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False Positives & False Negatives


So recently I have been getting a lot of requests to explain how we can be sure that when interpreting results, we aren’t getting a false positive or a false negative. To answer this question, we need to consider two general scenarios. 1. When there is no visible mold and we are trying to identify if mold exists in the walls. 2. Remediation has already been completed and we are doing a clearance test.


First, let me just say that when it comes to mold testing, unless you can physically collect a sample of visible mold, there is no way to say 100% that mold growth exists just by air testing. If no visible mold growth exists at the time of testing, when we test the air, the most we can say is that the lab results are either indicative of mold growth in the area, or that the results don’t indicate mold growth in the area. The word “Indicative” is important because all we can do is interpret what the results of air sampling might mean. Sometimes it is relatively straight forward and we can be confident in the conclusions we draw, and sometimes it can be difficult to draw conclusions. So in the end, we need to remind ourselves that though this is a generally reliable method for identifying if mold growth exists in a residence, this is an imperfect science and results can be misleading in some situations.


You may be asking yourself, “well then how does mold testing help if we won’t know anything for sure?” Well, let’s put on our forensic scientist hats and simplify this using an analogy. Let’s imagine we are at a crime scene in someone’s house and we are looking for fingerprints. We have a suspect in custody and he says he was never in this particular house. So we go around looking for fingerprints and let’s say we find a fingerprint that matches his. We can confidently say that the evidence indicates he was in this house at some point right? If he was never there, how did his fingerprint get in this house? Some of you who watch a lot of crime shows might be thinking that maybe someone planted it there somehow and that he could be getting framed. Let’s eliminate that as a possibility and say that no one is trying to frame our suspect. The only way his fingerprint ended up in this house is if he put it there himself with his own hand. Boom! Case closed, right? If only all cases were this simple.


Here is where it gets interesting. Let’s imagine that we are extremely good at searching for fingerprints and if his fingerprint WAS in the house, we would most likely find it. The only way that we wouldn’t find it is if it got wiped off or maybe he was wearing gloves. So what if we don’t find his fingerprints in this house, does that mean he wasn’t there? It doesn’t, it just means we didn’t find any evidence that he was there. In this instance we would not conclude that he was never there but we would say there is no evidence indicating he was there.


So let’s apply this to mold inspection and we will see why this answers our false positive vs. false negative question. I recently had a friend tell me that he had mold testing done in his bedroom which had a leak in the ceiling. He said they came out and tested the room and results showed highly elevated mold levels in his room when compared to the outdoor reference sample. He didn’t want to believe it so he got testing done by the same company the very next day and the results showed a much lower level. He then concluded that the first round of testing was a false positive and that there was never mold there in the first place. Here is why this is wrong. Much like the fingerprint at our pretend crime scene, the elevated mold levels had to come from somewhere. We know it didn’t come from the outdoor air (Because the outdoor reference showed much lower mold levels) and all those mold spores didn’t magically appear. We also know that there are actually some reasons why we might see big fluctuations when testing from one day to another. For example, if any area is very wet due to a leak, sometimes mold spores will stick to the moisture and won’t become air borne until it is dried out. Sometimes if the mold is inside a wall, for many reasons it might not be producing spores or maybe the spores being produced aren’t finding a path to get into the indoor air.


So, much like the crime scene, the absence of evidence does not mean it isn’t there, but the presence of evidence means it had to come from somewhere. Generally, if we see elevated levels it is likely coming from mold growth in that area. If we see low, normal mold levels then there is likely no mold growth in the area. But in the cases where testing is inconsistent or levels fall in a grey area, which, it is more likely that we get a false negative than a false positive.


The situation changes slightly when mold removal has already been completed. Please refer to my previous blog about what to expect from a legitimate mold removal company. Many mold removal companies are “scammers” to put it plainly. They don’t do a good job, they cross-contaminate a residence, and they cause more problems than they solve. If I am doing a clearance test and a mold removal company did not set up a containment area or run HEPA air filters to clean the air afterwards, then elevated mold levels might indeed show a false positive. By false positive I mean the results may indicate that mold growth still exists in the residence, while it has actually all been removed. In this circumstance, the elevated mold levels are real, however it is due to the carelessness of the mold removal company, not because actual mold growth still exists. This is why it is important to get mold removal professionally done by a legitimate company.


All that being said, this doesn’t mean mold testing isn’t reliable, it just means that we have to use it for what it is. This is why we also check for moisture intrusion and relative humidity levels. We gather as much information as possible so that if we do see something peculiar, we can put on our forensic scientist hats and determine what is most likely going on.

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