DIY Mould Test or Lab Test? An Honest Comparison

A DIY mould test kit from the DIY store costs 10 to 40 euros and takes minutes to set out. An air sample with lab analysis costs several times that and needs a pump. Why go to the trouble – and when is the cheap kit actually enough?

This article compares the three usual routes: what each test really answers, where it goes wrong, what it costs and when you need which one. The limits of our own method are in here too – no test does everything.

In short: petri dishes and quick tests show that mould is present – and in small amounts it always is. Whether your air carries more, or something different, than normal can only be answered by an air sample with a defined volume, an outdoor comparison and analysis in a lab. For simple curiosity the kit will do. As soon as the result has to decide something, you need the lab.

Three routes to a mould test

Anyone who wants to do a mould test themselves will find essentially three routes:

1. Petri dish (settle plate). A dish of nutrient medium stands open in the room for a set time, is then closed and incubated for a few days at room temperature. Whatever fell out of the air onto the medium grows into visible colonies. This is the classic mould test kit from DIY stores and online shops; some suppliers will analyse the grown plate in a lab for an extra fee.
2. Quick tests, contact slides and swab kits without a lab. A test strip, a contact slide or a swab is pressed onto the suspect spot. The result is a colour change after minutes or growth after a few days – a yes or no for exactly that spot.
3. Air sample with accredited lab analysis. A pump draws a defined volume of air through a sampling cassette and the spores stick to a coated slide. An accredited laboratory counts and identifies them under the microscope. You can take the sample yourself; the analysis you cannot.

The decisive difference is not the price but the question the test is able to answer at all.

What each test actually answers

The petri dish answers: “Does anything grow from what landed on the plate in that time?” The answer is almost always yes. Mould spores are a normal component of outdoor air and are always present in indoor air – so says the mould guideline of the German Environment Agency (UBA). How many spores per cubic metre are in the air, whether that is more than outside and which genera are behind it, the plate does not say.

The quick test or swab answers: “Is there mould on this spot?” That helps when you cannot place a dark stain. About the air in the room, hidden sources behind a wardrobe or wallpaper, or the genus, it says nothing.

The air sample with lab analysis answers: “How many spores of which genus per cubic metre are in this room – and how does that compare with the outdoor air?” The lab reports spores/m³ per genus, the indoor-versus-outdoor comparison and an elevation rating. That comparison is the basis of every professional assessment, because there is no legal limit value for mould spores in indoor air – neither in Germany nor in the UK.

Typical sources of error in DIY tests

The UBA guideline lists settle plates explicitly among the measurement methods it does not recommend: they do not deliver reproducible results. The reasons:

• No defined air volume. An open plate catches whatever happens to sink. Large, heavy spores land quickly; small, light ones – precisely the typical indoor genera such as Aspergillus and Penicillium – stay airborne for a long time. Draughts, the height of the plate and movement in the room change the result.
• No reference values. Twelve colonies – is that a lot? Without reference to an air volume, the number cannot be compared with anything. Rating tables in the kit are the manufacturer's figures, not a recognised rule of practice.
• No outdoor reference. In summer the outdoor air is full of spores. A plate next to a tilted window mostly shows you the garden.
• Growth on the plate is not growth in the room. The nutrient medium favours fast-growing species; others do not grow on it at all. Dead spores and fungal fragments – allergenic too – never show up.
• Mould is everywhere. Every plate will grow something. The question is never “is there mould?” but “is there more, or something different, than normal – and why?”

Much the same applies to quick tests: the guideline rates biochemical and molecular quick methods as not yet sufficiently validated and lists contact plates as not recommended for indoor rooms either. And without a site inspection by professionals, the UBA says, a proper assessment of mould growth is fundamentally not possible – that applies to every self-test, including ours.

Cost and time compared

The prices are far apart – and so is what you get. For the air sample, allow at least two samples: one in the room in question and one outdoor sample – without it the lab cannot assess anything.

A site visit by a surveyor comes on top where needed.

When a DIY test is enough – and when it has to be the lab

Honestly: often you do not need a test at all. If you can see the mould, the question is not “whether” but “why”.

