How to read a Certificate of Analysis (COA) for bacteriostatic water
A Certificate of Analysis (COA) reports the tests run on one specific batch, the method used for each, the acceptance criteria, the numerical result and a pass/fail conclusion, signed by an authorized person at a named laboratory. To read one, first match the batch number to the lot on your vial, then check that every test cites a method, shows a number against a limit and is dated and signed.
What is a Certificate of Analysis?
A Certificate of Analysis is a document that lists the tests performed on a particular sample, the results obtained and the acceptance criteria applied, followed by a statement of whether the sample complies with its specification [1]. The World Health Organization's model certificate notes that a COA is usually prepared for each batch of a substance or product [1]. It describes the lot in your hands, not the product line in general.
For bacteriostatic water, the COA shows whether a vial matches its label. The FDA-labeled prescription product, for reference, is described as containing 0.9% (9 mg/mL) or 1.1% (11 mg/mL) benzyl alcohol with a pH of 5.7 (4.5 to 7.0), and it is labeled Rx only [5]. Batch is a different product, sold for laboratory and research use only.
What are the parts of a COA?
Layouts vary, and WHO leaves the format to the issuing laboratory [1]. The content, though, should be recognizable. WHO's model lists these elements [1]:
- Issuing laboratory: name and address, a certificate ID and "page X of Y" numbering [1].
- Product identity: name, description, primary packaging and the original manufacturer's batch number [1].
- Dates: manufacture (if available), expiry or retest, and sample receipt [1].
- Manufacturer: the original manufacturer, plus any repacker or trader [1].
- Specification and method: for each test, the procedure reference and the acceptance criteria (limits) [1].
- Results: numerical where applicable, compared with the limit; subcontracted results identified [1].
- Conclusion and release: a pass/fail statement, dated and signed by an authorized person [1].
Read the specification and result columns side by side: the limit the batch had to meet, and what it actually measured.
Which tests should a bacteriostatic water COA report?
The panel depends on the manufacturer's specification. FDA guidance points to United States Pharmacopeia (USP) general chapters for these methods [2, 3, 4].
| Test | Typical method reference | What it measures | Why it matters |
|---|---|---|---|
| Benzyl alcohol identity and assay | Chromatography, e.g. GC or HPLC [7, 8] | That the preservative is benzyl alcohol, and how much is present | Confirms the labeled concentration, e.g. 0.9% (9 mg/mL) [5] |
| pH | Potentiometric pH measurement | Acidity of the solution | The FDA-labeled product states a pH range of 4.5 to 7.0 [5] |
| Sterility | USP <71> [2] | Whether viable microorganisms grow from sampled units | The primary check for microbial contamination |
| Bacterial endotoxins | USP <85> [3] | Endotoxin level, usually by a lysate-based test | Endotoxins are a separate concern from living organisms [3] |
| Particulate matter | USP <788> (subvisible); USP <790> (visible) [4] | Particles too small to see, and visible particles | Particle limits are a standard quality attribute of sterile solutions [4] |
| Appearance | Visual inspection | Clarity, color and visible particles | The FDA-labeled product says not to use it unless the solution is clear and the seal intact [5] |
Identity and assay of benzyl alcohol
Benzyl alcohol is routinely measured by gas chromatography; PubChem lists several GC-based analytical methods for it [7]. Reverse-phase HPLC with UV detection is also widely used for preservatives [8]. On the COA, look for a numerical result (for example, a percentage or mg/mL value) against a stated range, not just "conforms".
Sterility (USP <71>)
FDA describes USP <71> "Sterility Tests" as the principal source for sterility testing methods [2]. The test has limited sensitivity because only a small number of units are sampled; FDA gives the example that a 10,000-unit lot with a 0.1 percent contamination rate, tested with 20 units, has a 98 percent chance of passing [2]. If a sample inhibits microbial growth, the method has to be modified (for example by dilution, extra filter washes or inactivating agents) so that it does not give false negatives [2]. Benzyl alcohol is a preservative, so ask how the method was verified for a preserved sample.
Bacterial endotoxins (USP <85>)
FDA's guidance on pyrogen and endotoxins testing is built around USP <85> "Bacterial Endotoxins Test", which describes gel clot, photometric and kinetic methods [3]. FDA has accepted the Limulus amebocyte lysate (LAL) test for endotoxins in place of the older rabbit pyrogen test for decades [3]. A useful COA names the method type and reports a numerical result, in endotoxin units (EU) per milliliter, against the limit.
Particulate matter (USP <788>) and appearance
FDA describes subvisible particulates as those that cannot be seen with the naked eye and points to USP <788> for their control, while USP <790> covers inspection for visible particulates, with the goal that each lot is essentially free of them [4]. Appearance ties to the label instruction to use the solution only if it is clear [5]. For more on what a normal solution looks like, see is bacteriostatic water supposed to be clear?.
Does it matter who issued the COA?
Yes. A COA should name the laboratory that did the work [1]. Accreditation is the independent evaluation of a laboratory against recognized standards to confirm its impartiality and competence, and testing laboratories are accredited against ISO/IEC 17025 [6]. WHO's guidance says a laboratory accredited to ISO/IEC 17025 should reference that accreditation on the COA when it relates to the specific analysis [1]. If a repacker or trader reissues a certificate, it should still show the laboratory that performed the tests and the original manufacturer, with the original COA attached [1].
