Photograph of a mold test culture plate with active fungal growth Mold Cultures: Causes & Importance of Errors in Mold Test Kits

  • MOLD CULTURE TEST ERRORS - CONTENTS: What causes errors when using mold cultures and mold test kits? How severe are the errors in this mold test method? Home test kits for mold: culture plates vs. tape sampling. What are the sources of variation in the mold level that air tests can detect?
  • POST a QUESTION or READ FAQs about the validity of mold cultures as home test kits to screen for mold contamination indoors
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Mold test accuracy & error sources with culture plates:

This article describes the reliability and accuracy of home test kits for mold that rely on mold culture media.

We explain the causes and the significance of errors when using mold cultures or "home test kits" to screen a building for indoor mold contamination.

In sum, mold culture kits are unreliable as a means to screen a building for the presence of dangerous levels of mold contamination. Here we explain why that is so.

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Sources of Error in Mold Culture Plate or Settlement Plate Tests

- Daniel Friedman

Because serious health as well as financial risks are raised by the risk of building mold contamination, it is important that building screening for mold contamination be performed using reliable methods. In fact our experience and opinion argue that tests alone as a building screen for mold are fundamentally unreliable, particularly when negative (no mold problem) results are obtained. A visual inspection by an expert may be needed. See MOLD / ENVIRONMENTAL EXPERT, HIRE ? for help in deciding if it is appropriate to bring in an expert to examine your building for mold contamination.

Air sampling by culture plate, mold swab, or surface testing by swab are highly questionable when used alone as a screen to "test" a building for harmful mold contamination. Here we list the reasoning that explains that view.

The use, accuracy, and reliability of mold culture test kits for screening buildings for mold contamination are discussed at MOLD CULTURE TEST KIT VALIDITY and MOLD CULTURE SAMPLING METHOD


Photograph of a mold test culture plate with active fungal growth

Cultures may grow the "wrong" spore in a building sample: Similarly, tests which rely on culture to identify particles are at severe risk of giving a "false negative" result, missing a serious problem, or of giving a "misleading positive" result by asserting that a particular spore which grew on the culture is the problem in the building.

Fungal spores of different mold genera and species will grow at different rates on different culture media. Spore "A" may "overgrow" spore "B" in a particular test, obscuring the presence of spore "B" which might be the real problem in the building.

Mold spores are particles of significantly varying size and mass. This means that mold spore particles that are in the air will fall out of the air at rates that depend more on the particle size and mass than on the frequency of particles in the air.

Some mold spores that can be serious problems if at high indoor levels include species of Aspergillus and Penicillium that produce spores so tiny (1-4 microns) that they may in fact remain airborne almost as a gas. Even though these particular genera may grow readily on the test culture media, we cannot assume that the spores will "fall onto" the mold culture surface at anywhere near the same rate as larger, heavier mold spores that may be present in the same building.

This means that even though Aspergillus Sp. might be the more serious mold contaminant in a building, it may fail to appear on the culture surface or it may be underrepresented or even hidden by larger, heavier mold spores that may be present such as certain species of Cladosporium sp.

Mold spores are produced at different levels in air at different times. Changes in building conditions (fans on or off), temperature, humidity, even light and darkness, mean that mold growing on building surfaces may release detectable spores at enormously different rates at different times. So when and where you placed a culture plate makes a huge difference in the test result.

When inspecting and testing a college basement library where there was very extensive Aspergillus sp. contamination, we found very low levels of airborne mold until operations had installed dehumidifiers to drop the indoor humidity level. When the humidity level fell the growing mold colonies decided it was time to release their spores and suddenly, one morning, the airborne mold level shot up by several orders of magnitude. So the timing and conditions of when a building is sampled for mold contamination makes a large difference in the results obtained. This accuracy problem invades not just mold culture tests, but nearly all mold testing methods except for surface sampling of visible mold growth.

Mold grows differently on cultures than on building surfaces: Interestingly, the study of even some of the infamous "toxic indoor molds" that will grow on a mold culture plate is very misleading when a forensic scientists rely on texts documenting cultured mold genera and species for the purpose of mold identification.

