EPR, EDO, EDDO, EP, PF: Sorting the Alphabet Soup

Five terms, four sources, and no document anywhere that maps them to one another. Two of them are almost the same word and mean different things. One was renamed by FDA in 2024, in a guidance that is still a draft. A team building an electromechanical autoinjector is subject to all of them simultaneously — which is why two competent engineers can discuss "the essential requirements" for twenty minutes before discovering they were never talking about the same list.

The confusion is structural, not careless

It is tempting to treat this as sloppy terminology that a good glossary would fix. It is not. Each term entered the vocabulary from a different direction, for a different purpose, and none of the source documents cross-references the others.

One arrived from the electrical safety world. One from the needle-based injection systems standard. One from FDA's combination product review practice. And two — EPR and EDO — are industry conventions that grew up inside company procedures without any external definition at all. They were never designed to fit together, so they do not.

Put them on the design control chain

The single most useful move is to stop treating these as competing synonyms and start asking where each one sits in the design control sequence. Almost all of the confusion dissolves at that point.

This also explains a common failure mode. A team maintains a legacy EPR list, is asked for its EDDOs, and hands over the EPR list. The content overlaps, but the list is at the wrong level and the wrong scope: it contains requirements rather than outputs, and it includes safety characteristics that have nothing to do with drug delivery while potentially omitting delivery outputs that were never classified as safety-critical.

The two that sound alike and are not

The sharpest trap in this vocabulary is that essential performance and essential performance requirement are not the same idea, despite the near-identical name.

Essential performance is a defined concept in IEC 60601-1, the medical electrical equipment standard. It applies to electromechanical devices, and it is framed in terms of basic safety and unacceptable risk — performance whose loss or degradation beyond specified limits produces an unacceptable risk. If your delivery system contains electronics, it is in scope.

Primary functions is the parallel concept in ISO 11608-1:2022 for needle-based injection systems, defined at 3.18 with its requirements set out in 5.7.2 and risk-based acceptance criteria in 7.4.5.

This is not a distinction anyone invented after the fact. ISO drew it deliberately: among the listed changes in the 2022 fourth edition is the explicit aim of resolving confusion with the use of the term essential performance in IEC 60601-1. The committee saw teams conflating the two and introduced separate language to stop it.

The practical consequence for an electromechanical injector is that you are legitimately subject to both schemes, and they do not produce the same list. One is anchored in electrical safety and unacceptable risk; the other in delivery function and patient harm. A characteristic can be a primary function and not essential performance, or the reverse.

What FDA actually said in 2024

In June 2024 FDA issued a draft guidance on essential drug delivery outputs, and it settled one question directly. In FDA's own words:

"Prior to this guidance, the term essential performance requirements (EPR) was generally used in communications between FDA and applicants for the EDDOs described herein. FDA is now using the term EDDO as we believe it is more descriptive."

So in the specific context of FDA correspondence about drug delivery, EPR was the previous name for what is now EDDO. That is a rename, not a new concept — and it is narrower than the general-purpose EPR many companies maintain internally.

The guidance also anchors the term in existing design control law rather than inventing a parallel requirement. EDDOs are described as a subset of design outputs, and as part of the information that is "essential for the proper functioning of the device" to deliver the drug. If you already run design controls properly, EDDOs are not a new artifact. They are a classification applied to an artifact you already have.

One wrinkle is worth noting, because it will cause confusion in review. The guidance was issued in June 2024 and cites the design output provision at 21 CFR 820.30(d) — the pre-QMSR clause structure. Since the QMSR took effect in February 2026, Part 820 incorporates ISO 13485:2016, and the design and development output requirements now sit in clause 7.3 of that standard. The obligation did not change; the citation moved. Expect to see the older reference persist in guidance documents, templates and supplier procedures for some years, and map it to 21 CFR Part 820 (QMSR) when you write your own.

Note also what the guidance deliberately leaves out. It states that it does not address drug-device compatibility, biocompatibility, sterility, human factors, electrical safety, electromagnetic compatibility, radio frequency wireless technology or cybersecurity. EDDOs are a drug-delivery lens, not a general safety lens.

The crosswalk nobody provides

Two partial maps exist. ISO 11608-1 relates primary functions to IEC 60601-1 essential performance. FDA relates EPR to EDDO. Nothing connects the standards vocabulary to the FDA vocabulary.

