are diy agar petri dish tests reliable to sample mould?

Mould Plate Testing: Pros, Cons, and When to Use It

Mould plate testing, also known as gravity plate testing, is one of the simplest and most affordable ways to assess mould in your home and so often draws people in. It involves placing a petri dish filled with mould-friendly agar in the environment for a set period, allo wing airborne spores to settle and grow. Here’s a quick rundown of the pros and cons, but if we’re being honest, the list of cons is LONG, and as the saying goes - cheap isn’t usually best.

Pros of Mould Plate Testing

1. Affordable: Among the least expensive mould testing options.

2. Easy to Purchase: Available online.

3. Targeted Placement: If placed close enough to potential mould sources, it may help identify problem areas.

4. Lab Analysis: Some plates can be sent to labs for species identification (additional fee).

Cons of Mould Plate Testing

1. Limited to Viable Spores: Only picks up viable (living) mould spores, missing non-viable spores or fragments (both very important and both can make people sick).

2. Frequent Growth: Mould exists naturally everywhere, so these plates often show growth even when there isn’t a serious mould issue related to excess indoor moisture/water damage.

3. Contamination Concerns: Many factors—such as contamination from the air, improper handling, and lack of precise growth conditions—can skew the results.

4. No Self-Identification: You can’t tell mould species or toxicity just by looking—lab testing is necessary. Lab testing also quantifies the amount found.

5. Selective Growth: Growth depends on the type of agar used, meaning many species may not appear if they don’t grow well on the selected culture media.

6. Misses Heavier Spores: Doesn’t reliably capture heavier, stickier spores that don’t become airborne easily.

7. Not Suitable for Remediation Verification: Shouldn’t be used to confirm the success of mould remediation efforts.

While mould plate testing can be a useful initial and very basic tool, it’s best viewed as a preliminary step rather than any kind of comprehensive investigation - and it can sometimes lead to more confusion than clarity. Finding spores doesn’t tell you if they’re toxigenic without lab analysis, and even then, you won’t know their exact source. On the other hand, if no spores are found, you’re left wondering if there’s an issue with old, “dead,” non-viable mould that the test doesn’t detect. Investing in proven, legitimate testing methods is a better choice. If you’re facing extensive mould issues, health concerns, or need verification testing after remediation, professional testing is recommended for a complete, reliable analysis.