That's because even a very specific mold genera, species, and strain will often produce very different-looking growth structures, hyphae, sporulating structures, and even spores depending on what it is growing on. A mold growing on agar may look rather different from the same species growing on drywall paper or on a cardboard box or on a leather shoe in a wet basement.

A result of this confusing variation in growing mold is that many building investigators, aerobiologists, mycologists, and forensic microscopists collect huge photo reference libraries of what mold looks like growing not only in its canonical form on certain cultures but also what the same species looks like "in the real world" of growing on surfaces found in nature, and on surfaces of different building materials.

Indicator mold spores vs dominant or problem mold spores: Indicator mold spores such as Stachybotrys chartarum, Chaetomium globosum or other species, and some Cladosporium species are molds that we consider indicative of wet building conditions if they are found at significant levels in a building. And as larger heavier spores than Aspergillus sp. or Penicillium sp., they are more likely to fall more rapidly onto a culture plate surface than those lighter fungal spores. So even if we see significant growth of Stachybotrys sp., (an acknowledged harmful indoor mold) for example, it would be a mistake to presume that the cultured Stachybotrys sp. fungus is the only, or even the most significant health hazard or even the most prevalent mold in the building.

Cultures may fail to grow any of a problem mold in a building: some fungal spores (in fact about 90% of all fungal spores) won't grow at all in culture media under any conditions, (non-viable spores and many Ascospores) but may still be present at toxic or allergenic levels in a building.

Furthermore, mycologists have experimented with a great many different culture media formulae in order to try to find media that will support the growth of various mold genera or species under study. No single mold culture media grows all of the "culturable" mold genera/species (of the 10 % that will grow on media in the first place).

By this measure, a "mold culture" approach to screening a building for mold is 90% wrong before you even start. Luckily, many problematic indoor molds do grow in cultures, though still exposed to the earlier objection we raised above.

Comparing counts of spores or fungal colonies treats all molds as if they were equally toxic on a per-spore basis. As a collector of studies, papers, books on this topic, and as someone conducting our own studies, we have seen a very wide range of opinion among experts in the field.

Mold spore allergenicity or toxicity varies widely among fungal genera/species. So does the sensitivity of humans and other animals to fungal spores. So no single number will be absolutely correct. Just as spore toxicity varies by species, so does the physical size of individual spores.

The effect of breathing air contaminated by 5000 Penicillium sp. spores per cubic meter is unlikely to be identical to the effect of breathing 5000 Stachybotrys chartarum spores per cubic meter of air. Not only does their chemistry and toxicity vary, but a typical Pen/Asp spore is about 2 microns in diameter (1/25th the width of a typical human hair) while a typical Stachybotrys chartarum spore might be 8 x 12 microns -- much larger and thus providing more potentially harmful material per individual spore. You can see that writing federal or state standards for permissible fungal spore exposure by "count" or "levels" is difficult.

Even "bad" mold spores may not be toxic in a particular case: adding to the complexity of assessing health impact, individual spore genera, species, and strain within species will not necessarily produce toxins all the time. The toxicity of some molds varies depending on growing conditions such as the substrate upon which the mold is growing.

Eat different stuff, if you're a mold, and you may or may not produce toxins. We should still remove or clean up problem molds, but we cannot immediately know their actual toxicity or allergenicity without some more sophisticated (and uncommon) testing that are not normally performed, nor normally cost-justified.

Not only are there many variables to consider, but using currently popular air sampling or culture methods, even a low or "OK" test result cannot guarantee that there is no problem in the building.

Fortunately one can become reasonably confident about the level of mold or allergen risk in a building through competent visual inspection, judicious use of various sampling tools and methods, and competent laboratory determination work. Because this expertise is costly and the work time consuming, it should not be ordered without reasonable justification.

Differences in Test Results Between a Mold Culture Plate Surface Swab Test & Surface Particle Sample Analysis Using Adhesive Tape

Using a Culture Plate to Grow Particles from a Surface Swab

In a significant change from using culture plates to simply collect mold spores settling out of air (a mold culture "air test"), a large vendor of mold culture plate test kits currently recommends using the culture medium to try to grow particles collected by a cotton swab. The swab collects particles from a few square inches of a building surface and is then wiped or rolled across the culture medium.

