In-Process Assay Gate — Test Results that Unlock the Next Manufacturing Step

This topic is part of the SG Systems Global regulatory & operations glossary.

Updated November 2025 • Tests & Laboratory Analyses, Batch-Specific Potency, Test-Driven Setpoint Adjustment, Hold / Release Status • IPC, Potency, Gating, GMP

An in-process assay gate is a controlled point in the manufacturing process where production cannot proceed until specific test results – typically assay, potency, titer, solids or a related critical attribute – are available, reviewed and accepted. The gate “locks” the next operation behind laboratory or inline data. Only when the material meets predefined criteria is the operation released, setpoints updated and execution allowed to continue. In regulated environments, in-process assay gates are a central tool for turning quality control data into hard process controls rather than after-the-fact observations.

“An in-process assay gate is where the batch has to prove itself before you let it go any further.”

1) Definition and Purpose

An in-process assay gate is a process control and workflow construct, not just a test result. It combines three things:

• a defined in-process test (or set of tests) such as assay, potency, titer, % solids or concentration;
• predefined acceptance criteria and review requirements; and
• a technical interlock in MES or the control system that prevents the next operation from proceeding until the criteria are met.

The gate is placed at points where material strength or composition must be known before dosing, dilution, blending, filling or further processing. Rather than letting the process run and hoping assay data backs it up, an in-process assay gate ensures that the batch is actively controlled on the basis of measured data. This is strongly aligned with modern GMP expectations around “control strategy”, in-process controls (IPC) and science- and risk-based decision-making.

2) How It Differs from Routine In-Process Controls

Most processes have in-process controls – samples taken, results recorded, trends reviewed. In many plants, however, those results do not actively control the process: production continues, and results are checked later. An in-process assay gate is different: it is gating. If the test is not complete, not reviewed or not within limits, the system will not allow:

• the next batch step to start;
• a transfer, addition or dilution to proceed; or
• a change in status from hold to release.

In other words, an assay gate is a specific kind of IPC where there is a hard dependency between the test outcome and the permission to proceed. This makes the gate more akin to a hard-gating control than a simple monitoring tool. It also means that the design of assay gates needs to be carefully documented in the control strategy and validated as part of the computerised system and process validation.

3) Typical Locations for In-Process Assay Gates

In-process assay gates are typically deployed at inflection points where strength, purity or composition must be understood before the next action. Common locations include:

• after a concentration or evaporation step, before dilution or formulation;
• after synthesis or fermentation, before downstream purification or pooling;
• after creation of a concentrated intermediate, before transfer to final bulk tank;
• before an API or concentrate is dispensed into final dosage form;
• prior to a concentration-adjusted charge into a blend or reactor;
• before releasing a hold on a partially processed bulk for further processing or filling.

In each case, the gate prevents a situation where materials of unknown or out-of-spec potency are silently incorporated into downstream processing, where they are much harder to remove, rework or justify. It effectively shifts decision-making earlier in the process, when the cost and consequence of intervention are lower and easier to control.

4) Link to Batch-Specific Potency and Potency Adjustment Factors

Assay gates are closely tied to batch-specific potency. When the relevant test (assay, titer, % solids, LOD) is completed, reviewed and accepted, the result is used to set or update batch-specific potency for the lot or intermediate. That updated potency then feeds into:

• potency adjustment factors for solid or liquid additions;
• concentration-adjusted charges for solutions and slurries;
• corrected active content calculations for each transfer and addition.

The gate ensures that all potency-dependent steps behind it use up-to-date potency data, rather than placeholder or historical values. MES should prevent selection of lots that have not yet passed their assay gate, or that do not have a current potency value aligned with the required potency basis. This is particularly important when lots are re-tested or when stability information leads to a revised potency being assigned during processing.

5) Test-Driven Setpoint Adjustment Triggered by Assay Gates

In advanced control strategies, an in-process assay gate does more than simply permit or block the next step. It may also trigger test-driven setpoint adjustment. For example:

• if an intermediate is stronger than expected, the subsequent dilution or charge targets are automatically reduced;
• if it is weaker, the system may increase the charge or adjust batch size within predefined boundaries;
• if it falls outside a defined “adjustable” window, the gate may force a deviation decision instead of adjusting setpoints.

