Interpreting drug test results – “not all drug levels are comparable”

Posted by TDDA

As a drug tester or as an employer you might be familiar with the following scenario.

‘An employee’s drug test result has come back from the laboratory showing a positive test for Cannabis with a THC-COOH (the metabolite of THC, the main psychoactive compound in cannabis) level of 140ug/L (ng/ml). You’re aware that the cut-off level for THC–COOH in AS/NZS4308 is 15ug/L and this results is nearly 10 times the cut-off, but the employee is adamant that the positive test is the result of a single joint on the weekend.’

Is there a way to interpret this result? Is the employee telling the truth? What followup options are there for this test?

Firstly, unlike breath/blood alcohol results were a general broad interpretation of intoxication can be drawn from the result, for a urinary drug test, for any common abused drug, no correlation exists between drug level in urine and the degree of intoxication. The urine drug test identifies drug use and not impairment or frequency of use and therefore interpreting the result with respect to potential impairment is not possible.

Secondly, the drug test result does not take into consideration the donors level of hydration i.e. had they been drinking lots of fluid or had they not had any fluids for several hours? The concentration of urine varies depending on the state of hydration of the individual, which is most discernible from the colour; strong dark urine when dehydrated to almost clear colourless when over-hydrated (a ‘straw’ colour is suggested as normal). The level of drug in the urine sample also varies depending on the level of hydration, if the urine is very concentrated then the drug level will be much higher and conversely the drug level much lower in a sample from a well hydrated donor.

The drug testing laboratory can determine the relative concentration of urine by measuring the level of creatinine in the urine sample. Creatinine is a byproduct of energy metabolism primarily produced in muscles, and it is removed by the kidneys into the urine at a fairly constant rate. The creatinine level can be used by the laboratory to provide additional interpretation of the drug test result, specifically if the donor has provided samples over a period of time in a drug abuse rehabilitation situation.

A significant amount of scientific research has been conducted in the modelling and interpretation of drug test results associated with cannabis use. There are numerous variables that need to be considered in these interpretation (drug use pattern, BMI, metabolism, kidney function, etc.) and therefore it is impossible to tailor an interpretation to an individual. However, if drug testing has identified a cannabis user the following actions are appropriate:

Trying to ascertain if the individual is an infrequent (including one-off use) or habitual heavy user (daily or weekly) may provide some indication as to the period of time (days or weeks respectively) it will take for the drug to be flushed from his or her system.
It is advisable to re-test all individuals within two to four weeks of the first test and to ask the laboratory for a comparison of the results. This provides a reinforcement of the abstinence behaviour and subject to a negative test result might allow the individual to return to work under the conditions provided for in the company policy.
Subsequent tests should only be interpreted by a qualified toxicologist.
It is not advisable to have long extended periods were no testing occurs as this does provide an opportunity for drug re-use and interpretation of the normailsed results is best conducted on multiple test results collected regularly (e.g. fortnightly, monthly).

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