Reading a challenge test result
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A challenge test report is a table of colony counts and it is not self-explanatory. The useful reading is in the shape of the decline rather than in the final number.
What the report gives you
Five organisms, each inoculated separately, with counts at days 0, 2, 7, 14 and 28. Day 0 is what went in. Everything after is what your preservation system did about it.
Bacteria and fungi are assessed against different expectations, because they behave differently: bacteria should fall quickly and fungi are judged more on not increasing.
What a pass looks like
A steep early decline, then nothing recovering. The organisms should be dropping by day 2 and substantially gone by day 14, with day 28 confirming that nothing came back.
That last part matters more than it looks. A system can knock a population down and then exhaust itself, and the count at 28 days is what catches it.
Marginal is not a pass
A result that scrapes the criteria in a laboratory, from a freshly made sample, in a sealed container, is a result with no margin left for reality: a slightly different raw material lot, a filling room that was warmer, a customer's bathroom.
Treat marginal as a signal to change something rather than as permission to ship. Increasing the preservative is one option and usually not the best one; raising water activity discipline, adjusting pH into the range where your preservative is actually active, adding a chelator, or moving to a package that removes finger contact are all available.
What to do with a failure
Do not ship it, do not increase the dose and hope, and do not assume the preservative is at fault. A failure is a system result: pH may have moved it out of its active range, a metal ion may be present, the water activity may be higher than assumed, or the package may be contributing.
Change one variable, remake, and retest. It is slow and it is the only method that tells you what actually fixed it, which is what you will need when a raw material changes next year.