How accurate are HbA1C or Dexcom readings?

I have been using a Dexcom G6 for approximately five months with a t:slim pump. Before I switched to t:slim/Dexcom combination, my HbA1C was 59 (7.5?). According to Dexcom Clarity, my 90-day blood glucose is now estimated to be 6.5%. However, the blood test still shows HbA1C at 59, which I found somewhat disappointing.
Obviously I will need to tighten my control, but I am just wondering if these figures make sense to others?

EllieM said:

Hmm, maybe I'm asking a stupid question, but is there any chance there has been a mistake and that 59 is your old reading?

As regards the dexcom estimate, I find mine fairly accurate. Have you tried testing your dexcom against a glucometer to see of it is consistently under reading?

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I calibrate the Dexcom reading every morning, and other times when the pump or Dexcom app asks for it.

Those blood test for HbA1c were from 26th May and 8th November, both taken at the GP surgery.

EllieM said:

Hmm, could there be user error involved here? Someone has copied over the previous result instead of the current one?

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I'll ask the GP surgery receptionist to check.