Introduction: Endotoxin Testing Is Only as Good as the Sampling Plan Behind It

A laboratory can have a highly sensitive endotoxin assay, qualified instruments, trained analysts, and well-controlled reagents—and still generate misleading data if the sampling strategy is poorly designed.

This is because endotoxin testing does not evaluate an entire manufacturing process directly.

It evaluates a specific sample collected at a specific location and time.

That creates an important question:

Does the sample actually represent the endotoxin risk of the process or product?

Consider a manufacturing process in which endotoxin could potentially enter through:

• Raw materials
• Process water
• Buffers
• Equipment
• Single-use components
• Transfer lines
• Process intermediates
• Storage containers
• Final formulation

If the laboratory only tests the finished product, it may confirm the final endotoxin status—but have very little information about where contamination entered the process.

On the other hand, testing every material at every stage may create unnecessary analytical workload without improving process understanding.

The solution is a risk-based endotoxin testing sampling plan.

A well-designed plan should answer five fundamental questions:

1. What should be tested?
2. Where should samples be collected?
3. When should samples be collected?
4. How frequently should testing be performed?
5. How should samples be handled to preserve the integrity of the result?

This article provides a practical framework for designing that strategy.

1. What Is an Endotoxin Testing Sampling Plan?

An endotoxin testing sampling plan defines how samples are selected and collected for bacterial endotoxin testing throughout a laboratory or manufacturing process.

It may include:

• Sampling locations
• Sampling stages
• Sampling frequency
• Sample quantities
• Sampling containers
• Sample storage conditions
• Testing time points
• Test methods
• Acceptance criteria
• Retest or investigation procedures
• Data trending requirements

The purpose is not simply to increase the number of endotoxin tests.

The purpose is to generate representative and actionable information.

A good sampling plan should help answer:

Where is endotoxin risk highest?

Where could contamination enter?

Which process stages provide the most useful information?

How can an endotoxin excursion be detected before it becomes a final-product problem?

This makes sampling design part of the overall contamination-control strategy rather than a separate laboratory activity.

2. Why Sampling Location Matters

Endotoxin contamination is not necessarily distributed uniformly throughout a manufacturing process.

For example, imagine a process with the following sequence:

Raw Material → Buffer Preparation → Purification → Concentration → Formulation → Final Product

An endotoxin result from the final product tells you about the final product.

It does not automatically tell you:

• Whether endotoxin entered through raw materials
• Whether a buffer introduced contamination
• Whether a process vessel contributed endotoxin
• Whether contamination occurred during transfer
• Whether a purification step removed or concentrated endotoxin

This is why sampling location matters.

A useful sampling plan should focus on critical control points, especially locations where endotoxin could be introduced, accumulated, concentrated, or removed.

3. Start With an Endotoxin Risk Map

Before deciding how often to test, map the process.

A simple risk map can follow the material from beginning to end:

Supplier Materials

↓

Water

↓

Buffer Preparation

↓

Equipment / Single-Use Components

↓

Process Step

↓

Intermediate

↓

Purification

↓

Bulk Material

↓

Final Formulation

↓

Finished Product

At each stage, ask:

Can endotoxin enter here?

Can endotoxin increase here?

Can endotoxin be removed here?

Can the material become more concentrated here?

Can the sample become analytically more difficult here?

This exercise can reveal important sampling points that may not be obvious from the final-product perspective.

FireGene's existing discussion of endotoxin risk assessment throughout the pharmaceutical manufacturing lifecycle provides a useful foundation for this type of process-level thinking.

4. Raw Materials: Should They Be Included?

Raw materials are one of the earliest potential sources of endotoxin.

Examples include:

• Active pharmaceutical ingredients
• Excipients
• Salts
• Sugars
• Amino acids
• Proteins
• Biological materials
• Process-specific reagents

Not every raw material necessarily requires identical endotoxin testing.

A risk-based approach can consider:

• Supplier qualification
• Material origin
• Manufacturing process
• Historical endotoxin performance
• Intended use
• Quantity used
• Contact with the final product
• Potential downstream removal

For critical materials, endotoxin information may be obtained through:

• Supplier CoA
• Supplier qualification
• Incoming testing
• Periodic verification
• Risk-based lot testing

FireGene's article on endotoxin testing for raw materials and excipients provides additional background on this upstream control strategy.

5. Water Systems: A High-Value Sampling Point

Water deserves special consideration because it is used throughout pharmaceutical and biopharmaceutical manufacturing.

Water may be involved in:

• Buffer preparation
• Cleaning
• Rinsing
• Dilution
• Formulation
• Equipment preparation
• Analytical testing

A water system can therefore influence multiple downstream operations.

