The World Anti-Doping Agency and Six Nations Rugby have demanded the laboratory in charge All Black lock Patrick Tuipulotu's drug tests explain the discrepancy in their results.
Tuipulotu's A sample tested positive for a banned substance after the All Blacks test against Ireland on 5 November.
He was provisionally suspended from all rugby and returned home early from the end of year tour.
Yesterday it was revealed the Blues player's B-sample results had cleared him of any wrongdoing.
Drug Free Sport New Zealand boss Graeme Steel said Tuipulotu's case was a "one in 10,000" occurrence.
WADA and Six Nations, the northern hemisphere's equivalent of the Rugby Championship, which includes Wales, England, Scotland, Ireland, Italy and France, are investigating the discrepancy in Tuipulotu's A and B sample.
In a statement, WADA said they will now investigate the matter with the Sports Medicine Research and Testing Laboratory (SMRTL) in Salt Lake City in the United States, which performed the analysis.
"A sample collected from New Zealand player Patrick Tuipulotu immediately after that match was sent to the Sports Medicine Research and Testing Laboratory (SMRTL) in Salt Lake City for analysis," the statement said.
"SMRTL reported the presence of a specified substance in Mr Tuipulotu's A sample and so, on 16 November 2016, after conducting the necessary review and notification procedures, SNRL provisionally suspended Mr Tuipulotu, as required under World Rugby Regulation 21.
"Mr Tuipulotu subsequently requested that his B sample be analysed. SMRTL reported on 7 February 2017 that the B sample analysis did not confirm the A sample analysis.
"Accordingly, and again as required by World Rugby Regulation 21, SNRL confirmed to Mr Tuipulotu that the entire test is considered negative and that his provisional suspension has been lifted with immediate effect.
"SMRTL is currently investigating the reason for the discrepancy between Mr Tuipulotu's A and B samples."
Six Nations Rugby, who conducted the anti-doping test following the All Blacks match against Ireland in Dublin, made their first comment on the matter, issuing a statement that largely echoed the WADA release, only adding some detail surrounding the timing of the testing.
"Six Nations Rugby Limited (SNRL) conducted anti-doping testing at the Autumn Internationals match between Ireland and New Zealand on 5 November 2016, in accordance with the 2016 Autumn Internationals Disciplinary Rules and Anti-Doping Programme, which incorporate (in large part) World Rugby Regulation 21," the Six Nations Rugby statement said.
Tuipulotu is now free to return to rugby and addressed the media this morning.
He said he was relieved to have been cleared of taking a banned substance.
It had been a stressful time for his partner and family as well as for him, he said, and he was looking forward to getting back on the field to train with the Blues before the Super Rugby season.