You Received an AINN Letter from FDA. Here's What It Means — and What to Do Next.

If you've received an Additional Information Needed (AINN) letter from FDA during the review of your medical device submission, you're not alone — and you're not necessarily in trouble. But how you respond will determine whether your path to clearance or approval stays on track.

What Is an AINN Letter?

An AINN letter — also called an AI Hold letter or Additional Information (AI) Request — is FDA's formal mechanism for requesting clarification or additional data when deficiencies are identified during substantive review. It is most commonly encountered in 510(k) reviews, but the same mechanism applies to De Novo requests. For PMAs, FDA uses related but distinct letter types at the decision stage, though AI Holds can also occur mid-review.

For 510(k) submissions, the AINN letter typically arrives around day 60 of the 90-day review clock. When it does, the clock stops. You then have 180 calendar days to submit a complete response. No extensions are granted. Once FDA receives your response, the review clock resumes.

Receiving an AINN doesn't mean your submission failed. It means the FDA is engaged and has specific concerns that need to be addressed. That said, the quality of your response matters enormously.

Common Statistical Reasons for an AINN Letter

From a biostatistics standpoint, AINN letters frequently cite inadequate sample size justification, missing data handling issues, performance goal methodology concerns (unclear derivation of the OPC or performance goal), protocol deviations and inadequate sensitivity analyses, endpoint definition or analysis population concerns, and software or algorithm validation questions.

What to Do When You Receive One

1. Read it carefully — all of it. FDA's questions are often layered. A single item may contain multiple sub-questions. Map every question explicitly before drafting a single word of your response.

2. Don't rush — but don't wait. You have 180 calendar days to respond. Build your response timeline from day one.

3. Consider requesting a clarification meeting early. The AINN letter typically gives you 10 days to request a clarification meeting. For complex statistical issues, consider submitting a Submission Issue Request (SIR) — a type of Q-Submission that allows you to seek FDA's feedback on your planned approach before committing to a full written response.

4. Answer every question directly. Restate each question. Answer it directly. Provide supporting data or analysis. A response that buries the answer in narrative will generate a second AINN.

5. Don't introduce new problems. Have your biostatistician review the full response before submission, not just their section.

On the Statistical Response

If FDA's questions are statistical in nature, the response needs to go beyond narrative explanation. Depending on the question, you may need to re-run analyses with corrected methodology, provide simulation results supporting a revised power calculation, submit a revised Statistical Analysis Plan with tracked changes, provide a literature review supporting your performance goal derivation, or conduct and report sensitivity analyses that were missing or inadequate.

Vague assurances that your approach was "appropriate" will not satisfy a statistical reviewer. Show your work.

What FDA Needs to Close the Issue

A strong AINN response doesn't just answer the question asked — it anticipates the follow-up. For each deficiency: provide a direct answer stated plainly, supporting analysis or data, revised document language with tracked changes where applicable, and an explanation of how the revision affects downstream results if relevant.

If your original submission had a material flaw, acknowledge it directly and present the corrected approach.

BSC's Role in AINN Responses

At Biomedical Statistical Consulting®, we've helped sponsors navigate AINN letters across 510(k), PMA, and De Novo pathways. We review the letter, identify the underlying concern behind each question, develop the statistical response strategy, execute any required analyses, and draft the formal response language — all with an eye toward what FDA actually needs to see to close the issue.

If you've received an AINN letter with statistical components, time matters. Contact BSC and let's work through it together.