PUB-014 / v1.0
Water activity and interpretation
Water activity describes water availability in the actual matrix. A reliable decision requires a suitable method and representative sample.
What each source supports
An official monograph can establish formula and tests without publishing a complete stability study. A product label can support active-drug warnings without covering the compounded vehicle. Assess authority, exact edition and claim applicability. References identify their actual scope of support rather than serving as a decorative list.
Tests that answer the question
Appearance identifies some failures but does not establish strength, absence of degradants or microbiological suitability. Select method and criterion for the actual dosage form and use. A suspension needs redispersibility and device-delivered-dose assessment; at low strength, uniform mass does not imply uniform API. A critical specification failure requires quarantine and investigation.
Three different quantities
Water percentage, loss on drying and aw are not interchangeable. aw relates to water-vapour equilibrium at the measurement temperature. A binary-solution table does not automatically determine a gel, collodion or emulsion with additional ingredients. The 0.85 found in food regulations does not replace the 0.60 pharmaceutical classification threshold.
From measurement to decision
Check standards, temperature, equilibration, representativeness and volatile interference. Report uncertainty. SOP-017 uses a conservative policy assessing whether result plus uncertainty stays below 0.60. This is an explicit local decision rule, not a verbatim USP requirement.
Interpretation limit
Low aw does not establish sterility, API stability or packaging integrity. It must remain representative of intended storage and use. F-012 links the result to formula, batch, method and BUD.
Sources and references
- 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
- 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
- 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