A Not Substantially Equivalent letter can read like a verdict on the device itself. It isn’t one. FDA’s NSE determination is a finding about one specific comparison — your device against the predicate you cited, on the record you submitted — and reading it correctly is the difference between a wasted year and a clear next move.
What the letter is actually saying
The useful information in an NSE letter is which basis FDA cited, because it determines what fixes the problem. A different intended use is a harder gap to close than a technological difference FDA felt raised unanswered safety questions — the first often means the predicate itself was the wrong choice, while the second can sometimes be resolved with additional testing against the same predicate. A finding that the submission simply didn’t include enough data to establish substantial equivalence is different again, and often the easiest to fix. Read the letter for which of these it is before deciding what to do next — the same care that goes into choosing a predicate in the first place applies here in reverse.
The three ways forward
A new 510(k) with a different or additional predicate, or the same predicate with data addressing the specific gap FDA cited, is the most direct path when a suitable predicate genuinely exists. A De Novo request under Section 513(f)(2) fits when there’s no appropriate predicate at all and the device’s risk profile is low to moderate — often faster and less costly than the third option, a PMA, which is the right path when the device is legitimately higher risk and Class III designation reflects that risk accurately rather than just the absence of a predicate.
Where people get stuck
Resubmitting unchanged and hoping for a different reviewer
The NSE letter names specific deficiencies. A resubmission that doesn’t address them directly tends to draw the same result.
Reading NSE as “our device is unsafe”
It’s a comparison determination, not a safety finding on the device in isolation. Conflating the two can send a team toward the wrong fix.
Defaulting to a PMA out of frustration
When De Novo was genuinely available, a PMA is a much larger commitment than the situation required. Evaluate the risk profile honestly before choosing the heavier path.
An NSE letter is disappointing news delivered in careful, specific language, and the specificity is what makes it usable. Read it for exactly what it says about the comparison FDA actually reviewed — not the device generally — and the next step is usually clearer than the letter first feels. It’s also a reminder of how much weight the predicate choice and the underlying product classification carry before a submission ever goes in.
Sources & further reading
- 21 CFR 807.100 — FDA Action on a Premarket Notification Submission ecfr.gov
- FD&C Act — Section 513(f), Classification of Devices, 21 U.S.C. § 360c(f) fda.gov
- Regulatory Academy — How to Read a 510(k) Summary regulatoryacademy.com
- Regulatory Academy — How to Read an FDA De Novo Classification Order regulatoryacademy.com
- Regulatory Academy — How to Choose a 510(k) Predicate Device — and Defend It regulatoryacademy.com
This essay is provided for general educational purposes and reflects the regulatory landscape as of its publication date. It is not legal, regulatory, or career advice.