We applaud this change [from use of mold culture plates as an air settlement test] as it may make the mold culture test more relevant and interesting than a simple mold-culture by air sample approach when someone is screening a building for evidence of the presence problem mold that was not readily visible.

Surface Swab Cultures Can Detect Roughly 10% of Mold Species That Could Be Present

But what actually grows (and thus what can be detected) in a culture plate are those particular mold species that particularly likes that particular culture medium.

Even if a mold culture test kit uses a medium that will grow a number of molds that occur in buildings, mycologists note that only 10% of molds will grow in ANY culture media whatsoever (there are hundreds of culture media formulas) under ANY circumstances whatsoever.

So from that starting point, 90% of molds won't grow in a culture.

You are "90% wrong" or at best, "10% possibly correct" in characterizing a building mold presence or absence if you rely just on culture media.

Actually you may be a bit better than 10% right, because of the more than 1.5 million mold species that exist and are everywhere all the time [2], a smaller number of them (about 200) are commonly found in buildings and it's possible that a higher percentage of that 200 will grow on the culture plate.

But then again, probably not. Lists of the "most common building molds" are themselves subject to important sources of error including reliance on inexpert individuals who collected samples of what they saw. That means that dark colored molds growing in visible areas on common surfaces (drywall) are probably over-represented, and light-colored, hard-to-see molds and molds growing in more hidden areas (see FIBERGLASS INSULATION MOLD for an example) are probably under-represented.

Cultured Molds Differ in Growth Rate & Thus Detection

But even among molds that will grow in a culture media, different genera/species respond differently to the culture - so what grows the most abundantly is not necessarily the most abundant mold in the original sampled area. A mold species that likes the culture may overgrow another species that actually was more abundant but does not like the mold culture. This is one reason for careful timing of when the culture is "read" by the test lab.

Comparing Mold Culture Tests with Surface Dust Particle Analysis

About the best we could try would be to collect surface dust from a representative test surface in a building, and the use the cotton swab wipe and culture approach for a spot next to the taped sample using the method described at TEST KITS for DUST, MOLD, PARTICLE TESTS.

But no means should we expect the same test lab result between the two approaches.

So it should not be a surprise if the findings in lab analysis of a surface dust sample collected by adhesive tape differs significantly from the results of a mold culture test.

For screening building dust or surfaces to identify the types of particles present (mold, insect debris, animal dander, fibers of fiberglass or possibly asbestos, oil burner soot, dust mite fecals, and many other particles involved in indoor air quality investigations) a dust particle sample is likely to be much more accurate in detecting the range of particles actually present in the building - it not limited only to what will grow on the culture plate.

There is a place for both approaches, and mold cultures are often used in research, but the results of any building investigation that uses any mold tests need to be understood in terms of what the test is capable of detecting. \

Watch out: regardless of what test method you use to collect your mold for lab analysis, surface swab and culture plate or adhesive tape, everything depends on the surface you chose for collecting the sample. During IAQ investigations, we like collecting dust particles from a horizontal surface in an area of a building where people spend a lot of time.

While a detailed count-type quantitative analysis of either lab test doesn't really make much sense (there is just too much sample to sample variation), an approach that identifies the dominant particles present, or the presence of unusual particles that may indicate a nearby problem can be helpful.

Do not expect to obtain a reliable picture of the level of risk of an indoor mold problem if you rely only on a "mold test" of any kind. While each mold test method has its strengths and weaknesses (see ACCURACY OF VARIOUS MOLD TEST METHODS), for a reliable indication of the chances of a significant indoor mold (or other) problem a thorough, detailed visual inspection of the building, along with a competent history taking of the building and of complaints by its occupants are essential.

Do you need to hire an expert to inspect and test your building for mold?

Not necessarily. See MOLD EXPERT, WHEN TO HIRE.

So are Mold Cultures Totally Useless

Certainly not. Mold cultures are highly useful in medical and academic research.


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Questions & answers or comments about the validity of mold cultures as home test kits to screen for mold contamination indoors. The article above gives a brief tutorial about the accuracy of mold culture tests for the level of allergenic and toxic mold in residential buildings. Are culture plate mold colony or mold counts valid? Are mold cultures and mold swab tests valid tests? These critical questions are discussed above.

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