You Bought a Mold Test Kit and It Came Back “Positive.” Of Course It Did.
You saw the kits at Home Depot. $12–$30 for peace of mind. You set the plate out, waited the prescribed time, sealed it, and either read the results yourself or mailed it to a lab. The result came back positive for mold.
Now you’re more anxious than before you tested.
I’m Phil Sheridan. I own 4D Restoration in Edmond, Oklahoma. These kits aren’t fraudulent — they do detect mold. The problem is that detecting mold is meaningless, because mold is always there.
What the Kit Actually Measures
Most consumer mold test kits work in one of two ways:
Settle Plate (Gravity Sampling)
An open petri dish with a growth medium. You set it on a counter for a prescribed period (usually 48 hours), seal it, and wait for colonies to grow. After 3–7 days, you see growth on the plate and compare it to the included color chart.
What it tells you: Mold spores landed on the plate and germinated. This will happen in every room of every building on Earth.
What it can’t tell you: Whether the count is normal or elevated. Whether the species are concerning. Where the mold is growing.
Cassette Kit (Air Sampling — Sort Of)
A small cassette with a sticky collection strip. You crack the seal and leave it exposed for a specified time. Spores adhere to the strip. You send the cassette to a lab for analysis.
What the lab tells you: Species identification and approximate count — if you pay the additional lab fee ($40–$100 on top of the kit price).
What it still can’t tell you: Whether the count is elevated relative to outdoor conditions. Without a simultaneous outdoor sample, the indoor count has no context.
Why “Positive” Means Nothing
Every indoor environment contains mold spores. Indoor air in a clean, well-maintained home typically contains 200–500 spores per cubic meter. This is normal. This is expected. This is not a problem.
A settle plate in a “clean” room will grow colonies. A cassette in a “clean” room will capture spores. The test is designed to detect something that’s guaranteed to be present.
It’s equivalent to a test kit that detects dust. Of course there’s dust. The question isn’t whether dust exists — it’s whether there’s an abnormal amount.
The only way to determine whether indoor mold levels are abnormal is to compare indoor counts to outdoor counts taken at the same time, in the same conditions. This is called a baseline comparison, and it’s the methodology that professional mold testing uses.
How Professional Testing Works
Professional mold testing (spore trap analysis) uses calibrated air pumps that pull a known volume of air (typically 75 liters) through a cassette at a controlled flow rate. The cassettes are analyzed by a certified microbiology lab.
The critical difference: we take at least two samples — one inside the affected area and one outside. The lab reports:
- Total spore count per cubic meter — indoor vs. outdoor
- Species distribution — which mold species are present and in what proportion
- Comparison analysis — whether indoor counts exceed outdoor baseline
Example results:
| Sample | Total Spores/m³ | Dominant Species |
|---|---|---|
| Outdoor (baseline) | 800 | Cladosporium (500), Alternaria (200), Basidiospores (100) |
| Indoor — living room | 650 | Cladosporium (400), Alternaria (150), Basidiospores (100) |
| Indoor — master closet | 4,200 | Aspergillus/Penicillium (3,500), Cladosporium (400), Stachybotrys (300) |
The living room is fine — counts below outdoor baseline, same species distribution. The master closet has a problem — counts 5× outdoor levels with Aspergillus/Penicillium and Stachybotrys that aren’t present in outdoor air. Something in that closet is producing spores.
A home test kit would show “positive” in all three locations and tell you nothing about the difference between the living room and the closet.
When Professional Testing Is Worth It
Post-remediation clearance. After mold removal, air testing confirms that indoor counts have returned to or below outdoor levels. This is the most valuable use of testing.
Insurance documentation. If your insurer is questioning the extent of mold damage, lab results with species and concentration data provide objective evidence.
Unexplained symptoms. If someone in your household is experiencing persistent respiratory symptoms and you suspect mold but can’t find a visible source, air testing can detect elevated concentrations from hidden growth.
Real estate transactions. Pre-purchase mold testing gives you data about the indoor environment before you commit.
When Testing Isn’t Necessary
You can already see the mold. If there’s visible mold on drywall, wood, or other surfaces, you already know it needs remediation. Testing will confirm what your eyes already told you. Spend the testing money on the actual removal.
Small surface mold on non-porous material. Mold on bathroom tile or a window frame doesn’t need testing. Clean it with appropriate fungicide and improve ventilation.
What to Do Instead
If you’re concerned about mold in your home:
- Skip the hardware store test kit. It will tell you mold exists. You already knew that.
- Do a visual and olfactory assessment. Look for visible growth. Sniff for musty or earthy odors, particularly in enclosed spaces (closets, under sinks, behind appliances).
- If you find something or smell something, call 405-896-9088. I’ll assess visually, take moisture readings to identify the water source, and recommend testing only if the situation warrants it — not as a default upsell.
Phil Sheridan. Owner, 4D Restoration. IICRC Certified. 405-896-9088.