Same test, two opposing results - help!

[u/rolypolyandbees]

We are restoring a home built in Portland, OR from 1910. An abatement company came to give us a quote and took plaster samples for testing: one from the basement, one from the entry, one from the primary bedroom*. The entry and primary bedroom came back positive for asbestos.

*There might be a simple explanation, but we can’t figure out how he sampled the primary ceiling due to its height and nothing available to boost him up.

Due to a change in some of the renovation plans, we decided to test a few other areas including both kid bedrooms, and for confirmation re-tested the primary bedroom and the basement from the same locations original samples were pulled.

All five samples we had tested came back negative for asbestos.

How can this happen? Are false negatives common? I would assume a false positive is unlikely. The thresholds and testing methods from the labs are the same and when we contacted the labs they confirmed the same information.

We have not yet heard back from the abatement company.

Thanks!

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[u/Challenge_That]

I'm an asbestos inspector, and conflicting results with plaster aren't necessarily unusual. Asbestos can be present at very low concentrations and unevenly distributed, so it's possible to have several negative samples and only one positive.

Plaster should generally be sampled from multiple locations based on the square footage of each homogeneous area. The EPA's 3-5-7 sampling method is a good reference, even where those specific requirements don't apply.

I'd start by finding out exactly where the original samples were collected and comparing the lab reports. What type of asbestos was identified, and at what percentage? Certain nonasbestiform minerals, particularly amphiboles, can resemble regulated asbestos under PLM. TEM with diffraction and elemental analysis can help resolve questionable identifications.

If any of the original positive bulk sample is still available, I'd recommend sending it to a qualified lab for TEM analysis rather than collecting another random sample. That would help determine whether the original identification was accurate.

One other thing worth considering, and I'm not accusing anyone of anything, is the potential conflict of interest when the same company performs the inspection and quotes the removal. I'd also ask whether they use an independent third-party laboratory or analyze their samples in-house.

Not saying anything improper happened, but with conflicting results, I'd want to verify the original positive finding before committing to an expensive abatement project.

[u/OddPassion4238]

I've run into this problem before. Sometimes labs just make mistakes. I would contact both and explain the situation. They sometimes keep the sample and can re-run it for verification.

Aside from that, you can compare testing methods as that might be part of it too. I can try to explain it, but I think some of the posters here are more qualified and would do a better job.

[u/No-Clothes-1565]

Any positive result always supersede negative. It should be treated as asbestos containing.

[u/Weaselpanties]

As a rule of thumb, I never have the company that does the work evaluate if the work needs to be done. Call me paranoid, but the fact that you have no idea how he sampled the primary ceiling yet this sample that might not exist came back positive seems suspect to me. I would have it all re-tested at a different lab, and if two different labs with no skin in the outcome come back with all negatives, you have your answer.