If a sample tests positive, does the lab do a second test to ensure it wasn't a false positive?

I was once a participant in a class that was conducted by someone that has experience working in the lab(s) that conduct the actual analysis on urine samples. It was briefed that ANY positive tests are automatically ran again to confirm the positive test and to ensure the initial machine is running correctly. This Soldier.....not innocent of cocaine usage.

No, urinalysis testing has been proven to be nearly perfect in clinical studies. Cocaine is one of the harder drugs to catch as the drug is out of your system in 2-4 days. A positive result for cocaine does not need to be confirmed as they are run twice at the lab to confirm with 2 different machines. The soldier used cocaine, everyone knows that

The first test is a reagent test, and they can be false positives because it's a very broad test so anything similar to it triggers a red flag. Then the sample is sent to the gas chromatography chamber where the EXACT chemical composition of the chemicals and their levels is measured. They don't give false positives, that's not how gas chromatography works. Then they test the same sample a second time. Plus, the levels have to meet a certain threshold. Cocaine has a half life of about 20 hours. It takes about five half life cycles to eliminate most drugs. By the fifth day that level is too low to meet the threshold to set off a uranalysis. If that testing sample had a positive it's because there was positive urine in the sample, and enough of it at a high enough concentration to raise alarms