What a COA is for
A certificate of analysis is the lab report for the specific batch in your hand. In licensed markets it is not optional: a batch cannot be sold until an accredited third-party laboratory has tested it and the results have passed the state's thresholds.
It exists to answer two separate questions. What is in this, and is anything in it that should not be? Most people read only the first half.
Check the identifiers first
Before any result matters, confirm the document describes the thing you bought. A COA is batch-specific, and a producer with twenty batches has twenty certificates.
Match the batch or lot number on the package to the one on the report. Check the sample date — flower degrades, and a certificate from fourteen months ago describes a product that no longer exists in quite that form. Check that a named, accredited laboratory issued it, not the producer's own facility.
The cannabinoid panel
This is the potency section. Expect THCA and delta-9 THC listed separately, with a calculated total; the same treatment for CBDA and CBD; and often minor cannabinoids such as CBG, CBN and CBC.
Two conventions are worth knowing. Results are given per gram for flower and concentrates, but per package or per serving for edibles — reading one as the other is how people take ten times what they meant to. And "ND" means not detected, while "<LOQ" means present but below the limit of quantitation, which is not the same as absent.
CBN is worth a glance for a different reason: it forms as THC oxidises. A high CBN reading on flower is a reasonable proxy for age or poor storage.
The safety panels are the point
Four screens matter here, and a pass on each is the actual value of the document.
Pesticides. State lists run to dozens of compounds with individual action limits. This is the panel that most often fails, and the one least visible to a buyer without a report.
Residual solvents. Relevant to anything extracted with butane, propane or ethanol. Solventless products — rosin, dry sift, ice water hash — should show nothing, and if they do, something has gone wrong upstream.
Heavy metals. Cannabis is a hyperaccumulator: it draws lead, cadmium, arsenic and mercury out of soil and concentrates them. This panel is about where the plant grew.
Microbials and mycotoxins. Yeast, mould, aspergillus, E. coli, salmonella. Most relevant for flower, and the reason poorly cured product is a health question rather than a taste one.
Terpenes, when they bother
Terpene analysis is optional in most states, so its presence signals a producer who expects to be read closely. Totals usually fall between 1% and 4%, with the top three accounting for most of the aroma.
This is the section to read if you are trying to work out why you liked something, because it is the part that varies audibly between two jars with identical potency.
Reasonable suspicion
A few things should slow you down. A certificate with no laboratory name or accreditation number. Potency reported without any safety panels at all. A sample date long before the packaging date. An image of a report rather than a document you can verify — most labs publish results against a sample ID, and a producer confident in the batch will tell you where to look.
None of these prove anything is wrong. They do tell you how much the producer expects to be checked.