It's an interesting one this, I had always believed that the 5008 fluid balance system was tested thoroughly enough in T1 not to require an accuracy check. But it is possible for the UF pump to under deliver it's volume due to detachment of the membrane, and still operate well enough to pass the negative pressure holding test. P05 and P06 pumps must have the same weakness but will have conductivity monitoring to pick up the problem.
Are there any other machines that don't recommend a UF accuracy check at service?
From my own experience of UF pump failure I didn't read into it as deeply as you appear to have.
What I have seen in the past is for the machine to simply alarm UF pump failure - dialysis continues but the UF option is highlighted in a red box and there's nothing that can be done to bring it back on line.
If the pump fails to deliver completely and is picked up by S11 and S36 contacts then yes UF will be disabled during treatment. However the system that detects operation of the UF pump does not verify the calibrated volume of the UF pump is delivered on each stroke, so if that volume changes by detachment of the UF pump membrane under negative pressure the membrane gets distorted and the volume delivered is lower. As the stroke volume is only around 1.2ml a small change in volume delivered makes a big difference to the total UF volume accuracy on the negative side.
It took over a week of patients coming off heavy for staff to tell us about this machine, and they only casually mentioned it when they reported the machine for a separate problem When I spoke to staff about it they said they would only report a relatively large UF discrepancy, far more than the machine + weighing scale tolerance so I don't think you can rely on this fault being picked up with the regular pre/post weighing.
It's probably low incidence of a low risk fault, but I think it's important to highlight that it can happen as I didn't believe it could. I assumed the integrity of the pump was fully checked in T1 and it isn't.
I think the only way to pick it up is by a simulated treatment UF check
As you mention Chris, any test is only as good as the moment it is taken. Hopefully Fresenius may tweak their T1 to include some kind of test although other than a pressure test I'm not sure how they'll pull it off. I would expect the nursing staff to pick it up but they'll probably assume the patient didn't weigh themselves correctly - perhaps wearing different amounts of clothing etc... domestic staff giving too much food and drink or the patient sneaking extras in or something.
I remember on the 4008 we would pulse X strokes and measure the volume - perhaps that will come back in.