The World Anti-Doping Agency (WADA) reinstated the lab in July, ending a month-long suspension that had threatened to embarrass the host country, and additional security is in place, such as a feed of footage of the doors to the storage freezer.
"It was rebuilt and accredited by WADA. Because of a mistake, the accreditation was suspended in a most inopportune time," the IOC's medical chief Richard Budgett said.
"The lab passed a very intensive stress test by WADA (prior to the Games). Blind samples tested for all kinds of substances. They passed that stress test and the lab has been performing superbly."
News that makes sense
Your trusted source for staying up-to-date with the world around you. Get free daily news updates and analysis, straight to your inbox.
Budgett, a Briton who won a rowing gold at the 1984 Olympics, gave no details of the mistake the cost the lab its accreditation, but the head of the facility said in July there had been problems with the implementation of new equipment.
The lab will be processing about 6,000 blood and urine tests during the Games and any suggestion of it not being up to standard could have further overshadowed an Olympics already grappling with huge doping issues.
Dozens of Russian athletes, including virtually the entire track and field team, were suspended as part of sanctions against the country of its systematic state-backed doping programme that also included the Sochi 2014 Winter Olympics.
The Russian lab was used to switch clean samples for positive doping samples of Russian athletes with the help of the Russian secret service and the head of the lab, as highlighted in a report commissioned by WADA.
"At the time we thought we had great expertise in place but when you have director of lab involved then that is a challenge," Budgett said.
"Could we have done things differently? Yes, but at the time we had more external scrutiny than (there) normally is."
Budgett said a number of security protocols were in place in Rio, including guards, access restrictions and cameras, to eliminate any possibility of tampering with samples.
"There is security presence from the national guard, private security at the lab, an extensive network of surveillance cameras. I get the footage of the door of the freezer," he said.
Only authorised personnel, in pairs, can enter the freezer to access samples, he said.
International experts were also present inside the facility at the remote five-storey building on the island campus of the Federal University of Rio de Janeiro, along with observers, to make sure the process is executed correctly.
"The whole thing is very carefully controlled," Budgett said.
So far three athletes have tested positive for banned substances at the Games.
(Reporting by Karolos Grohmann; Editing by Alison Williams)
