The Pitfalls of Crossed Matrix Drug Testing

A testing matrix refers to the base substance from which an analysis occurs. In Australia there are two matrices used for Drug Testing and for which there is an Australian Testing Standard (AS/NZS4760 - Oral Fluid Testing and AS/NZS4308 Urine Testing).

Crossed Matrix Testing refers to the use of one matrix for a screening test (eg Oral Fluid) with confirmatory analysis performed on an alternative matrix (eg Urine).

Choosing a Matrix for Testing

The two principal testing matrices used for regular testing in Australia and New Zealand are Oral Fluid and Urine. Both are equally accurate systems for testing, incorporating screening onsite and analysis in a laboratory.

Some Australian case law provides an insight into the application of each matrix. In summary, CFMEU v Port Kembla Coal (BHP) (FWC2015) deals with the choice to test using both matrices and Endeavour Energy v CEPU (FWC2012) deals with the obligation to test for drugs including alcohol. The Commissioner in the 2012 case found that "it is precisely because it only detects for recent use that oral fluid testing is a better indicator of the likely impairment as a result of smoking cannabis."

In the 2015 case, the Commissioner remarked, "... oral fluid testing is more focussed on acute impairment, whereas urine testing is more likely to uncover patterns of drug use which may lead to levels of impairment and safety concerns."

Selection of a testing matrix should be entirely dependent on your organisational risk profile and the importance of identifying recent use versus a pattern of longer term cannabis use (for example). All of the other drug classes are somewhat similar in detection periods.

The above chart demonstrates the significant detection period difference in Cannabis between urine and oral fluid. THC in urine can be detected for up to 28 days after cessation in extreme cases and in oral fluid generally for a maximum of some hours.

Why some testing programs employ crossed matrix testing

From time to time, organisations decide to employ a Crossed Matrix Testing regime. In general, this takes the form of an initial onsite screening using oral fluid with confirmatory analysis using urine.

The reasons given by these organisations is often that oral fluid testing as a screening test is quicker and more efficient as well as less invasive.

Urine on the other hand, it is felt, is more accurate and tests for a wider range of drugs.

I will endeavour in the following to address the reasons why crossed matrix testing is not a good choice as a testing regime.

Relative Accuracy

That oral fluid is less accurate than urine as a testing matrix is a myth. Both matrices are accurate bases for testing when used in conjunction with laboratory confirmatory analysis. Both matrices are subject to limitations in screening. Confirmatory analysis is extremely accurate for both matrices. In terms of screening accuracy, oral fluid tends to be more subject to some cross contamination issues whereas urine tends to be more susceptible to tampering and masking agents.

Selection of a Testing Approach

The decision to employ drug and alcohol testing in a workplace needs to be strategic and consider carefully the risks associated with drugs and alcohol in your workplace. Factors for consideration include:

• The risks to others from a pattern of use: Testing can be relatively costly. Regular testing is not always possible. Using urine as a test matrix can alleviate the risk of cannabis use in the intervening period between testing encounters.
• The relative invasiveness of a test: Urine is generally considered a quite invasive test compared to oral fluid and many organisations prefer oral fluid testing for this reason. Urine testing requires access to toilets that have been specially prepared to remove access to water sources and are secure.
• Time factors: Oral fluid testing is generally more rapid for testing for a variety of reasons. Our data suggests that an oral fluid test can be performed in about 75% of the time of a urine test.
• Regulatory or other factors: Some industries have a regulatory or similar requirement that mandates one form of testing over another.

The reasons that you select a screening approach based on these factors should also hold true for the reasons you would select a confirmatory matrix.

The Standards Framework

Whilst testing Standards are similar in construction, they don't in fact envisage screening using one Standard and then confirming using an alternative Standard.

Properly, a confirmation analysis is designed to confirm the results of a screening test. If a screening test is performed substantially in conformance with Appendix A, AS/NZS4760:2019, collection of a sample would be performed in conformance with Section 2 of that Standard, it follows then that confirmatory analysis should be performed in conformance with Section 5 of that Standard. It would not make sense to swap to the equivalent sections of AS/NZS4308:2023 at sample collection. It also avoids any potential compliance gaps between the relevant Standards.

No Improvement

There is no evidence that would suggest that a better result is obtained using one test matrix over the other. Neither testing Standard is superior. The two Standards have slightly different drug classes as standard tests with capacity to test additional classes in accordance with the relevant appendices of those Standards.

Speed and Efficiency

Oral fluid is certainly easier and quicker to test than urine and requires less facilities. The devices for each testing matrix develop a result at roughly the same speed. The added time delays involved in obtaining a urine sample in a toilet cubicle means that generally, an oral fluid test takes about 75% of the time it takes to perform a urine test. Using oral fluid for screening and urine for analysis compounds the time taken for a test making the process of dealing with a laboratory referral for a "not negative" a more protracted process.

Results Confusion

Given the difference in cannabis detection windows and the different substances tested for as noted in the table above, a laboratory report may vary widely from the results obtained in a screening test, which can create significant confusion. For example, oral fluid screening may yield an opiate result (say for some codeine ingested earlier in the day) but urine confirmatory analysis may yield both an opiate result (as expected) and a cannabis result from a week ago when the donor was on annual leave.

The end result can lead to having to explain complex pharmacokinetics, questioning the accuracy of initial or subsequent results or both. This can place the Requesting Authority in an extremely complex and difficult position.

Complexity

Experienced testers will have no difficulty in navigating the complexity of running two processes, however crossed matrix testing does add a level of complexity which might trip up an inexperienced tester.

The process would look something like this:

1. Screening is commenced substantially in compliance with App A AS/NZS4760 including inspection of the oral cavity and use of a swab with a verified device.
2. A "not negative" is identified.
3. Confirmatory sample collection is then performed in compliance with Sect 2 AS/NZS4308 using a cup that does not show the urine screening results but does allow for an assessment of the integrity of the sample including temperature, appearance and masking agents.
4. Aliquot of the sample to a laboratory device for despatch to confirmatory analysis with relevant documentation
5. Confirmatory analysis and reporting in accordance with Sect 6 AS/NZS4308

These additional steps particularly at 3 and 4 above can create the potential for human error. The analysis additionally should involve two forms of test/analysis to be compliant under Sect 6.5(c).

In Summary

At least at some level, a testing program relies on a sense of consistency in results and approach. The potential for significant confusion around results for no appreciable improvement in outcome means that there is really nothing to be gained from employing a crossed matrix approach. The potential for human error increasing with each additional step in the process can create an indefensible situation where workers lose confidence in the process.

Conclusion

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