The laboratory that carried out doping tests on Marion Jones is expected to report in the next two weeks on why the 'A' and 'B' samples did not match.
Jones' initial sample was given at the US Championships in June
"The laboratory concerned will conduct a discovery analysis to know why this occurred," said World Anti-Doping Agency director general David Howman.
Ex-Olympic champion Jones's 'A' sample tested positive for blood-boosting drug erythropoietin (EPO) in June.
But Jones was cleared in September after her 'B' sample proved negative.
Howman added: "In any case where the 'B' sample does not match the 'A' sample, we ask the laboratory for a full report.
"The lab and the technician involved give a total analysis of why."