Short answer
A mold laboratory report lists the spore types identified in each sample and their concentration, usually in spores per cubic meter of air. The conclusion comes from comparing your indoor sample against the outdoor baseline taken the same day: indoor levels and spore mixes similar to outdoors generally suggest no distinct indoor source, while indoor levels substantially higher than outdoors — or dominated by water-indicating types that are scarce outdoors — point to an indoor moisture problem. The lab reports the numbers; interpreting them against the building is the assessor's job.
The report has two halves, and people read the wrong one
Every credible lab report contains raw data and, separately, an interpretation. Most people open it, find the largest number, and start worrying. The number by itself is close to meaningless.
The laboratory's job is to identify and count accurately. It does not know your building, whether your air handler was running, or that the sample was taken beside an open slider. That context is what turns a count into a finding, and it comes from the assessor, not the lab.
Sample identifiers and why locations matter
Each sample carries an identifier tied to a location — 'Sample 1: Master bedroom,' 'Sample 2: Exterior control.' Before reading any numbers, confirm the report includes an exterior or outdoor control sample. If it does not, the indoor numbers cannot be interpreted.
Check that the locations match where you actually had concerns. A report full of clean results from rooms you were never worried about does not tell you anything about the room you were.
Spore counts, raw counts, and what the units mean
You will usually see a raw count — how many spores of each type the analyst actually observed on the slide — and a calculated concentration, typically expressed as spores per cubic meter of air. The concentration accounts for how much air was drawn through the sampler.
Pay attention to the raw count alongside the concentration. A very low raw count extrapolated across a small air volume can produce a concentration figure that looks dramatic while resting on a handful of observed spores. Reputable labs note detection limits for exactly this reason.
'Non-detect' or 'ND' means none of that type were observed in that sample. It does not mean the organism is absent from the building.
Genus names, and the ones that suggest water
Results are typically reported by genus — Cladosporium, Penicillium/Aspergillus, Curvularia, Stachybotrys, and others. Spore-trap analysis usually cannot distinguish Penicillium from Aspergillus visually, which is why they are commonly grouped.
Some genera are extremely common outdoors and routinely appear indoors simply because outdoor air comes inside. Others are more strongly associated with sustained wet materials. Finding a water-indicating genus indoors at levels well above the outdoor baseline is a meaningful signal — not because of what the organism might do to a person, but because it suggests something in the building has been wet long enough to support it.
That is the correct way to read any genus result: as evidence about the building, not as a verdict about health.
The comparison that produces the conclusion
The analysis is fundamentally comparative. If indoor concentrations are similar to or lower than outdoors and the mix of types resembles the outdoor mix, indoor air is generally reflecting the outdoor environment. If indoor concentrations are substantially higher, or the indoor mix is dominated by types that barely register outdoors, that pattern points toward an indoor source.
A conclusion built on this comparison, cross-referenced against moisture readings and thermal imaging from the same visit, is defensible. A conclusion built on an indoor number alone is not.
Red flags that a report is being oversold
Be skeptical of any report that assigns health outcomes to your results. Laboratories analyze samples; they do not diagnose people, and neither do assessors. A report that tells you what your symptoms are caused by has left its lane.
Be skeptical of a report with no outdoor control, of 'mold-free' or 'safe' certifications, and of a report that arrives attached to a remediation quote from the same company. In Florida, assessment and remediation are separately licensed activities, and the same party is restricted from performing both on the same project — a combined report-and-quote should prompt questions.
Finally, be skeptical of urgency. A legitimate finding is still legitimate tomorrow. Pressure to sign immediately is a sales technique, not a building-science conclusion.
What you should walk away able to answer
After reading your report with your assessor, you should be able to state: whether indoor levels differ meaningfully from outdoors, whether a moisture source was identified and where, what specifically is recommended next, and what the report does not cover.
If you cannot answer those four, the report has not been explained to you properly yet — and asking is entirely reasonable. Sterling includes a call to walk through findings with every inspection for exactly this reason.