Small, superficial growth under about half a square metre with a known cause can, according to the UBA, usually be removed by the occupants themselves – provided you do not suffer from allergies or respiratory conditions.

A DIY test is enough when you want to place a visible stain (mould or dirt?) and the answer has no consequences – or when you are simply curious and know the limits of the method.

A lab test is needed as soon as the result has to decide something:

• Evidence. Towards a landlord, managing agent, insurer or a court, what counts is a documented sampling with a defined method, an outdoor reference and a lab report – a grown petri dish has no evidential value. How to proceed as a tenant is covered in Mould: tenant rights and evidence; for legal steps, turn to a tenants' advice service or a solicitor.
• Health complaints. Airway irritation that is worse at home than elsewhere belongs with a doctor. An air sample gives that conversation a factual basis – it is not a diagnosis.
• Clearance after remediation. Whether the remediation worked cannot be seen on a repainted wall – a clearance test compares indoor air, outdoor air and an unaffected room.
• Hidden mould. A musty smell but nothing to see: elevated levels of typical moisture-indicator genera in the air point to a source behind cladding, screed or wallpaper.
• Buying or selling a property. A documented condition before handover saves the argument later.

How the AirTrap mould test works

AirTrap is the third route: you take the sample yourself, and the accredited lab does the analysis. The complete set contains the pump, cassettes with the analysis included and the prepaid return envelope.

1. Set the pump up in the middle of the room, insert the AirTrap XL cassette, remove the caps, start the timer: 15 l/min for 5 minutes, i.e. 75 litres. Close the caps and label the cassette with the room and the time. The steps in detail: step-by-step sampling guide.
2. Take an outdoor sample the same day – same method, same duration. Without it there is no assessment.
3. Register the samples online and post them in the return envelope.
4. The slides are analysed in the ISO/IEC 17025-accredited Sporecyte laboratory: AI-assisted spore identification, every result checked by a specialist. The report arrives by e-mail within 48 hours of the samples reaching the lab. How the analysis works is described in Lab analysis; how to read the numbers is explained in Understanding your mould lab report.

This method has limits too. The sample is a snapshot – five minutes on one day, dependent on ventilation, season and weather. Identification is to genus level, not species: Aspergillus and Penicillium are reported together, and viable spores are not distinguished from dead ones.

And the report does not replace a site inspection: it shows whether and how strongly the air is affected – the cause is found by a competent person on site. The whole process is on How it works.

Frequently asked questions about mould tests

Is a mould test from the DIY store reliable?
As a yes/no for a visible spot: usable. As a statement about the air in your home: no. The German Environment Agency does not recommend settle plates: no reproducible results, no defined air volume, no outdoor reference – and something grows on every plate.

What does a lab mould test cost?
DIY kits cost 10 to 40 euros. An air sample with accredited analysis from AirTrap starts at around 60 euros per sample in the refill pack; the complete set with the pump and cassettes starts at around 800 euros in the EU shop. A typical flat needs two to four samples including the outdoor sample.

Can I take the air sample myself?
Yes – that is exactly what AirTrap is made for: you sample following the guide, the accredited lab analyses. What matters: outdoor sample on the same day, windows closed for a few hours beforehand, a label on every cassette. If the result has to stand up in court or towards an insurer, it is better to have an independent surveyor take and document the samples.

How do I test my house for mould when there is nothing to see?
One air sample per suspect room, plus an outdoor sample and, if possible, an unaffected reference room. If the report shows clearly more, or different, genera in one room than outside and in the reference room, that room has a source – and the search for the cause begins on site.

Are there limit values for mould spores in indoor air?
No – neither in Germany nor in the UK. Assessment is comparative: indoor against outdoor, conspicuous genera, reference rooms. That is why a test without an outdoor reference cannot be assessed – whatever the method.

See complete sets   How it works

Written by the AirTrap team. Sources: UBA guideline on the prevention, detection and remediation of mould in buildings (2017, updated 2024), DIN ISO 16000-19 and DIN ISO 16000-20. This article does not replace medical advice, legal advice or the assessment of a competent person.