The French international tested positive for a fat-burning substance -- higenamine --
in a sample taken after a Europa League match with Liverpool against Manchester United on March 17, 2016.
And in a detailed and highly critical report published by UEFA Wednesday revealed the case would not have come to light had the Sakho's sample been sent to another laboratory.
UEFA revealed that the WADA-accredited laboratory in Lausanne did not test for the substance at all while Cologne, where the former Liverpool defender's sample was sent, did.
Sakho was provisionally suspended for 30 days on April 28, but UEFA ruled in July that it had "decided to dismiss the case against the player" in light of the evidence.
The suspension caused the 27-year-old, who this season has been on loan at Crystal Palace having fallen out of favor with Liverpool manager Jurgen Klopp, to miss the Europa League final as well as Euro 2016, having been omitted from France's squad during his temporary suspension.
The France international accepted that higenamine was in his system but insisted it had not been an anti-doping violation as the substance was not on WADA's prohibited list, stating it was not a beta-2 agonist under Category S3, all of which are banned by the doping organization.
UEFA ruled that higenamine was "not expressly mentioned by name on WADA's prohibited list", calling on Cologne Laboratory's Dr. Hans Geyer and Lausanne lab boss Dr. Martial Saugy to give evidence.
In its findings, UEFA said Dr. Saugy had expressed "his own doubts about the categorization of higenamine as a beta-2 agonist, and also questioned the steps that WADA had taken to reach its conclusion."
Meanwhile, the report added Dr. Geyer had "explained that after the sample tested positive for higenamine, he needed to check with WADA if higenamine was actually a prohibited substance before making his determination."
In conclusion, UEFA stated: "Fundamentally, it is unreasonable to expect an athlete to have a greater understanding of a substance than a WADA accredited laboratory and its scientists."
However, WADA has defended its position over the Sakho case.
In a statement to CNN, a spokesman for the agency said: "With regards to the case of Mr Mamadou Sakho, WADA, with the support of its list expert group, thoroughly reviewed the full case file along with recently published articles on higenamine.
"WADA supported the list expert group's unanimous view that higenamine is a beta-2 agonist and does indeed fall within the S3 class of the prohibited list.
"It was decided, however, after careful review of the specific circumstances of the case that WADA not lodge an appeal."
However, WADA also said that in early August 2016,it had "requested its network of accredited laboratories to implement systematic testing for higenamine."