray, rather than just bombard staff with information’ (Junior doctor, H1).
			'Specific training should be targeted to relevant groups rather than lots of different types of mandatory training’ (Operation Department Practitioner, H1).
		'I think a lot of it is to do with the training, I was talking to a few junior doctors in respiratory and a lot of them haven’t even heard about the training package on the website, but they’re putting tubes down.’ (Nurse, H2).
			'I think the (e-learning) package would be good…If its interactive people are more likely to do it’ (Junior doctor, H2).
Beliefs about capabilities	Low levels of confidence for checking the pH level	'…people just aren’t checking the aspirate and we almost need to get them to just check and then even if they are unsure, fine send for an x-ray, but if you see that those then correlate and you see that more and more often, then your confidence might increase.’ (Junior Doctor, H2).	'Another way to bring it across would be to have a teaching event or something’ (Nurse, H2).
		'I think confidence would increase if staff knew they were learning the correct skills’ (Senior nurse, H1).
Environmental context and resources	The lack of resources, such as pH paper or lack of forms for documentation, often leaves doctors with no choice but to send for an x-ray in order to make the decision to feed	'We’re still having problems getting strips; was looking for some this morning and there weren’t any in the cupboard so I had to pinch some from another patient’ (Junior doctor, H1).	'Can you get it in the packs? Like the IV catheter packs? You’ve got all the stuff for your aseptic technique…maybe you need a similar NG pack so people don’t forget that here’s your 20 ml syringe that you aspirate with; here’s your litmus paper…’ (H2: junior doctor).
		'I believe that some of the problems come about where to document it…so it's getting the pH and where do you document that…’ (Nurse, H3).
			'Someone developed these catheter packs that have all the equipment you need. Could there not be an NG tubes pack with all the necessary equipment for everyone to follow in a specific order?’ (H1, junior doctor).
Emotion	Certain staff do not want to rely on the pH value and feel more comfortable if they have sent for an x-ray	'I think the nurses are still quite anxious because it’s so big even now I think they’re still anxious about pH and they just want to know that it’s in the right place’ (Junior doctor, H1).	'We could provide junior doctors with information about the use of x-rays and potential problems these cause’ (Junior doctor, H1).
		I think there is very much a fear isn’t there, once you can’t get that thing back it’s, you know… (Nurse, H2).
			'I think also the 50% of the deaths that occurred were from misinterpretation of x-rays. I think if you told F1’s that, even that on a poster, I think, you know, if you caught that out of the side of your eye as an F1…’ (Junior doctor, H2);
			'I would look at it as I went past if it was an x-ray…because a lot of questions that come from the requirement for x-rays are not seen by the people who interpret the x-rays so I think that (a poster with information regarding misinterpretation of X-rays) would be really good’ (Junior doctor, H2).