horse blood test for pregnancy

Question
Hi sorry not sure if you would know this or not,but i have a 18hh shire mare which i believe is in foal,i had the vet do a blood test at 220 days after breeding as she is moody,more tired then normal and her stomach has a dropped look,she would be due middle of june,the blood test came back negative,do you know if that would be accurate as i have read alot about the test since having it done and it says its less accurate after 100 days which my vet never told me about,they palped her 2 years ago as believed she was pregnant then and they said she was not in foal then 3 months later had a filly from her so dont want to palp her again as they missed it last time.So not not sure if the blood test is right or not now.
thanks

Answer
Hi Kaylie.
  Here, most mares are ultrasounded at 14-16 days after being bred, then again at 45 days. I can't understand why anyone would wait 220 days! I think an ultrasound would be your best bet.
  Good luck--hope you get the answer/result you want!
Anne Stepien