Composite meth testing is a sampling strategy that combines multiple surface wipe samples into a single laboratory analysis, reducing the cost of testing while still detecting whether methamphetamine contamination is present. There are two approaches — laboratory composite and field composite — and the difference between them matters when a positive result requires further investigation.
What Is Composite Meth Testing?
In a standard (discrete) meth test, each surface wipe sample is analysed individually by the laboratory. This gives you a precise result for every surface sampled — but it means every sample incurs a separate laboratory analysis fee. For a property with 10–15 sampling locations, this can be expensive.
Composite testing addresses this by combining multiple surface wipes into a single laboratory sample. If none of the combined surfaces are contaminated, the composite sample returns a negative result — and you have screened the entire property for the cost of one analysis. If the composite sample returns a positive result, you know contamination is present somewhere in the group, and further testing is needed to identify exactly where.
Composite testing is defined and permitted under NZS 8510:2017 as a screening-level approach, using NIOSH 9111 surface wipe methodology.
Field Composite Testing
In field composite testing, the assessor physically combines multiple surface wipes into a single sample container at the property. For example, the assessor might wipe surfaces in the living room, kitchen, and bedroom, then place all three wipes into one container for a single laboratory analysis.
The advantage of field composite testing is cost — one analysis covers multiple rooms. The disadvantage is that if the composite result comes back positive, the assessor must return to the property to collect individual (discrete) samples from each surface to identify which room is contaminated. This means a second site visit, additional sampling, and additional laboratory fees.
Laboratory Composite Testing
In laboratory composite testing, the assessor collects each surface wipe separately — each in its own container — and the laboratory combines the extracts for a single analysis. The key advantage: if the composite result is positive, the laboratory already has the individual wipes and can re-analyse them individually without the assessor needing to return to the property.
This makes laboratory composite testing the more efficient and cost-effective approach in practice. You get the screening benefit of a single analysis, and if contamination is detected, the laboratory can immediately identify which specific surface is responsible — saving a return site visit and the associated costs.
Composite vs Discrete: Which to Choose?
The choice depends on your objective:
- Composite testing (screening): Best for initial assessment — pre-tenancy baseline tests, pre-purchase checks, or routine property screening where you want to confirm the property is clean at the lowest cost.
- Discrete testing (detailed assessment): Best when you need to know exactly which surfaces are contaminated and at what concentration — for decontamination planning, legal evidence, insurance claims, or clearance testing after remediation.
In practice, most professional assessments start with composite screening. If the composite result is positive, discrete testing follows to identify the specific contaminated surfaces. This staged approach gives you the most information for the lowest cost.
Reading Composite Results in a Lab Report
When you receive a laboratory report, composite results will show a single value representing the combined sample. If this value is below the relevant threshold, all surfaces in the composite group are considered to meet the standard. For non-rental situations, the NZS 8510:2017 threshold is 1.5 µg/100 cm²; for rental properties, the 2026 Residential Tenancies regulations set a contamination threshold of 15 µg/100 cm². If the composite value exceeds the applicable threshold, individual surfaces need to be tested to determine which are contaminated. For a detailed guide on interpreting laboratory reports, see how to read a meth toxicology report.