Montreal - The World Anti-Doping Agency (WADA) says samples handled by three laboratories that were recently suspended will be re-tested.
WADA has in the past month temporarily shut down underperforming labs in China, Portugal and South Africa, disrupting testing ahead of the Rio de Janeiro Olympics.
WADA president Craig Reedie says clean athletes "need not be concerned" that testing has been compromised.
Reedie says the labs must "reassess results of past analysis and reanalyse past athlete samples from a determined period of time as needed."
He says the 31 other WADA-accredited labs worldwide will handle more samples, "thereby ensuring that there are no gaps" in testing.
WADA has not explained how the labs failed, but suggested that outdated equipment and returning "false negatives" as possible reasons.