“As part of testing, monitoring hemoglobin A1C can better reflect long-term control in people with diabetes or prediabetes.”

Source: https://www.huffpost.com/entry/slow-down-biological-age_l_654aa443e4b0e63c9dc15057

So, I will start with answer to your first part of the question regarding the fetal hemoglobin versus the number of red blood cells and all these.

Source: https://seekingalpha.com/article/4600640-editas-medicine-inc-edit-q1-2023-earnings-call-transcript?source=feed_all_articles

The system also helped reduce levels of glycated hemoglobin, or HbA1c, which provides a snapshot of blood sugar levels over time.

Source: https://www.gjsentinel.com/lifestyle/health_and_wellness/artificial-pancreas-device-may-help-folks-with-type-2-diabetes/article_948bcd0a-6c11-5693-ac52-d50de348fbc9.html