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SOP-017 / SOP v1.0

Water activity measurement

Classification of a nonsterile finished matrix or intermediate requiring specific aw evidence.

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Procedure

  1. Record complete formula, lot, final water/solvents, container and process history. Define the classification question before testing.
  2. Select an internal or external instrument demonstrably suitable for the matrix. Volatile solvents require documented interference assessment, sensor protection and representative sampling.
  3. Check the instrument using traceable standards bracketing the relevant range according to its manual. Record values, temperature and acceptance before and after the series.
  4. Define representative positions and independent samples. Do not dry, dilute or heat a sample to obtain a lower reading. Prevent moisture exchange during transfer.
  5. Allow sample/chamber equilibration according to the instrument criterion. Record every reading, temperature, repeatability and uncertainty. Loss on drying or Karl Fischer does not replace aw.
  6. This template’s conservative policy accepts low aw only if the highest representative result plus reported uncertainty is <0.60. Borderline results require reassessment, not downward rounding. The uncertainty rule is editorial.
  7. Link the decision to packaging and duration: the category must remain applicable through storage/use. Ratio, supplier or process changes trigger reassessment. aw <0.60 does not establish chemical stability or sterility.
Stop criterion

Unsuitable method, failed standards or insufficient margin below 0.60: no low-aw classification.

Related forms

F-012, F-006

Sources and references

  1. FDA — Water Activity (aw) in FoodsInspection Technical Guide 39, 16 April 1984; website copy accessed 2026-09-12Definition; sorption behaviour; measurement equipmentMetrological principles only. Food regulatory thresholds and old instrument recommendations are not pharmaceutical BUD rules2026-09-12
  2. USP — Compounding Standards and Beyond-Use Dates, 2022 updateNovember 2022, published by USP; public copy hosted by Mississippi Boardpp. 1–2; nonsterile tableRisk-based default categories; not product-specific stability or Greek law2026-09-12
  3. PIC/S — Guide to Good Practices for the Preparation of Medicinal Products in Healthcare EstablishmentsPE 010-4, 1 March 2014Chapters 1–9; Annex 2 (nonsterile liquids, creams, ointments)Professional quality-system framework; not automatic national legal adoption or certification2026-09-12