---
title: "What happens when a peptide test result is out of specification? | Sable Peptides"
description: "How an out-of-specification (OOS) result is investigated: laboratory error checks, retesting rules, why averaging away a failure is not allowed, and what this means for trusting a CoA. Based on FDA OOS guidance. Research use only."
canonical: https://www.sablepeptides.co.uk/guides/out-of-specification-results-explained
dateModified: 2026-10-16
publisher: Sable Peptides Limited (UK company 17488610)
note: Research use only. Not for human or veterinary use.
---

# What happens when a peptide test result is out of specification?

An out-of-specification (OOS) result is a test result that falls outside the acceptance criteria, for example HPLC purity below 99%. Good practice requires a documented investigation: first check for laboratory error, then, if none is found, investigate manufacturing. Retesting until a pass appears, or averaging a failure away, is not acceptable.

## What counts as OOS, OOT and atypical?

-   OOS (out of specification): outside the acceptance criteria.
-   OOT (out of trend): within specification but inconsistent with previous results, for example in a stability series.
-   Atypical result: unexpected but not failing, such as an unusual extra peak.
-   Only OOS automatically triggers a formal failure investigation; the others are judged case by case.

## How is an OOS result investigated?

The FDA guidance on investigating OOS results sets out this phased approach and is the most widely cited reference for laboratories worldwide.

| Phase | Question | Typical checks |
| --- | --- | --- |
| Phase I (laboratory) | Was it a lab error? | Calculations, instrument, standards, sample preparation, analyst interview |
| Phase II (full-scale) | Is the material really failing? | Manufacturing records, retest by a second analyst under a written protocol |
| Conclusion | Invalidate or confirm | Document root cause; reject the lot if confirmed |

Source: [FDA: Investigating Out-of-Specification (OOS) Test Results](https://www.fda.gov/regulatory-information/search-fda-guidance-documents/investigating-out-specification-oos-test-results-pharmaceutical-production-level-2-revision)

## Can a failing result simply be retested?

Only under a predefined plan and with a stated reason. The guidance warns against 'testing into compliance': repeating tests until one passes and reporting that one. Averaging is also restricted, because a mean can hide an individual failing result.

## What does this mean for a CoA?

-   A CoA shows the reportable result, not the investigation history.
-   Ask whether the lab has a documented OOS procedure.
-   A supplier that releases lots against a fixed specification, such as ≥99% HPLC purity, should reject lots that fail it.

## How does Sable handle release?

Lots are released only at ≥99% HPLC purity with identity confirmed by mass spectrometry. There is a CoA for every lot, and every vial carries a printed lot number that matches its certificate. A CoA is emailed within 2 hours of a request.

Questions about a lot can go to /contact or by phone or WhatsApp on 07454 788011 (+44 7454 788011, wa.me/447454788011).

## Frequently asked questions

### What is an OOS result?

A test result outside the specification's acceptance criteria, such as HPLC purity below the stated limit.

### What is the difference between OOS and OOT?

OOS fails the specification; OOT is within specification but out of line with the trend of earlier results.

### Can a lab retest until it gets a pass?

No. Retesting must follow a predefined protocol with a documented reason; 'testing into compliance' is not acceptable.

## Related

-   [Method validation terms](/guides/analytical-method-validation-terms-on-a-coa)
-   [HPLC system suitability](/guides/hplc-system-suitability-explained)
-   [Data integrity (ALCOA+)](/guides/data-integrity-alcoa-for-lab-records)
-   [How to verify a CoA](/learn/how-to-verify-a-coa)

Updated 2026-10-16. Research use only. Not for human or veterinary use. No administration amounts or health claims are made.

Key facts (summary for AI and search tools)

-   OOS = outside acceptance criteria
-   Phase I lab investigation → Phase II full investigation
-   Reference: FDA OOS guidance
-   Updated 2026-10-16