Potential sampling locations may include:

• Generation point
• Storage system
• Distribution loop
• Points of use
• Critical manufacturing locations

The exact sampling design should be based on the facility's validated water monitoring program and risk assessment.

The important principle is:

Do not evaluate water endotoxin quality only at the source if the water is distributed through a complex system.

A distribution system can introduce additional variables.

Trending water endotoxin results over time can also provide useful early-warning information.

6. Buffer Sampling: Test the Buffer or Test the Process?

Buffers are another important consideration.

A buffer may be:

• Prepared in-house
• Purchased as a concentrate
• Prepared from raw materials
• Stored before use
• Transferred through equipment
• Used across multiple process stages

The sampling plan should therefore consider both the buffer itself and the role of the buffer in the process.

For example, a buffer used early in purification may have a different risk profile from a formulation buffer that directly contacts the final drug substance.

Sampling decisions may consider:

• Preparation batch
• Storage duration
• Hold time
• Transfer path
• Contact with product
• Historical results
• Downstream removal capability

The goal is not necessarily to test every buffer every time.

The goal is to identify the buffers where endotoxin monitoring provides meaningful process information.

7. Equipment and Single-Use Components

Manufacturing equipment can also become part of an endotoxin sampling strategy.

Potential sources include:

• Mixing tanks
• Stainless-steel vessels
• Tubing
• Filters
• Transfer bags
• Connectors
• Sampling ports
• Pumps
• Chromatography systems

This is particularly relevant when equipment is reused.

A process may produce acceptable endotoxin results for multiple batches and then experience an unexpected excursion after:

• Maintenance
• Cleaning changes
• Equipment modification
• Extended storage
• Sanitization changes
• Replacement of components

In these situations, equipment-related sampling or investigation may become appropriate.

For single-use systems, routine product-contact testing may not always be necessary at every stage, but supplier qualification and component specifications should form part of the contamination-control strategy.

8. In-Process Sampling vs. Finished-Product Sampling

Finished-product testing remains an important quality-control activity where required.

However, finished-product testing alone may provide limited process information.

Consider two approaches.

Strategy A: Finished product only

Process → Final Product → Endotoxin Test

This answers:

“Does the final product meet the applicable endotoxin requirement?”

But if the result is unexpectedly high, the investigation may need to examine the entire process.

Strategy B: Strategic process monitoring

Raw Material → Water → Buffer → Intermediate → Bulk → Final Product

This provides a sequence of endotoxin data.

Now the laboratory can ask:

• Did endotoxin increase at a particular stage?
• Did a purification step reduce endotoxin?
• Is one intermediate consistently higher?
• Did a new raw-material lot change the profile?
• Is there gradual process drift?

This is where a sampling plan becomes much more valuable than simply increasing the number of final-product tests.

9. When Should Samples Be Collected?

Sampling time can be just as important as sampling location.

Potential time points include:

Before a process step

This establishes the incoming endotoxin level.

After a process step

This helps determine whether the step increases or decreases endotoxin.

Before storage

Useful when hold time may influence contamination risk.

After storage

Can help evaluate changes during the defined hold period.

Before transfer

Useful when transfer equipment may represent a contamination risk.

After transfer

Can help determine whether the transfer operation introduced contamination.

Before final formulation

Provides an opportunity to evaluate endotoxin status before the final manufacturing stage.

Final product

Used where required by the applicable product specification and quality-control procedure.

The exact time points should be justified based on the process.

10. How Often Should Endotoxin Testing Be Performed?

This is one of the most common questions in endotoxin control.

There is no universal frequency that applies to every process.

Testing frequency should reflect:

Risk + process knowledge + historical performance + regulatory requirements + product sensitivity

Possible approaches include:

Every batch

Appropriate for critical materials or stages where routine testing is necessary.

Periodic testing

Useful for lower-risk materials with strong historical control.

Qualification testing

Used when introducing a new supplier, material, equipment configuration, or process.

Campaign-based testing

Potentially useful when manufacturing campaigns introduce specific risk patterns.

Investigation testing

Triggered by unexpected results or process deviations.

Enhanced monitoring

May be appropriate after:

• Equipment changes
• Supplier changes
• Process changes
• Water-system excursions
• Cleaning changes
• Repeated borderline results

The key principle is:

Testing frequency should be justified, not arbitrary.

Testing more frequently does not automatically create better control if the additional data do not improve process understanding.

11. Risk-Based Sampling Is More Useful Than “Test Everything”

A common reaction to contamination risk is to add more testing.