The in-process assay gate is the moment at which the test results are interpreted and turned into updated setpoints for downstream steps. In a system like V5, this logic can live in a recipe phase, rule set or process control plan, with the gate ensuring that calculations are only executed once valid data are present and reviewed under appropriate roles and permissions.

6) Interaction with Hold / Release Status

In-process assay gates often interact with quality status such as quarantine / quality hold and hold / release / QA disposition. A typical pattern:

• material produced up to an intermediate step is automatically placed on hold;
• an in-process assay gate requires completion and approval of assay and related tests;
• if results are within limits, QA (or an authorised role) changes status from hold to released for further processing;
• MES prevents any downstream steps from consuming the material until status is released and the gate is marked as passed.

This provides a clear audit trail: the system can show that no additional steps were performed and no blending, dilution or filling occurred while the intermediate was on hold awaiting assay data. The gate thus becomes both a process safeguard and a documentation anchor for QA disposition decisions within the eBMR.

7) Data Flow: LIMS, Analytical Lot Links and MES

In-process assay gates depend on reliable data flow between laboratory systems and MES/ERP. A robust design typically includes:

• test configuration and results in LIMS or equivalent lab system;
• linkage of results to the specific intermediate or lot via an analytical lot link;
• automatic transfer of key results (assay, % solids, titer, LOD, potency) into MES or master data;
• gate logic in MES that checks for presence, status and acceptance of those results before allowing the next step;
• audit trails capturing changes to data and gate status.

In this architecture, operators do not type assay results into MES manually. Instead, LIMS publishes approved results, and MES subscribes to them. The in-process assay gate becomes a formalised handshake between lab and manufacturing, rather than a loose, manual dependency conveyed via paper printouts or emails.

8) In-Process Assay Gates in Potency-Adjusted Batching

In the context of potency-adjusted batching, assay gates are particularly important when a batch step depends on the potency of an intermediate created on-site (for example, a concentrated bulk intermediate, a reconstituted API solution or a pre-blend). A typical sequence:

• an intermediate is prepared according to a recipe;
• the intermediate is sampled and tested for potency, titer or % solids;
• the in-process assay gate requires these results and QA or supervisor review;
• once accepted, batch-specific potency is updated for the intermediate lot;
• subsequent concentration-adjusted charges into final bulk use this potency.

Without such a gate, intermediate potency might be assumed or approximated, resulting in mis-dosing when that material is used. With a gate, MES can prove that downstream dosing logic is based on measured, reviewed data, not on assumptions – which is exactly the kind of evidence inspectors expect when reviewing control strategies for strength and label claim in complex processes.

9) Exception Handling and Deviation Management

Not all assay results will fall neatly within the expected window. In-process assay gate design should therefore include clear rules for exceptions, such as:

• criteria for when automatic setpoint adjustments are allowed versus when they are disallowed;
• upper and lower boundaries beyond which the gate cannot be passed without a deviation or NCR;
• options for rework, reprocessing or discarding the intermediate if potency is out of range;
• requirements for QA approval before overriding gate logic in exceptional cases.

In MES, this typically appears as alternative workflows or branches that are only available under specific roles and with mandatory justification and electronic signatures under 21 CFR Part 11/Annex 11 expectations. The assay gate thus becomes not just a technical interlock, but a focal point for documented risk assessment and decision-making when reality diverges from the plan.

10) Data Integrity and ALCOA+ Considerations

Because in-process assay gates directly impact whether batches progress or are held, their design is heavily scrutinised through a data integrity lens. Key expectations include:

• results must be attributable to a specific sample, test method, analyst and instrument;
• gate decisions (pass/fail, adjustments, overrides) must be recorded with user identity, timestamp and reason;
• no uncontrolled manual transcriptions of assay results into MES – data should flow via validated interfaces;
• changes to limits, algorithms or linkage rules should be under formal change control;
• audit trails should allow reconstruction of who changed what, when and why.

From an ALCOA+ standpoint, in-process assay gate events (including the underlying test results and decisions) must be attributable, legible, contemporaneous, original and accurate. Treating the gate as a first-class object in the control strategy and eBMR – not just as an implicit understanding between lab and operations – is central to meeting these expectations.

