Dissolution Testing in the Pharmacopoeia (USP vs. Ph. Eur.): an audit-ready practical guide

Dissolution testing is one of the assays that is easiest to describe and one of the easiest to challenge during an audit: many variables, many data points, and a direct impact on release specifications. That is why it is not enough to simply “follow the pharmacopoeia.” You need to demonstrate control, suitability, and robustness of execution in your own context, including instrument, analysts, and product.

To perform a dissolution test correctly in compliance with USP and Ph. Eur., you must:

• use the apparatus and conditions specified in the monograph,
• prepare the medium with controlled pH, volume, and temperature and degas when necessary,
• sample at the prescribed time points with correct filtration, including evaluation of filter adsorption,
• apply the multi-stage S1/S2/S3 criteria without shortcuts,
• ensure qualification, calibration, and data integrity.

Contents

• Why dissolution is so critical in audits
• Apparatus: what is harmonized and what is not
• Medium: preparation, pH, volume, and degassing
• Operating conditions: rpm, temperature, timing, and sampling
• Filters and “stopping dissolution”: the error that generates false OOS
• Acceptance criteria S1–S2–S3, with examples
• Apparatus qualification and calibration: what FDA and EMA expect
• Raw data and data integrity: how to make the package inspection-ready
• Frequent errors and quick preventive actions
• FAQ

1) Why dissolution is so critical in audits

Dissolution is often under the spotlight because:

• it is a routine test for many oral solid dosage forms
• it is inherently variable, due to apparatus, medium, operator, accessories, and sampling
• it is a typical source of laboratory investigations, deviations, and OOS, and inspectors look for evidence of control parameter by parameter

Key QA/QC message: an auditor does not want to hear “we followed USP/Ph. Eur.” They want to see how you made the method repeatable and defensible.

2) Apparatus: what is harmonized and what is not, USP vs. Ph. Eur.

For oral solid dosage forms, the most common apparatus are:

• Apparatus 1, Basket
• Apparatus 2, Paddle

These are substantially aligned between USP and Ph. Eur., and Ph. Eur. 2.9.3 uses the same “Apparatus 1/2” terminology.

Also recognized are:

• Apparatus 3, Reciprocating Cylinder, useful for modified release and medium change sequences, such as acid to buffer
• Apparatus 4, Flow-Through Cell, useful for low-solubility products or when a controlled flow approach is advantageous

Practical multi-market note: the comparative table in the guide shows that the structure of the chapter, including apparatus 1–4 and the logic of the criteria, is harmonized. Problems mainly arise from monographs, especially around medium, time points, and Q values.

3) Medium: preparation, pH, volume, and degassing, the point that drives variability

Volume and tolerances

A typical volume is often 900 mL, unless otherwise specified, and volume tolerance is strict, for example ±1%.

QC best practice: measure volume with a suitable instrument, or by mass with defined density, and record the measurement method used.

pH: control it like a critical specification

Many organizations underestimate medium pH. In reality, it is a direct driver of solubility and therefore of the result. The guide highlights the importance of consistent pH control and recording, with tight practical tolerances, for example ±0.05 as a prudent internal operational rule.

Degas, or deaeration: when to do it and how to defend it

USP and Ph. Eur. provide substantially equivalent recommendations: dissolved air can alter dissolution, generate bubbles, increase variability, and create questionable results. Degassing should therefore be applied whenever necessary to ensure reproducibility.

What makes the choice audit-ready

• if you degas: use a standardized procedure, such as heating plus vacuum filtration, or a validated automated system, and document it
• if you do not degas: have a documented rationale, for example a surfactant-containing medium where degassing causes foaming and worsens execution, plus evidence of robustness

4) Operating conditions: rpm, temperature, timing, and sampling

Temperature

Standard condition: 37.0 ± 0.5 °C. Inspectors verify stability before and during the run, including use of evaporation covers and start/end or trended temperature records.

Speed, rpm

Small deviations become big problems. 50 vs. 75 rpm is not a detail. It is a change in compendial conditions. If you change it, you must justify and govern it through change control, and regulatory assessment if dossier impact exists.

Sampling

Frequently challenged aspects include:

• sampling at the exact required time, within an internally defined and consistent window
• immediate filtration
• correct handling of replacement volume where applicable
• traceability linking vessel, time point, analyst, and UV or HPLC raw data

5) Filters and “stopping dissolution”: the error that creates false OOS

Two typical failure modes are:

• the filter adsorbs active ingredient, leading to underestimation and a false OOS
• the pore size or material is unsuitable, so particles pass through or interferences occur

The guide suggests a mature laboratory approach: recovery studies, correct selection of filter material and pore size, and, where necessary, discarding the first filtered volume to saturate binding.

Audit tip: if you have had dissolution OOS, have filter compatibility data ready. This is one of the pieces of evidence that separates an operational error from a true product issue.

6) Acceptance criteria S1–S2–S3, with clear examples

On this point, USP and Ph. Eur. are aligned: multi-stage acceptance.

Example with Q = 75%

• S1, 6 units: each unit must be ≥ Q + 5%, so ≥ 80%
• S2, 12 units: average must be ≥ Q, so ≥ 75%, and no unit may be < Q – 15%, so < 60%
• S3, 24 units: average must be ≥ Q, no more than 2 units may be < Q – 15%, and no unit may be < Q – 25%

Errors inspectors detect immediately

• declaring compliance after 6 units even though one unit is below Q+5, when you should have proceeded to S2
• testing 12 or 24 units automatically without respecting stage logic
• using invented criteria, such as “75% ±10%,” instead of the official rules

7) Apparatus qualification and calibration: what FDA and EMA expect

The guide highlights a highly inspectable practical point: today, a defensible approach is centered on rigorous mechanical calibration, including leveling, alignment, wobble or eccentricity, speed, and temperature, rather than relying only on older “calibrator tablet” logic.

What should be ready during an audit

• an up-to-date qualification and calibration plan with records
• post-maintenance evidence, such as re-checks
• clear rules that an expired instrument is not usable

8) Raw data and data integrity: how to make the package inspection-ready

A strong data package includes:

• start time and sampling times for each vessel
• documented temperatures
• attached UV or HPLC reports, not copied values
• active and reviewed audit trail, with no unjustified reprocessing
• transparent calculations, using validated software or a controlled spreadsheet

9) Frequent errors and quick preventive actions, mini-checklist

✅ Medium: volume, pH, and temperature recorded, with degassing handled appropriately
✅ rpm: verified and consistent with the method
✅ Filters: correct material plus demonstrated recovery
✅ Stages: S1/S2/S3 applied with no “interpretations”
✅ Data: raw data traceable and QA review documented

FAQ

Is dissolution testing always mandatory?
It depends on the monograph and the market. If the product is covered by applicable compendial requirements, you must comply with them, unless an authorized alternative exists.

If I use a compendial method, do I need to validate it?
Not from scratch, but you do need to perform method verification or suitability under your actual conditions, including instrument, product, and analysts.

Can I change the medium or rpm because “it works better that way”?
Only with scientific justification and governance, including change control and often regulatory approval.

If you want to go beyond theory and have audit-ready checklists, examples for multi-pharmacopoeia management, typical corrective actions, and criteria that are defensible in inspection, the complete guide “Guide to Pharmacopoeias: Dissolution Testing and Compendial Methods” is available on guidegxp.com.