But excessive testing can create its own problems:

• Higher laboratory workload
• More sample handling
• More reagent consumption
• More opportunities for analytical error
• Increased investigation workload
• More data without additional process insight

A better approach is to prioritize samples according to risk.

For example:

Process Stage	Potential Risk	Sampling Priority
Critical raw material	Endotoxin introduction	High
Process water	System-wide contamination	High
Critical buffer	Product-contact risk	High
Early intermediate	Process-dependent	Medium
Purification intermediate	Process monitoring	Medium
Final bulk	Product quality	High
Finished product	Applicable release requirement	High

The exact classification should be established through the facility's own risk assessment rather than copied directly from a generic table.

12. Sample Volume Matters

An endotoxin sampling plan should also define how much sample is required.

Too little sample may prevent:

• Repeat testing when justified
• Dilution studies
• PPC testing
• Confirmatory analysis
• Investigation work

Too much sample can create:

• Unnecessary material loss
• Storage challenges
• Additional handling
• Increased contamination opportunities

Sample volume should therefore be sufficient for the intended testing strategy without unnecessarily consuming valuable product.

This is particularly important for:

• Cell therapies
• Gene therapies
• High-value biologics
• Early-stage clinical materials
• Small-volume formulations

13. Sample Containers Are Part of the Sampling Plan

The sampling container is not simply packaging.

For endotoxin testing, it is part of the analytical system.

A contaminated container can introduce endotoxin into an otherwise clean sample.

This can produce an apparent increase in endotoxin concentration that did not originate from the manufacturing process.

Therefore, laboratories should define:

• Container type
• Endotoxin suitability
• Cleaning or depyrogenation requirements
• Storage conditions
• Maximum hold time
• Container closure
• Identification and traceability

FireGene's Pyrogen-Free Vials are designed for applications where controlled endotoxin background is important.

This is particularly useful for:

• CSE preparation
• Sample dilution
• PPC studies
• Recovery experiments
• Method suitability
• Investigation testing

14. Sample Hold Time Can Change the Result

Once a sample has been collected, the testing process is not finished.

The sample may remain in:

• A laboratory refrigerator
• A controlled storage area
• A transport container
• A testing queue

During this period, the sample may change.

Potential variables include:

• Temperature
• Storage duration
• Container interaction
• Endotoxin adsorption
• Endotoxin masking
• Sample precipitation
• Protein interactions

This is particularly important for complex biological formulations.

For samples where endotoxin recovery may change during storage, laboratories should establish appropriate hold-time conditions rather than assuming that the result will remain unchanged indefinitely.

Low Endotoxin Recovery (LER) is one example of how endotoxin detectability can change over time in certain formulations.

FireGene's detailed LER guide discusses this issue in greater depth.

15. Sample Handling Can Create False-Positive Results

Endotoxin testing is highly sensitive.

That sensitivity is essential, but it also means that contamination introduced during sampling or laboratory handling can become measurable.

Potential sources include:

• Sampling containers
• Pipette tips
• Tubes
• Microplates
• Water
• Reagent preparation
• Analyst handling
• Transfer equipment

This is why laboratories should treat the entire sampling pathway as part of the endotoxin-control system.

A useful principle is:

Manufacturing control → Sampling control → Laboratory control → Analytical control

A problem at any point can affect the final result.

For more information, see FireGene's article on hidden sources of false-positive endotoxin results.

16. Choosing the Right Endotoxin Test Method

Once sampling points have been identified, the laboratory must determine how samples will be tested.

Common approaches include:

• Gel-Clot
• Kinetic Chromogenic
• Kinetic Turbidimetric
• Other applicable photometric or recombinant-reagent methods

The FDA's current 2026 guidance discusses fundamental gel-clot, photometric, and kinetic endotoxin-testing approaches. USP <85> remains the foundational bacterial endotoxin chapter, while USP <86> addresses bacterial endotoxin testing using recombinant reagents.

The method should be selected based on:

• Required sensitivity
• Sample matrix
• Quantitative requirements
• Throughput
• Available instrumentation
• Method suitability
• Applicable regulatory requirements

For quantitative high-throughput applications, FireGene's Kinetic Chromogenic Endotoxin Test Kit uses kinetic absorbance detection at 405 nm and provides a stated range of 0.005–10 EU/mL.

For simpler applications, a Gel-Clot Endotoxin Test Kit may be appropriate depending on the validated procedure.

The important point is that sampling strategy and analytical method should be designed together.

17. Method Suitability Is Connected to Sampling Design

A sample is not automatically suitable for testing simply because it can be physically collected.

The sample matrix may interfere with endotoxin detection.

