When a lab report says your air sample was run by direct microscopy, ASTM D7391 is usually the method behind it. Knowing what that method does — and what it cannot do — keeps you from over-reading a spore-trap result in your narrative.
What is ASTM D7391 spore trap analysis?
ASTM D7391 is the standard test method for categorizing and quantifying airborne fungal structures collected by spore-trap (impaction) sampling and counted by direct microscopy (ASTM International, D7391). It is the analytical procedure a lab follows when you submit an Air-O-Cell-type cassette: the captured material is mounted, examined under a microscope, and the spores are identified to genus or group and counted to produce a concentration in spores per cubic meter.
The defining feature is "direct microscopy." The analyst counts what is on the slide, viable or not. That makes it fast and inclusive of dead spores and fragments, which is why it is the dominant air method for mold screening — but it also sets the limits described below.
How is direct microscopy different from a culturable analysis?
D7391 counts total spores by sight; a culturable (viable) analysis grows what can grow. The two answer different questions, and a report should not treat them as interchangeable. Direct microscopy under D7391 captures everything that landed on the trap — living spores, dead spores, and fragments — and reports a total structure count. A culturable method grows colonies on media and reports only organisms that were alive and able to grow, identified often to species.
Direct microscopy is faster and cheaper and does not miss non-viable material, which matters because dead spores still carry allergens. Its tradeoff is resolution: many genera that look alike under the microscope get grouped, the classic example being the combined "Aspergillus/Penicillium" category, because their small round spores are not distinguishable by light microscopy alone. If a report needs species-level identification, culturable analysis or molecular methods are the right tools, not D7391.
How do you cite ASTM D7391 in a report?
Cite it as the analytical method, attributed to the lab. The accurate phrasing is that the air samples were analyzed by direct microscopy per ASTM D7391, and the specific edition lives on the lab's report — confirm it there rather than guessing a year. The lab's own chain-of-custody and analytical report is your primary citation for the method actually used; your job is to report it faithfully, not to assert a method the lab did not run.
ASTM standards are purchased documents, so quote only the method designation and scope you can verify from the lab report or the publicly accessible ASTM abstract — never invent a clause. The AIHA mold reference and the EPA mold resources are appropriate freely accessible companions for the interpretation side, where D7391 itself is silent (AIHA, Recognition, Evaluation, and Control of Indoor Mold; EPA, Mold and Moisture).
What does a D7391 result not tell you?
A D7391 spore count is a snapshot of airborne structures at one place and time, and it sets no pass/fail line. The method quantifies; it does not interpret. There is no ASTM, EPA, or federal numeric threshold that turns a spore count into a result (EPA, Mold and Moisture). Interpretation comes from comparison — indoor versus a same-day outdoor control, the presence of indicator organisms, and the building conditions — which is the AIHA and ACGIH framework, not D7391.
So a report that says "the D7391 result exceeded the acceptable limit" is asserting a limit that does not exist. The defensible reading describes the indoor-to-outdoor relationship and flags indicator species, using the count as evidence rather than a verdict. MoldMind ingests the lab's structured results — method, species or group, count per cubic meter — and builds the indoor-versus-outdoor comparison into the analysis rather than leaving a raw count to speak for itself. See the sample report and the spore-trap concept guide for how methods map to questions.
Sources
- ASTM International, D7391 Standard Test Method for Categorization and Quantification of Airborne Fungal Structures — direct-microscopy spore-trap analysis.
- EPA, Mold and Moisture — no federal numeric mold limit; interpretation is comparative.
- AIHA, Recognition, Evaluation, and Control of Indoor Mold — interpretation framework companion.