ESR's Bioanalytical Programme manager Paul Fitzmaurice said on-site testing by drug testing companies used a method similar to pregnancy tests; in which the employee's urine was tested for any synthetic product.
If positive, that sample was sent to ESR for more complex testing.
Dr Fitzmaurice said if they were aware of the product and how the body processed it, they were able to identify it for the employer.
"(However) they [products] almost evolve on a monthly basis or they certainly evolve once the Ministry of Health has banned one of the substances - it changes to something else.
"The unfortunate situation we find ourselves in is New Zealand is leading the world in this regard, and we're evolving synthetic cannabinoids into the market quicker than anywhere else."
Users of the drug kept themselves informed through website forums where people discussed which drugs were more likely to beat the tests, Dr Fitzmaurice said.
New Zealand Drug Testing Service director Jo Kirk said they were getting business from all types of industries - "from market gardens, to supermarkets, to timber mills".
She said feedback from laboratories was that there were many products they could not pick up on because of the changing face of the drugs.
"The labs are saying they can't find anything, but the instant (initial) tests are saying they have failed."
If an employee was sacked over the initial test they could take their case to court, where they would probably win, Ms Kirk said.
Employers and Manufacturers Union chief executive Kim Campbell said the industry was "rotten and devious".
While many people associated blue collar workers with the drug use, it could be just as prevalent with white collar workers, Mr Campbell said.
"The inventiveness of the people in the business are faster and more inventive than the law was set up to catch."
Employers were losing the battle against workplace synthetic drug use, Mr Campbell said.
There needed to be a more regulatory process, "otherwise we'll keep sticking our fingers in the dyke".
