Testing on the blood sample from Olympic horse Waterford Crystal will get under way in a New York laboratory on Monday.
However, The Irish Times has learnt that if the results are negative, the case against Irish Olympic gold medallist Cian O'Connor will almost certainly be dropped, in spite of the positive result from the A urine sample.
Following the theft of the B urine sample en route to a laboratory in Newmarket, in England, the International Equestrian Federation (FEI) has arranged for the remaining blood sample to undergo confirmatory analysis at the USA Equestrian Drug Testing and Research Laboratory in New York.
However, it is believed that the prohibited substances found in Waterford Crystal's urine were not traceable in the blood when the A samples were tested in Paris.
If this is confirmed when the B blood sample is tested, the case will be dropped - official sources have indicated - and Cian O'Connor will retain Ireland's only gold medal from the Athens Olympics.
In a separate development, it has emerged that the driver assigned to deliver Waterford Crystal's B urine sample to the Horseracing Forensic Laboratory (HFL) in Cambridgeshire was told twice to change the method of delivery.
"The driver had a call from our customer services department telling him to change the delivery from the specified address to a representative of HFL," an executive of the freight delivery company DHL told The Irish Times.
"He refused to carry out that change because he said he needed more authority," said Mr Matthew Zamoyski, DHL's communications director in Britain.
"Half an hour later, he was recontacted by customer services saying that those instructions were to be followed and that authority had been given to deliver the package to a person in the driveway. So he delivered it as instructed to someone who claimed to be a representative of HFL, who showed him what appeared to be HFL identification."
Sources at the laboratories in Cambridgeshire and Paris said that the sample was sent alone.
The sources at the two laboratories said samples are anonymous when they arrive and when they leave, and that internal identification procedures ensured that no one at either lab would know the identity of any sample.
Waterford Crystal's sample would have been issued with a barcode sticker that would be registered by the FEI as belonging specifically to Waterford Crystal, a HFL source said.
"At no time can any laboratory that deals with a sample identify where the sample comes from," she said.