Your question is unclear. An A1c [HBA1C] test can be taken anytime. You do not need prior fasting or postprandial results to do so. HTH
HBA1C measures average blood sugar over the past 3 months. It can be a useful screening test and for monitoring diabetes progress / progression. However, it can also be misleading as highs and lows can average out to give something that looks normal, but actually isn't.
Fasting and 2 h ours post eating are also useful and can guide diet and treatment on a day to day basis. If you are diabetic both fasting, post prandial and HBA1C are useful to be monitored.
Please ask more specific question and the reason for your question and we may be able to answer you better.