- USP <645> sets a Stage 1 in-line conductivity limit of 1.3 µS/cm at 25°C for both Purified Water and WFI, with a three-stage test procedure if that limit is exceeded (Beckman, 2025).
- USP <643> governs Total Organic Carbon testing, a standardized method for confirming pharmaceutical water purity beyond what conductivity alone catches.
- Typical USP Purified Water microbial limits run 50 CFU/mL as an alert level and 100 CFU/mL as the action limit.
What Conductivity Limit Applies to USP Water?
USP <645> sets a Stage 1 in-line conductivity limit of 1.3 µS/cm at 25°C, applying equally to Purified Water and Water for Injection (Beckman, 2025). If an in-line, point-of-use measurement exceeds that Stage 1 limit, the standard specifies a three-stage escalation: offline Stage 2 measurement, and if that also fails, a Stage 3 procedure comparing pH against conductivity/pH reference tables.
This staged approach matters operationally: a single momentary Stage 1 exceedance doesn’t automatically mean out-of-spec water, but it does trigger the follow-up testing that determines whether it actually is.
What Does USP <643> Require for Total Organic Carbon?
USP <643> provides the standardized method for measuring Total Organic Carbon (TOC) in pharmaceutical water, catching organic contamination that conductivity testing alone won’t detect. TOC and conductivity work as complementary checks, not substitutes for each other, a water system can pass conductivity limits while still carrying organic contamination that only TOC testing reveals.
What Are the Microbial Limits for USP Water?
A typical USP Purified Water system sets an alert limit around 50 CFU/mL and an action limit at 100 CFU/mL. Alert and action limits serve different purposes: an alert triggers investigation before the water is actually out of spec, while exceeding the action limit means the water fails USP requirements outright and triggers a formal deviation.
Water for Injection carries stricter microbial and endotoxin limits than Purified Water, reflecting its use in sterile and injectable applications where the same contamination tolerance simply doesn’t apply.
How Do USP Water Grades Compare?
| Parameter | Purified Water (PW) | Water for Injection (WFI) |
|---|---|---|
| Conductivity (Stage 1, 25°C) | ≤1.3 µS/cm | ≤1.3 µS/cm |
| TOC testing method | USP <643> | USP <643> |
| Microbial action limit | ~100 CFU/mL | Stricter; typically ≤10 CFU/mL |
| Endotoxin limit | Not specified | ≤0.25 EU/mL |
| Typical use | Non-sterile formulation, equipment cleaning | Sterile injectable and biologic production |
For the full production-method comparison between these grades, see our pharmaceutical water systems guide.
Frequently Asked Questions
What is the USP conductivity limit for purified water?
USP <645> sets a Stage 1 in-line conductivity limit of 1.3 µS/cm at 25°C for both Purified Water and Water for Injection.
What is USP <643> used for?
USP <643> provides the standardized test method for Total Organic Carbon in pharmaceutical water, catching organic contamination that conductivity testing doesn’t detect on its own.
What are typical microbial limits for USP purified water?
A typical system sets an alert limit around 50 CFU/mL and an action limit at 100 CFU/mL, with Water for Injection held to stricter limits given its use in sterile applications.
SKE & Eagle USP-Compliant Water Systems
SKE & Eagle designs purified water systems and WFI systems engineered to meet USP <645> conductivity and <643> TOC requirements, with continuous monitoring built in to catch limit exceedances before they become deviations.