There are a couple of reasons why I think you will be disappointed.
Firstly, muscle aches alone are not really a symptom of a B12 deficiency. So it's unlikely it is the cause of your problems. Secondly, your blood levels are already much higher than normal and almost certainly have been for 30 years. So it looks as if a lot of B12 hasn't made much difference.
Taking 20 mcg a day will probably not make much difference to your B12 levels. Not because your blood can only hold a certain amount, but because you're unlikely to absorb much more than you are now.
The absorption of B12 from the gut has a limited capacity of about 10 mcg at a time. So you are probably absorbing most of your 10 mcg tablet. But it cannot possibly absorb all of a 20 mcg dose. So the amount you absorb will not increase by much.
And the amount that you eliminate in the urine is dependent on the amount in the blood. So, if you do absorb any more B12, most of it is likely to be rapidly eliminated.
Your B12 levels are so high because you're taking 4 times the RDA (plus whatever you eat in food).
Testing MMA and hCys would tell you if you're suffering from a functional deficiency (where there's lots of B12 in the blood, but it's not getting to the right places in the cell). But I'd bet that's not the case as it's fairly rare and it doesn't match the symptoms you're describing.
MMA can be done via a blood test or a urine test. The levels can be raised by things other that a B12 deficiency - like liver or kidney problems. The urine test allows them to check kidney function at the same time. But you've already eliminated that. So the blood test should be suitable.
hCys is also a blood test. But it's not simple as the assay has to be done within a very short time of the sample being taken. So it needs to be done at a hospital with its own pathology lab that has the equipment set up. I've only had it done once, at the National Hospital for Neurology and Neurosurgery.
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