The most common test is Creatine Phosphokinase levels. Levels for those with normal dystrophin are generally in the 100 units/L range. Those affected by DMD can see readings in the thousands or even tens of thousands per litre.
I can share that CPK levels are variable. They can vary from facility to facility. I believe they are measured by color but I could be wrong on that. I do know that they can change easily - that is, if your son has had alot of activity, then it will go up. I thought that it would go down once our son was on deflazacort but it didn't go down that much. The deflazacort allows boys to be more active so that is what causes the CPK not to go down dramatically. I think our son's went down 3k. I also think that they go down as boy ages - less muscle leads to this. CPK is the measurement of muscle enzyme that leads out of the muscle cell. The CPK test is also a measurement for heart attach patients.
Another biomarker is vitamin D level. DMD boys are lower in this and if you combine with steroid use, then it is very low. The idea is to keep vitamin D levels up to prevent bone loss - osteopedia - loss loss in kids. By watching this, less bone breakage and hopefully more or longer ambulation. But, I know that is not always the case.
Carbon dioxcide levels are also important. Too much means tired, hard to concentrate. I think this is measured during a sleep study.
Cardio markers are measured with echo and cardic MRI's. These are used to detect any changes in the cardic muscle.
These are the ones that come to mind.
No doubt CPK is a simple marker. However this is what I was able to dig out
1. Ongoing project which is working on identifying all the markers that need to be monitored with DMD and during the trials
2, There is a huge differnece in the genes of DMD kids who are using Prednizone as compared to Deflazacort or using sterods as compared to ones not using steroids (see attached file). "59 probes representing 50 annotated genes were identified that optimally distinguished DMD-STEROID from DMD"
I got a comment from my doctor that possible apart form the clinical measurements of monitoring the physical abilities, identifing these markers is key to manage the progression.