Since we have done work with creatine and creatinine in urine, see appliction note we been asked about measuring levels of these compounds in EDTA plasma samples. It  should be possible unless there is some interference from the EDTA; we have not measured EDTA retention under these conditions (where we determined creatine and creatinine).

Obviously plasma is a different matrix from what we used but having mobile phase A to contain 50:50 MeOH/water will most likely be extremely helpful. We cannot provide a guarantee, just a good chance that it will work and if you run into problems we can help.