11) Benefits for Investigations, PQRs and CPV

Well-implemented in-process assay gates make investigations and ongoing reviews significantly easier:

• Investigations: investigators can quickly see the assay history, gate decisions and any setpoint adjustments for each batch, rather than inferring them from scattered lab reports and operator notes.
• Product quality reviews (PQR): gates provide clear structure for summarising where in the process assay controls are applied and how often adjustments or deviations occur.
• Continued process verification (CPV): gate-related data (assay distributions, adjustments, gate failures) can be trended to detect process drift or emerging risk.

By making assay gates explicit and structured, organisations turn what might otherwise be ad-hoc decisions into a consistent, analysable dataset that supports continuous improvement, risk management and, where appropriate, QbD-style lifecycle management.

12) Typical Use Cases Across Industries

In-process assay gates are relevant across regulated and quality-critical sectors:

• Pharmaceuticals and biologics: potency or titer checks on intermediates before pooling, dilution, formulation or filling; assay-based decisions before concentration or purification steps.
• Dietary supplements: assay of vitamin or botanical pre-blends before inclusion in final blends or compression; solids-based checks on syrups or liquid concentrates.
• Food and beverage: enzyme activity checks before dosing into large batches; solids and concentration checks on fortification premixes and flavours.
• Cosmetics and personal care: active concentration checks on bulk intermediates before emulsification, dilution or filling.
• Chemicals and speciality materials: active or functional group assay on intermediates before reaction, neutralisation or blending with downstream components.

In each case, the fundamental logic is the same: material quality and strength must be confirmed – and in some cases, converted into updated setpoints – before critical downstream operations are allowed to proceed.

13) Practical Implementation Steps

To introduce in-process assay gates in a controlled way, organisations typically:

• Map the process to identify decision points that depend on assay, potency, titer or solids results.
• Define which of those points must be gated versus simply monitored, based on risk and impact.
• Specify the tests, limits and review requirements associated with each gate.
• Implement gate logic in MES, including status checks, role-based approvals and links to LIMS data.
• Validate the gate behaviour as part of computer system validation (CSV) and document it in control strategy, SOPs and training.

Once these steps are complete, in-process assay gates become a normal part of the manufacturing rhythm: samples are taken, results flow from lab to MES, gates open or remain closed based on those results, and the resulting decisions are recorded transparently in the eBMR for later review and defence.

FAQ

Q1. How is an in-process assay gate different from a simple lab hold?
A lab hold may rely on procedural discipline to prevent further processing. An in-process assay gate embeds that dependency into the system: MES or the control system will not allow the next step to proceed until test results are available and accepted, providing a technical as well as procedural barrier.

Q2. Do all in-process tests need to be configured as assay gates?
No. Only those tests whose results must directly influence whether the process can continue – especially potency, strength or critical composition checks – typically justify a formal gate. Other tests may remain as monitoring-only IPCs.

Q3. Can operators override an in-process assay gate?
In exceptional circumstances, systems may allow controlled overrides, but only under defined roles with electronic signatures, documented justification and often a linked deviation. Routine use of overrides would defeat the purpose of the gate and raise serious data integrity concerns.

Q4. Does an assay gate always imply a QA decision?
Not necessarily. Some gates are owned by production or technical roles if the risk is lower. However, for potency-critical or safety-critical decisions, QA involvement or final disposition is usually expected and should be reflected in roles and workflows.

Q5. What is a practical first step to introduce in-process assay gates?
Start by selecting one potency-critical intermediate where downstream dose depends strongly on assay. Define the test, limits and gate behaviour, implement it in MES with proper LIMS integration, validate the behaviour, and then systematically expand the pattern to other high-risk points in the process.

Related Reading
• Potency & Batching: Batch-Specific Potency | Potency Adjustment Factor | Corrected Active Content | Concentration-Adjusted Charge
• Test-Driven Control: Tests & Laboratory Analyses | Test-Driven Setpoint Adjustment | Dynamic Recipe Scaling
• Status, Records & Review: Quarantine / Quality Hold | Release Status | Electronic Batch Record (eBMR) | Product Quality Review (PQR)