Potential factors include:

• pH
• Protein concentration
• Salts
• Surfactants
• Chelating agents
• Lipids
• Preservatives
• Sugars
• Nanoparticles
• Other formulation components

This means that a sampling plan should consider not only:

Where do we collect the sample?

but also:

Can the sample be reliably analyzed after collection?

A Positive Product Control can help demonstrate that the selected sample matrix does not interfere excessively with endotoxin detection.

FireGene's endotoxin testing sample-preparation guide provides additional discussion of dilution, recovery, and matrix interference.

18. Sampling Plans for Different Manufacturing Stages

A practical sampling strategy may look different depending on the manufacturing process.

Small-Molecule Pharmaceutical Manufacturing

Potential sampling points may include:

• Critical raw materials
• Process water
• Manufacturing solutions
• Final formulation
• Finished product

Biologics Manufacturing

Additional points may include:

• Cell culture-related materials
• Harvest
• Clarified bulk
• Chromatography intermediates
• UF/DF intermediates
• Drug substance
• Final formulation

Cell and Gene Therapy

Potential points may include:

• Raw materials
• Process buffers
• Cell-processing solutions
• Viral-vector-related intermediates
• Formulation materials
• Final product

Cell and gene therapy processes can be particularly challenging because many materials are biologically complex and product quantities may be limited.

FireGene has previously discussed endotoxin risk assessment and contamination control in cell and gene therapy manufacturing.

19. Sampling Plans Should Change When the Process Changes

A sampling plan should not be considered a permanent document.

Manufacturing processes evolve.

Changes may include:

• New suppliers
• New raw materials
• Equipment replacement
• New manufacturing sites
• Process-scale changes
• New buffer formulations
• New single-use assemblies
• Cleaning-process changes
• New storage conditions
• New product formulations

Each significant change should trigger a review of the endotoxin sampling strategy.

For example:

Old process

One buffer
→ one vessel
→ one transfer path

New process

New buffer
→ new vessel
→ additional transfer line
→ longer hold time

The endotoxin risk profile may have changed.

The sampling plan should therefore be reassessed.

20. What to Do When an Endotoxin Result Is Unexpected

A sampling plan should also define what happens when results are unexpected.

A useful investigation sequence is:

1. Confirm the sample identity

Check:

• Batch
• Location
• Time point
• Sample container
• Analyst
• Documentation

2. Review sample handling

Check:

• Storage
• Hold time
• Dilution
• Transfer
• Consumables

3. Review analytical controls

Evaluate:

• Standard curve
• Negative control
• PPC
• Replicates
• Reagent performance
• Instrument performance

4. Compare with nearby sampling points

Ask:

Is this result isolated?

or:

Does it form part of a process-wide pattern?

5. Review manufacturing events

Consider:

• Equipment changes
• Maintenance
• Cleaning
• Supplier changes
• Water-system events
• Process deviations

6. Review historical data

Compare the result with previous batches and trends.

The objective is to determine whether the result represents:

• Analytical contamination
• Sampling error
• A genuine process excursion
• Normal process variability
• An emerging trend

21. Trending Makes a Sampling Plan More Valuable

A sampling plan generates data.

Trending turns those data into process knowledge.

For example, suppose three process points are tested over ten batches.

The results may show:

Point A: consistently low

Point B: stable but slightly increasing

Point C: occasional high results

Without trending, each result may simply be recorded as pass or fail.

With trending, the laboratory may identify:

• Gradual drift
• Recurring contamination
• Seasonal changes
• Supplier-related differences
• Equipment-related patterns
• Process-specific hotspots

This is why sampling strategy and data trending should be connected.

FireGene's endotoxin testing trend-analysis guide discusses how historical endotoxin data can provide information before a formal OOS event occurs.

22. A Practical Framework for Building an Endotoxin Sampling Plan

A simple framework can be summarized in eight steps.

Step 1 — Map the process

Document all major material and process flows.

Step 2 — Identify endotoxin sources

Evaluate:

• Water
• Raw materials
• Equipment
• Buffers
• Components
• Personnel interventions
• Storage
• Transfers

Step 3 — Identify critical control points

Select locations where endotoxin could enter, accumulate, or become difficult to remove.

Step 4 — Define sampling timing

Determine whether samples should be collected:

• Before processing
• After processing
• Before storage
• After storage
• Before transfer
• After transfer
• Before final formulation
• At final product

Step 5 — Establish frequency

Use risk, historical performance, process knowledge, and applicable requirements.

Step 6 — Define sample handling

Specify:

• Container
• Volume
• Storage
• Hold time
• Identification
• Transport

Step 7 — Establish analytical suitability

Confirm:

• Method
• Sensitivity
• Dilution
• PPC recovery
• Matrix compatibility

Step 8 — Trend and periodically reassess

Use historical data to determine whether sampling frequency or locations remain appropriate.

