Growth promotion testing: media performance and result management

Growth promotion testing (GPT) checks whether a medium supports the required growth under defined conditions. It does not establish every property of the medium or replace method suitability testing in the presence of the product.

The operational question is specific: what performance must this lot support, for which use and with what evidence? The following matrix helps document the answer; it introduces neither compendial specifications nor a cultivation procedure.

1. Separate the properties being checked

Growth promotion concerns the ability to support growth. Selectivity concerns inhibition of non-target microorganisms; indicative or differential properties allow a characteristic response to be recognised. A medium may support growth while losing a necessary selective or indicative function.

A medium sterility check looks for unwanted contamination in the material examined: it does not measure growth promotion. Appearance, volume and pH are other relevant attributes, but a compliant physical value alone does not demonstrate microbiological performance. WHO good practices, 2011, section 5.2, address these properties separately.

2. Start with the method and intended use

Identify the medium, formulation, format, application and applicable requirement. Enumeration of non-sterile products, tests for specified microorganisms and sterility testing do not automatically share the same controls. Use the effective text of the relevant chapter and the approved method conditions, without transferring criteria from another application.

Distinguish purchased ready-to-use media from media prepared in-house. WHO allows reliance on a performance certificate for fully prepared media under defined conditions: a qualified supplier, a certificate for each batch, qualified transport and periodic verification. This is not a general exemption and does not override applicable compendial obligations. Where preparation or processing takes place in the laboratory, the powder certificate does not document the final lot's performance.

3. Make lots and controls traceable

Link the medium lot, supplier, receipt, any preparation, packaging and use status. For reference strains and materials, record identity, origin, lot, expiry, storage and the relevant history required by the procedure. An expired or untraceable reference may compromise the test before it is read.

Associate the verification with the method and revision, operators, equipment, materials, actual conditions and original results. When a control is prepared locally, retain the link to its starting material and required checks. Traceability also helps distinguish a medium problem from a test-system problem.

4. Plan verification with a matrix

Select controls, microorganisms, quantities and conditions from the source that actually applies. Additional risk-based checks require justification and do not replace prescribed checks. Before testing, define criteria, the comparator, test validity and anomaly handling.

The rows show how to construct a plan, not a complete list of tests. Keep the uninoculated medium control distinct where required. Do not use one result to cover properties that were not actually assessed.

5. Interpret qualitative and quantitative evidence

Read a qualitative response against the expected growth or characteristic. Quantitative verification requires clearly identified counts, units, comparator and calculation rules. A ratio is interpretable only when the numerator and denominator are understood and the comparison is valid.

Consider variability in control materials, count distribution and reading. Do not introduce a tolerance after seeing the result or make a doubtful outcome compliant through convenient rounding. Retain anomalous observations and initial data. To distinguish method recovery from medium performance, see bioburden and method suitability.

6. Manage a failed result

Place the affected lot under control, prevent further unauthorised use and open an investigation. Reconstruct materials, conditions, records and controls before attributing the cause to the medium. If the test itself is invalid, document why; that does not establish that the medium complies.

Simulated case. A ready-to-use lot fails the specified recovery criterion. It has already been used for two product batches. The laboratory blocks it, identifies the affected tests and checks the reference material, comparator, storage and transport conditions. The supplier's passing certificate is evidence to compare with local data, not a reason to erase them.

Containment is immediate; a conclusion requires evidence. Assess the validity of previous tests and product impact with QA under the applicable procedure. Any repeat must address a documented hypothesis under an approved plan: a later favourable result does not automatically cancel the initial one.

7. Maintain performance over time

Connect acceptance, storage, expiry and conditions of use. An initially suitable medium may be damaged after testing. Review excursions, compromised packaging and changes in supplier, formulation, preparation or transport before assuming performance remains equivalent.

Trend by medium, lot, control and anomaly type; an overall average may hide a deteriorating function. Define who authorises use and what evidence closes the deviation. As of 30 September 2026, the Pharmeuropa 38.3 revisions of 2.6.12, 2.6.13 and 5.1.4 are consultation proposals, not official standards to apply early.

Sources and consultation limits

• WHO, TRS 961, Annex 2, 2011: public full text, section 5.2 on media.
• USP 61, 2024 record, USP 62, 2017 record and USP 71, 2017 record: public scope previews; verify conditions and criteria in the effective edition through authorised access.
• EDQM, Pharmeuropa 38.3, 1 July 2026: proposal status, consultation until 30 September 2026.