Mould test kits are sold in New Zealand hardware stores and online for somewhere between the price of a coffee and the price of a takeaway. The kit is not the problem. The problem is what you do with the result, because most kits answer the wrong question. This guide sets out what each type of kit actually measures, which one is genuinely worth posting to a laboratory, and the point at which a kit stops being useful and a professional assessment starts being the cheaper option.
The three kinds of kit sold in New Zealand
- Petri dish (culture) kits. A dish of growth medium is left open in a room for an hour or so, sealed, kept warm for a few days, and then inspected for colonies. Some brands let you post the dish to a laboratory for a fee. These are also sold as "air test kits".
- Swab kits. A sterile swab is wiped across a surface and sealed in a tube. A few come with a colour-change strip that claims to detect mould on the spot; most are meant to be posted to a laboratory.
- Tape-lift kits. A piece of clear adhesive tape (or a pre-mounted adhesive slide) is pressed onto the growth, lifted off, and posted to a laboratory to be examined under a microscope. This is the same method a professional uses for surface sampling.
The first type is a different thing from the other two. It grows mould. The other two collect it for identification. That difference decides whether the result means anything.
What a petri dish kit actually measures
A petri dish left open on a bench is a settle plate. It collects whatever spores happen to drop onto it by gravity in the time it is open. It does not draw a known volume of air, so it cannot give you a concentration, and it has nothing to compare against. Every occupied building in New Zealand has airborne mould spores, all year round, because outdoor air is full of them. So the dish will grow something. That is not a finding; it is a certainty.
Culture also favours some moulds over others. Fast-growing genera such as Penicillium and Aspergillus race across the medium and crowd out slower ones. Some of the genera that matter most in a water-damaged building, Stachybotrys among them, grow poorly or not at all on a general-purpose kit medium. A fuzzy, colourful plate looks alarming and tells you almost nothing about the room it sat in.
In our experience the settle-plate kit is the one most likely to leave a household more worried and no better informed. If you have already used one, do not read too much into it either way.
Swab and tape kits: the DIY route that works, if a laboratory reads them
A tape lift examined under a microscope shows the analyst the mould as it sits on the surface: spores, the hyphae that connect them, and the structures that produce them. That is how a laboratory tells active growth from spores that have simply settled out of the air. Surface samples are reported as fungal structures per square centimetre, the genera present are identified, and the analyst's notes say what the picture under the lens actually looked like. No growth medium, no waiting for colonies, and no guesswork about what would have grown if given the chance.
This is why our own post-in test is a tape lift, and there are two ways to do it. The cheapest is to take the lift yourself with ordinary clear household tape, following the one-page instructions (PDF), and post it to us: $150 + GST for the first sample and $100 + GST for each additional one. If you would rather have purpose-made slides, or there are a few spots to check, we post a test kit of three Bio-Tape slides for $250 + GST with the analysis of all three included. Either way the sample is read in our Hamilton laboratory with the result back in three business days, and the chain-of-custody form that travels with it is what makes the result usable with a landlord, an insurer or a builder rather than just a curiosity.
Tape lifts do have limits. A tape lift only tells you about the spot you pressed it onto, and it tells you what is growing, not why. If the mould is behind a wall lining or under a floor, no surface kit will reach it.
Air testing is not a kit job
A proper mould air test draws a measured volume of air through a spore-trap cassette with a calibrated pump, and it is always paired with an outdoor reference sample taken the same day, because an indoor count only means something against the air outside. That equipment is not in a kit, and a kit that describes leaving a dish open as "air testing" is describing a settle plate. If the question is about the air rather than a visible patch, read how mould air testing works and what it can and cannot tell you.
When a kit stops being the cheap option
A post-in tape lift is the right first step when there is a small patch of visible mould and the question is simply "what is this?". It stops being the right tool when the situation is any of the following:
- Someone in the household has respiratory symptoms, or is immunocompromised
- The affected area is larger than about one square metre, or there is more than one affected room
- The mould keeps coming back after cleaning
- There is a musty smell but nothing visible, which usually means the growth is concealed
- You are buying or selling the property, or the matter involves a tenancy dispute or an insurance claim
- Remediation has been done and you need to confirm it worked
In those cases a kit produces one data point, and the argument that follows is about everything the kit did not measure. A professional mould testing visit takes air and surface samples with an outdoor reference, maps the moisture that is feeding the growth, and reports each area against the ANSI/IICRC S520 condition scale. A typical residential assessment is $800 to $1,200 ex GST, and it settles the question in one report rather than in a series of kits. If the kit has already been posted and the result is elevated, that report is usually the next step anyway.
A simple rule of thumb
- Visible patch, one room, no symptoms: post a tape lift to a laboratory. Skip the petri dish.
- Smell without a patch, symptoms, or more than one room: book an assessment, because the answer is not on a surface you can reach.
- Money, tenancy or insurance on the line: get the assessment first. The report is the evidence.