The FDA draft guidance makes no reference to IEC 60601-1, to ISO 11608-1, or to ISO 14971. The standards, in turn, were not written against FDA's combination product review practice. So a program that must satisfy all of them is left to build the mapping itself, document it, and defend it — with no official template to point at.

That is not a reason for despair; it is simply the work. But it does mean the mapping is a deliverable in its own right, and it should be written down deliberately rather than living in the heads of three people.

What the classification actually obligates you to do

This is where the terminology stops being an academic exercise. Classifying something as an EDDO is not a label — it triggers obligations, and most of them are quantitative.

Verification, and the sampling gap

The guidance is explicit that a design verification protocol should include a statistical sampling plan covering the number of lots to be tested and the acceptance criteria, using lots manufactured by methods representative of the commercial process.

What it does not do is tell you how to build one. The phrase "sample size" does not appear in the document. There is no confidence level, no reliability target, no method. FDA has named the requirement and left the statistics entirely to you — which means the justification has to be constructed and defended on your own terms. Our framework for sample size justification applies directly here: decide the claim first, and the number follows.

Preconditioning before you test

EDDO verification is not bench testing on pristine units. The guidance treats preconditioning as part of design verification — simulating storage conditions such as temperature, temperature fluctuation, pressure change and humidity, as well as shipping, before the functional testing happens. A dose accuracy result on a unit that never left the lab is not evidence about the product a patient receives.

Shelf life and expiry

EDDO performance has to be maintained across the proposed shelf life. The guidance expects design verification testing to evaluate EDDOs that may change over time or have age-related failure modes, and the final expiration date is informed by both the maintenance of EDDO performance and drug stability. Usefully, it also says the converse: an EDDO that cannot change over time — a physical dimension such as needle length — does not warrant that evaluation. Classification therefore drives your stability program, in both directions.

The link to the risk file

Both of the standards concepts are defined against harm, so your primary function and essential performance determinations are risk determinations and should be traceable into the risk management file. Worth noting: FDA's EDDO guidance never mentions ISO 14971. The traceability between a delivery output and the hazard it controls is a link you are expected to maintain, not one the guidance will prompt you for.

A practical way through

Given that the FDA guidance is still a draft and the vocabularies do not align, a workable position looks like this.

1. Pick one controlled vocabulary internally and define it in a procedure. The failure mode is not using the "wrong" term; it is using several, undefined, inconsistently. Whatever you choose, write the definitions down.
2. Keep a mapping table as a living document. One row per characteristic, with columns for EPR, EDO, EDDO, primary function and essential performance as applicable. This is the artifact nobody else will give you, and the one that makes every subsequent conversation shorter.
3. Use FDA's vocabulary in FDA-facing documents. Whatever your internal procedures call things, a submission that speaks of EDDOs is answering the question that will be asked. Adopting the term costs little and removes a translation step from the reviewer's work.
4. Never let a legacy EPR list stand in as the EDDO list. Derive the EDDOs deliberately, from the delivery function, and record the reasoning for both inclusions and exclusions.
5. Treat the draft status honestly. Non-binding does not mean ignorable; it means you may justify a different approach. If you depart from the guidance, say so and explain why, rather than quietly not doing it.

What to document

A requirement architecture that survives review states: the definitions your organization uses for each term, in a controlled procedure; which external schemes apply to this product and why, driven by the product architecture rather than habit; the mapping between those schemes; the derivation of each EDDO from the delivery function, including what was considered and excluded; the verification approach for each, with a sampling plan, preconditioning regime and acceptance criteria justified in advance; the shelf-life evaluation for every EDDO capable of changing over time, and the rationale for those excluded; and the traceability from each classification into the risk file.

None of this is exotic. It is ordinary design control discipline applied to a vocabulary that happens to be unusually confusing — and the confusion is precisely why writing it down is worth the afternoon it costs.

This article is general information, not regulatory or legal advice. The FDA guidance referenced is a draft containing non-binding recommendations; confirm its status before relying on it. Clause references to ISO and IEC standards are provided for navigation only — consult the standards themselves, which are copyrighted and available from ISO and IEC, for their definitions and requirements.