The K-number doesn't track today's device

A 510(k) clearance record reads like a fixed description of a device: a K-number, an applicant, a product code, a decision date. It is tempting to read it as current — as if the device sold under that number today is the device FDA reviewed on the date the letter was signed. For a device that has been on the market for years, that reading is often wrong, and it is wrong for a reason the regulation states on purpose, not by accident.

The threshold the rule actually sets

21 CFR 807.81(a)(3) requires a new premarket notification only when a device already in commercial distribution is about to change in one of two specific ways: a change or modification that could significantly affect the device’s safety or effectiveness — design, material, chemical composition, energy source, or manufacturing process are the rule’s own examples — or a major change or modification in the device’s intended use. Anything a manufacturer changes that stays under both of those bars triggers nothing. No new K-number, no new decision date, no new record of any kind.

The decision that never leaves the building

Deciding which side of that threshold a specific change falls on is the manufacturer’s own call, made under a structured, risk-based framework FDA laid out in its 2017 final guidance on deciding when to submit a 510(k) for a device change — itself a replacement for guidance FDA had run since 1997. When the firm’s own analysis concludes a new 510(k) isn’t required, standard practice is to write that conclusion down: a letter to file, sometimes called a memo to file, recording what changed, why, and the reasoning that kept it under the regulatory threshold. FDA does not receive that document when it is written. 21 CFR 820.35 governs the records a device manufacturer’s quality system has to control and keep available — readily accessible for FDA review, which is a requirement that a letter to file exist and hold up, not a requirement that it be filed with anyone in advance.

A 510(k) clearance describes the device FDA reviewed on the day the letter was signed. It says nothing about whether that description still matches what shipped last week — only that if it doesn’t, the gap stayed under a threshold the manufacturer measured itself.

What the public record was never built to carry

The 510(k) database entry for a given K-number doesn’t change after clearance. It carries no field for a subsequent letter to file, no change log, no count of how many times a firm has run this analysis since the decision date. Two devices selling under the same K-number today — one whose manufacturer has documented five design or manufacturing changes since clearance, one that hasn’t touched the device at all — look identical from the public side. The database was built to record a clearance decision, not to track a device’s condition afterward.

The practice

This cuts two ways for a regulatory or quality operator. On your own device, the letter to file is the document an inspector asks for by name during a design-control or CAPA review — §820.35 requires that it exist and be available, which means the rationale has to be written and defensible at the time of the change, not reconstructed afterward under questioning. On a competitor’s device, or on a predicate you’re citing in your own submission, reading an old K-number as a live specification is the mistake this rule exists to explain: the clearance is a floor the device met once, not a running record of what it is today. FDA Radar’s own 510(k) corpus mirrors the clearance record exactly as openFDA publishes it — K-number, applicant, product code, decision date — because that is the public record; a device’s later, internal change history was never going to be in it, because no source publishes that.

Whether a specific change to a specific device crosses the 807.81(a)(3) threshold is a risk-based judgment for that device’s own manufacturer to make and document, under FDA’s guidance and their own regulatory professional’s reasoning — regulatory intelligence and method, never a determination this post makes for anyone’s specific device.