When someone enters a room and says "Hi" I reply the same way. No problem.

I never know how to reply when someone enters and says Hi, how are things? I haven't seen you in a while?" My doctor said that at one of my appointments. Last time I saw him was 6 months previous. I managed to say Hi, I'm okay ( I always say I'm ok, fine, good even if I feel like stressed, nervous).

What do I say in response to " I haven't seen you in a while?"