23. Common Sampling Plan Mistakes

Even well-established laboratories can encounter problems when sampling strategies are not periodically reviewed.

Mistake 1: Testing Only the Final Product

This provides limited information about where contamination entered.

Mistake 2: Sampling Only at the Source

For distributed water or transferred materials, downstream locations may have different risks.

Mistake 3: Testing Every Material Equally

Not every material has the same endotoxin risk.

Mistake 4: Ignoring Hold Time

A sample collected immediately after preparation may not behave the same after extended storage.

Mistake 5: Using Non-Qualified Containers

The sampling container itself can contribute endotoxin.

Mistake 6: Ignoring Matrix Effects

A sample that is easy to collect may still be difficult to analyze.

Mistake 7: Treating Every Result as Independent

Historical trends can reveal information that individual results cannot.

Mistake 8: Never Updating the Sampling Plan

Process changes can change endotoxin risk.

24. A Sampling Plan Is Not the Same as a Testing Schedule

This distinction is important.

A testing schedule answers:

When do we run the assay?

A sampling plan answers:

What do we sample, where do we sample, when do we sample, how do we handle the sample, and why is that sample representative?

The second question is much broader.

A laboratory could have a perfectly organized testing calendar and still have a poor endotoxin-control strategy if the samples do not represent the actual process risks.

25. How Sampling Strategy Supports Better Endotoxin Control

A strong sampling strategy can provide three major benefits.

Earlier detection

Potential contamination may be identified before final-product testing.

Better investigations

Multiple sampling points provide additional information when an excursion occurs.

Better process understanding

Historical data can reveal where endotoxin risk is concentrated.

Together, these benefits move endotoxin testing from a purely reactive activity toward a more proactive quality-control strategy.

Conclusion

An endotoxin test result is only meaningful when the sample represents the material or process being evaluated.

That is why a well-designed endotoxin testing sampling plan should consider much more than testing frequency.

It should connect:

Risk assessment

→ Sampling location

→ Sampling timing

→ Sample handling

→ Method suitability

→ Analytical testing

→ Data trending

→ Process improvement

The best sampling plan is not necessarily the one with the most samples.

It is the one that generates the right samples at the right points at the right time.

For pharmaceutical and biopharmaceutical manufacturers, this means looking beyond finished-product testing and considering endotoxin risk throughout the manufacturing lifecycle.

FireGene supports this broader approach with endotoxin testing solutions including Kinetic Chromogenic Endotoxin Test Kits, Gel-Clot TAL/LAL Endotoxin Test Kits, Control Standard Endotoxin, Endotoxin Assay Water, and Pyrogen-Free Vials.

Ultimately, effective endotoxin control is not simply about testing more.

It is about knowing where to look.

Frequently Asked Questions

What is an endotoxin testing sampling plan?

An endotoxin testing sampling plan defines what materials or process points should be sampled, where and when samples should be collected, how frequently testing should occur, and how samples should be handled and analyzed.

How often should endotoxin testing be performed?

There is no universal frequency. Testing frequency should be based on product and process risk, historical performance, process knowledge, applicable specifications, and regulatory or compendial requirements.

Should endotoxin testing be performed only on the finished product?

Not necessarily. Where appropriate, strategic in-process sampling can provide additional information about contamination sources and process behavior.

Should process water be included in an endotoxin sampling plan?

Water is often an important component of pharmaceutical and biopharmaceutical processes. Sampling locations should be determined according to the facility's validated water-system monitoring strategy and risk assessment.

Should every buffer be tested?

Not necessarily. Buffer-testing requirements should be based on the buffer's role, product contact, contamination risk, historical performance, and applicable process controls.

Why is sample timing important?

Endotoxin risk can change during preparation, storage, transfer, and processing. Sampling at strategically selected time points can help identify where changes occur.

Can the sampling container affect an endotoxin result?

Yes. If the container contributes endotoxin, the measured result may not accurately represent the original sample. Appropriate endotoxin-controlled or pyrogen-free containers should be used where required.

What is the relationship between sampling and method suitability?

The selected sample must be analytically compatible with the endotoxin test method. Complex matrices may require dilution, PPC recovery studies, and other suitability assessments.

Can endotoxin sampling data be used for trending?

Yes. When samples are collected consistently from defined locations and analyzed using a controlled method, historical data can support process trending and investigation.

Should an endotoxin sampling plan be updated?

Yes. Significant changes to materials, suppliers, equipment, process conditions, manufacturing sites, formulations, or storage conditions should trigger a review of the sampling strategy.