A bacteriostatic water label says "sterile" and "nonpyrogenic" as two separate words because they are two separate claims, tested by two separate methods, and it is entirely possible for a product to pass one and fail the other. Five minutes with this page saves a lot of confusion later.
Sterile
Claim: the sealed contents contain no viable microorganisms: no bacteria, yeasts or molds capable of growing.
How it is achieved: sterilizing filtration and aseptic filling, or terminal sterilization of the sealed vial.
How it is tested: samples from the lot are filtered onto membranes, rinsed to remove the preservative, and incubated in two growth media for 14 days. No growth means the lot complies. See sterility testing.
What it protects against: living organisms in the water multiplying in whatever is prepared with it.
Nonpyrogenic
Claim: the contents contain less than a defined amount of pyrogens, in practice bacterial endotoxin, the heat-stable lipopolysaccharide fragments left behind by Gram-negative bacteria. For bacteriostatic water the limit is 0.5 endotoxin units per millilitre.
How it is achieved: making the product from water that has been purified by distillation, reverse osmosis or ultrafiltration in a system operated to prevent bacterial growth, in containers that have been depyrogenated (glass, by dry heat at about 250 °C).
How it is tested: the LAL (Limulus amebocyte lysate) test, in gel-clot, turbidimetric or chromogenic format, against a reference standard. See endotoxin testing.
What it protects against: the biological activity of endotoxin in downstream work, which persists whether or not the bacteria that made it are alive.
Why they are independent
- Sterile but pyrogenic: water that grew bacteria in a storage tank, then was sterilized. The bacteria are dead; their endotoxin survived sterilization and passes through sterilizing filters. Passes the sterility test, fails the endotoxin test.
- Nonpyrogenic but not sterile: water freshly purified by a good RO system, then dispensed into a clean but non-sterile bottle and left open. Low endotoxin, but organisms present. Passes the endotoxin test, fails sterility.
A product that must be both, as bacteriostatic water must, has to be made in a way that controls both, and tested for both.
Side by side
| Sterile | Nonpyrogenic | |
|---|---|---|
| Concerns | Living organisms | Endotoxin (non-living) |
| Removed by | Sterilization, sterile filtration | Distillation, RO, ultrafiltration, depyrogenation of glass |
| Survives | Nothing living survives sterilization | Autoclaving, boiling, 0.22 µm filtration |
| Test | Sterility test, 14 days | LAL test, hours |
| Limit | No growth | < 0.5 EU/mL (bacteriostatic water) |
| Lost by | Opening the vial; poor handling | Contamination with bacterial debris; not by opening as such |
A third word: bacteriostatic
Not a purity claim at all. It describes the preservative, which inhibits the growth of organisms introduced after the vial is opened. It does not make the contents sterile, and it has no effect on endotoxin. See what is bacteriostatic water.
On the certificate of analysis
Look for both lines: "Sterility: complies" and "Bacterial endotoxins: < 0.5 EU/mL" (or a numeric result). A COA with one and not the other is incomplete. See how to read a COA. The vocabulary is collected in the glossary.
All BAC Water Supply products are supplied for laboratory research use only and are not for human or veterinary use.