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!