Water for Injection Under Stringent Unannounced Inspections

Water for Injection (WFI) carries the highest risk classification among all process waters in the pharmaceutical industry. It is widely used in the preparation of sterile preparations such as injections and eye drops, as well as in the final rinsing of containers that come into direct contact with drug solutions. Its quality directly determines the safety and efficacy of the final drug products. 

Under the framework of Good Manufacturing Practice (GMP), WFI systems are a primary focus of unannounced inspections—commonly known as "flight inspections." Yet, among the numerous water quality parameters, hardness testing is frequently undervalued. In reality, it serves as a critical safeguard for ensuring the stable operation of WFI systems and mitigating compliance risks during inspections.

Pharmacopoeial Standards and GMP Requirements

The 2025 edition of the Chinese Pharmacopoeia mandates nine core tests for WFI, covering appearance, pH, ammonia, nitrate, nitrite, conductivity, total organic carbon, bacterial endotoxins, and microbial limits. Notably, hardness is not included among these finished product release tests. However, this does not diminish its importance—hardness serves as a critical in-process control parameter for the water generation system, occupying an irreplaceable position within the GMP compliance framework.

Article 100 of the Good Manufacturing Practice explicitly requires that pharmaceutical water and source water quality be regularly monitored, with corresponding records maintained. In actual GMP compliance inspections and unannounced audits, the absence of routine water quality monitoring—such as failing to regularly monitor and record source water quality—has been consistently cited as a common non-compliance finding related to pharmaceutical water. 

This means that although hardness testing is not a release criterion for finished WFI, it remains an essential process control requirement spanning the entire chain—from source water and pretreatment to the water generation system itself.

WFI is typically produced by distilling purified water. The generation equipment has specific feed water requirements: the conductivity of deionized water must remain below 2 μS/cm; otherwise, pipe scaling may occur, compromising both the quality and output of WFI and potentially leading to pipe blockages in severe cases.

Calcium and magnesium ions are the primary contributors to water hardness. In WFI preparation systems, elevated hardness in source water or purified water poses multiple hazards to core equipment—including multi-effect distillation units, reverse osmosis membranes, and electrodeionization devices. Hardness ions precipitate during high-temperature distillation, forming scale on equipment coils and inner walls.