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Testing Methods: Laboratory Composite Verses Cumulative Sampling

Laboratory Composite Analysis

With Laboratory Composite Analysis, individual or discrete samples are collected from within a dwelling and forwarded to the Laboratory as individual samples. This means that a separate wipe is used for each sample and gloves are changed between each sample.

This approach allows for multiple samples (up to 10) to be initially screened as a group to determine whether there is evidence of methamphetamine being present.

Sub-samples of the individuals are retained by the laboratory for testing in the event of a high result.  This avoids the high initial cost of individually testing all samples, while also ensuring that individual samples are retained and available for testing if needed without having to return to the property.

This method is a recommended method in the New Zealand Standard NZS8510.  It states that each discrete wipe shall constitute a single 100cm² sampling area.

Cumulative Sampling

This method is not mentioned in the standard (NZS:8510) and uses a single wipe in multiple locations in a dwelling.  You see some methamphetamine screening companies advertising that they take at least eight samples in a house using this method.  This is extremely misleading because in fact, they are taking one sample in eight different locations.  As the name suggests it is a cumulative sample, so the greater the number of locations sampled with the single wipe, the higher the contamination or residue level will be, however, it is often reported as micrograms per 100 square centimetres (µg/100cm²).  This again is misleading and purports to be able to be compared with the NZS:8510 maximum residue level of 1.5µg/100cm² when in fact if there are eight cumulative samples on the one swab then it is µg/800cm² and if there are 10 it will be µg/1000cm² or 1 square metre.

Laboratory Composite Sampling is Better by Far for the Following Reasons:

  • It gives a true average residue level that can be compared to the NZS8510 maximum residue level (Provided the sample is taken from a 100cm² area using the NIOSH 9111 method)
  • The cost to the sampling company is exactly the same because in both methods only analysis fee is charged.
  • It is not misleading and follows the New Zealand Standard by using discrete or individual wipes.
  • With cumulative sampling, if the first sample is cross-contaminated in any way then all samples are cross contaminated.

At Safe Zones we only use Laboratory Composite Analysis for screening tests and individual wipe analysis for detailed and post-remediation tests.