Hcg levels


I’m a bit confused. This app says I’m 5 weeks. But I got my blood work back and it says my hcg levels are at 851. That would put me at 4 weeks I do believe. Is that a good number for 4 weeks? My last period was March 29th.

Edit: Doctor emailed me back. Said going by my last period, I’d be 5 weeks and 1 day. She said my hcg levels are a bit low for being 5 weeks. :/. A bit nervous now.