Are home mold test kits accurate?

Three kinds of kits, three different answers

“Home mold test kit” covers a few quite different products. How useful the result is depends on which one you bought.

Kit type	How it works	What it can tell you	Main limit
Settle plate	Open a dish of growth medium for a set time, then watch it	That some mold spores landed on the dish	No volume, no outdoor comparison, no location
Tape or swab kit with lab analysis	Sample a visible spot and mail it to a lab	What a specific stain is, and whether it is mold	Says nothing about the air or the rest of the house
Mail-in dust DNA kit	Collect settled dust by cloth or vacuum, mail it to a lab for ERMI or HERTSMI-2	Which molds have built up in the dust over time	No inspection to find the source if the score is high

Why a settle plate almost always “fails”

Mold spores drift into every house through doors, windows, clothing and pets. Leave a nutrient dish open long enough and some of them will land and grow. A plate covered in colonies after 48 hours is the expected result in most homes, including homes that don’t have a problem.

The dish also plays favorites. Molds that grow quickly on that particular medium spread across it and crowd out the others. Some of the molds most tied to long-term water damage grow slowly or poorly on a standard plate, so they may never show up at all, even when they’re in the house.

Why a clean dish doesn’t clear the house either

A plate with little growth can be just as misleading. Mold growing inside a wall cavity or under flooring may release very few spores into the room. Stachybotrys, the mold people usually mean by “black mold,” has heavy, sticky spores that don’t stay airborne well. A settle plate on the kitchen counter can’t reach either one. The risk with a clean result is false reassurance: a real problem left in place because the kit said the air looked fine.

Some kits offer to have the lab identify whatever grew on the plate, for an added fee. That tells you the names of the molds on the dish. It still can’t turn an open plate into a concentration, and it still has no outdoor sample to compare against, so the core problem remains.

When a kit is reasonable

• Identifying a visible spot. A tape or swab kit analyzed by an accredited lab can confirm whether a stain is mold. That’s a narrow question, and the kit answers it.
• Dust DNA screening. Several labs sell mail-in ERMI and HERTSMI-2 kits. Follow the collection instructions exactly, because the result depends on them.
• Curiosity with no warning signs. A settle plate won’t hurt, as long as you don’t spend money on remediation because of it.

A better next step than a second kit

Before buying another kit, spend twenty minutes looking for moisture, because that’s what an inspector does first. Check under sinks, around toilets and tubs, along basement walls and below upstairs bathrooms. Put a simple hygrometer in the rooms that worry you. The EPA recommends keeping indoor humidity below 60 percent, ideally between 30 and 50 percent. A musty smell, staining or a humidity reading that stays high is a better reason to test than a grown-over dish.

If you do move to professional testing, the report will show counts compared with outdoor air, which is a different kind of number. Our answer on what mold spore counts mean explains how to read one.