What are the red flags on a COA?
- No lot match: the batch number on the COA differs from the lot printed on your vial or carton.
- No methods: tests are listed with no method reference, even though WHO expects a reference to the procedure used [1].
- "Conforms" with no numbers: WHO expects results in numerical form where applicable [1]. "Pass" alone cannot be checked.
- Undated, unsigned, or no named issuing laboratory [1].
- One COA for every lot: the same certificate, with the same batch number or none, offered for different lots. A COA is meant to describe one batch [1].
How do I match the lot on my vial to the COA?
- Compare the lot number and expiry on the vial or carton, character for character, with the COA's batch number and expiry or retest date [1].
- Check that the product and packaging described match what you received [1].
- If anything differs, ask for the COA that belongs to your lot.
At Batch, COAs are issued per batch. To request one, send the lot number printed on your vial as described under Batch quality and testing, or contact Batch support. Specifications for Batch reconstitution solution are on the product page.
What does a COA look like in practice?
Illustrative example — not a Batch result. Columns follow WHO's model certificate [1]; bracketed values are placeholders.
| Test | Method reference | Acceptance criteria | Result | Complies? |
|---|---|---|---|---|
| Appearance | [Visual method ref.] | [Clear, colorless solution] | [Observed appearance] | [Yes / No] |
| Benzyl alcohol assay | [GC or HPLC method ref.] | [Lower limit to upper limit, mg/mL] | [Measured value, mg/mL] | [Yes / No] |
| pH | [pH method ref.] | [Lower to upper pH] | [Measured pH] | [Yes / No] |
| Bacterial endotoxins | [USP <85> method type] | [Limit, EU/mL] | [Measured value, EU/mL] | [Yes / No] |
| Sterility | [USP <71>] | [No growth] | [Observed result] | [Yes / No] |
| Particulate matter | [USP <788> method] | [Limits by particle size] | [Counts by particle size] | [Yes / No] |
Quick COA checklist
| Check | What good looks like |
|---|---|
| Batch number | Matches the lot on your vial exactly [1] |
| Dates | Manufacture (if available), expiry or retest, and approval dates present [1] |
| Laboratory | Named lab with address; accreditation referenced if it applies [1, 6] |
| Methods | Every test cites a procedure, e.g. a USP chapter [1] |
| Limits and results | Each result is a number shown against its limit [1] |
| Conclusion | Clear pass/fail statement for the batch [1] |
| Approval | Signed and dated by an authorized person [1] |
| Test panel | Covers assay, pH, sterility, endotoxins, particulates and appearance [2, 3, 4, 5] |
Key takeaways
- A COA describes one batch. Match the batch number to your vial first [1].
- Every test should show a method, a limit and a numerical result [1].
- Sterility, endotoxin and particulate tests usually cite USP <71>, <85> and <788> [2, 3, 4].
- Sterility tests have limited sensitivity; preserved samples need a verified method [2].
- Check who issued the COA and whether ISO/IEC 17025 accreditation covers the test [1, 6].
Frequently asked questions
What does "conforms" mean on a COA?
It means the result met the limit, but it does not show the value. WHO's model expects numerical results where applicable, so a number is more useful than a word for anything measurable [1].
Does a COA have to come from a third-party lab?
Not necessarily. WHO's model is written mainly for labs independent of the manufacturer, and notes that a manufacturer's own lab may keep some details in other quality documents [1]. What matters is that the issuing lab is named, subcontracted results are identified, and any applicable accreditation is stated [1].
How do I get the COA for my Batch vial?
To request a COA, send the lot number printed on your vial as described under Batch quality and testing. For how storage affects the dates on a COA, see bacteriostatic water shelf life and storage.
For laboratory and research use only. Not for human or veterinary use. Not intended to diagnose, treat, cure, or prevent any disease.
Sources
- Annex 4: Model certificate of analysis (WHO Technical Report Series, No. 1010) — World Health Organization. Accessed October 24, 2026.
- Guidance for Industry: Sterile Drug Products Produced by Aseptic Processing — Current Good Manufacturing Practice — U.S. Food and Drug Administration. Accessed October 24, 2026.
- Pyrogen and Endotoxins Testing: Questions and Answers (Edition 2) — Guidance for Industry — U.S. Food and Drug Administration. Accessed October 24, 2026.
- Inspection of Injectable Products for Visible Particulates — Draft Guidance for Industry — U.S. Food and Drug Administration. Accessed October 24, 2026.
- Label: Bacteriostatic Water for Injection, USP (Hospira, Inc.) — DailyMed, U.S. National Library of Medicine. Accessed October 24, 2026.
- About ILAC — International Laboratory Accreditation Cooperation (ILAC). Accessed October 24, 2026.
- Benzyl alcohol (CID 244) — PubChem, U.S. National Library of Medicine. Accessed October 24, 2026.
- Cosmetics Preservation: A Review on Present Strategies (Halla et al., Molecules, 2018) — PubMed Central, U.S. National Library of Medicine. Accessed October 24, 2026.