A laboratory in New York has confirmed that it has received the B blood sample from Cian O'Connor's horse, Waterford Crystal.
The laboratory will test the sample to determine whether it contained illegal drugs.
However, the US Equestrian Drug Testing & Research Laboratory said the result of the tests would not be known until later.
Mr O'Connor is likely to retain his Olympic gold medal for showjumping even if the test results are positive because the urine part of the B sample is missing.
It was revealed last Monday that a urine sample from Waterford Crystal had been stolen while en route to a laboratory in England last month.
Initially the sample was to be tested in England. However, the Equestrian Federation of Ireland was informed last week that the sample was being re-routed to New York.
