A second sample - taken at the same time as the first - is tested if the driver or his team request it within 72 hours of being notified of the results of the first. The purpose is to rule out any question of contamination or mishandling of the sample, and allows the driver or his legal representative to be present and observe the entire testing process.
There has been no disclosure of the substance that was detected in the initial test. Banned substances in NASCAR competition can include otherwise perfectly legal over-the-counter and prescription medications.
After a competitor is informed of a positive result in the test, NASCAR's medical review officer contacts the person concerned to see whether there is any acceptable explanation for the finding. If the review officer is not satisfied, then the driver is placed on temporary suspension pending any 'B' sample test. If that also tests positive then the competitor's suspension becomes 'indefinite' while the person completed a mandatory, bespoke rehabilitation program.
While there have been a number of suspensions for substance abuse violations among other NASCAR workers and team personnel, only three drivers have previously been suspended since random drug testing was introduced in 2009.
None of those three drivers were subsequently able to return to NASCAR competition after the suspension.
"Obviously, we support the NASCAR program of drug-free drivers, mechanics and crew members and we need to abide by that," said team owner Roger Penske on Sunday after he returned from holiday in Europe to attend the IndyCar race in Toronto. "To me, it's a big speed bump for us.
"At the end of the day, we've had situations before that we've had to deal with and we're going to be professional, we are going to support the sport and we don't want to let our sponsors down," he said. "At this point, we'll just wait and see."