Something to bear in mind is that PDK4 is not even a "DCM gene." It's just a completely unrelated mutation which has been noticed to occur in a statistically-significant number of American-bloodline Dobermans which develop DCM. There's not even an identified correlation between the PDK4 mutation and a statistically-significant elevated incidence of DCM in the European Doberman cohort.
All this test means is that there was likely one popular bloodline responsible for both the mutation and ONE hereditary factor (of many) that cause DCM, and it's fairly widespread.
There is no absolute, "If your dog tests positive, it will develop this type of DCM" conclusion which may be reached, nor should anyone conclude that there's any such result as "clear of DCM".
The only context in which this test should be used is to help broaden the sample set and understanding of one potential cause. At worst, it creates unnecessary anxiety with a positive result, and bad breeders tend to use negative results to instill an utterly false sense of confidence in gullible buyers who don't understand the science.
I can't say I'm sold on the predictive value of the Holter unless there's already cause for concern. The value of Holter Monitoring in humans is pretty well-researched, and there's been a pretty common theme outlined:
--Imaging techniques (echocardiograms) are best, hands-down, for identifying the physical changes to the heart that are cause for concern.
--Holter Monitoring (ambulatory ECG) is great, in conjunction with imaging diagnostics, to understand more about what's going on and how a heart condition is progressing, WHEN such a condition has been identified.
--HM is not particularly useful as a positive predictive diagnostic tool that sudden cardiac death is imminent, but it may be useful as a NEGATIVE predictive diagnostic tool to identify patients at low risk of sudden cardiac death (again, in conjunction with other diagnostics).
Also, SCD is not a huge concern of mine. It kills some Dobes, sure, but that's a relatively small problem compared to congestive heart failure that comes on slowly as a result of DCM.
The bottom line is that the purpose, scope, and implications of tests need to be understood and leveraged appropriately. They should not be done in ignorance, just to feel good about having done them or to satisfy a checklist.
My testing plan is to continue as I have been-- regular screenings where the vet just listens for abnormalities. Periodic echocardiograms to have a specialist see if there's something to be concerned about. As far as Holter, I'd certainly do that if there was a cause for concern from an Echo, to better understand the problem. If no concerns are presenting, I wouldn't bother, because the best result is simply inconclusive as far as DCM. The worst, which is the typical result, is inconclusive on both DCM and SCD risks.