Loss on Drying, commonly called LOD, is one of the most frequently checked quality parameters in pharmaceutical manufacturing.
If you work in granulation, drying, compression, quality control, or in-process quality assurance, you will hear statements such as:
“Check the LOD before unloading the FBD.”
“LOD is still high. Continue drying.”
“Granules are over-dried.”
“LOD is within limit. Release the material for milling.”
These statements may sound simple, but LOD has a direct effect on the behavior of granules and powders during further processing.
In wet granulation, LOD is commonly used to decide whether the granules have reached the required drying endpoint. If the granules contain too much moisture, they may create problems during milling, blending, compression, or storage.
On the other hand, removing too much moisture can also create problems. Over-dried granules may become brittle, generate more fines, show poor compressibility, or create tablet defects.
Therefore, understanding LOD is important not only for laboratory personnel but also for production operators, production officers, QA personnel, and anyone involved in pharmaceutical manufacturing.
This article explains Loss on Drying in simple language, including its principle, procedure, calculation, acceptance criteria, practical manufacturing importance, common mistakes, troubleshooting, documentation requirements, and frequently asked interview questions.
What Is Loss on Drying (LOD)?
Loss on Drying (LOD) is the percentage of weight lost from a sample after drying it under specified conditions of temperature and time.
The weight loss is mainly due to the removal of moisture from the sample.
However, an important technical point must be understood.
LOD is not always the same as water content.
During drying, a sample may lose:
- Water
- Residual solvents
- Other volatile substances
Therefore, LOD measures the total amount of volatile matter removed under the specified test conditions.
In many pharmaceutical manufacturing processes, particularly wet granulation, the major component removed during drying is water. For this reason, LOD is commonly used as a practical indicator of the moisture level of granules.
Why Is LOD Important in Pharmaceutical Manufacturing?
Moisture can significantly affect the processing behavior and quality of pharmaceutical materials.
A small variation in moisture may change:
- Powder flow
- Granule strength
- Compressibility
- Lubrication behavior
- Tablet hardness
- Friability
- Disintegration
- Dissolution
- Chemical stability
- Microbial growth risk
- Final product quality
This is why LOD is often monitored as an in-process control parameter during manufacturing.
The purpose is not simply to obtain the lowest possible moisture value.
The objective is to achieve the validated moisture range required for consistent downstream processing and product quality.
LOD in the Wet Granulation Process
LOD is especially important during wet granulation.
A typical wet granulation process follows this sequence:
Raw Material Dispensing → Sifting → Dry Mixing → Binder Preparation → Binder Addition → Wet Massing → Drying → LOD Testing → Milling → Final Blending → Compression
After wet granulation, the wet granules contain a considerable amount of moisture.
The granules are normally dried using equipment such as:
- Fluid Bed Dryer (FBD)
- Tray Dryer
- Vacuum Dryer
- Other validated drying systems
During drying, samples are collected according to the approved Batch Manufacturing Record or SOP.
LOD is checked to determine whether the granules have reached the required drying endpoint.
If the LOD is above the specified limit, drying is continued.
If the LOD is within the specified range, the batch may proceed to the next manufacturing stage after completion of the required checks and documentation.
If the LOD is below the validated range, the material may be considered over-dried and should be handled according to the approved procedure.
Practical Example of LOD During FBD Drying
Consider a batch of wet granules being dried in a Fluid Bed Dryer.
The Batch Manufacturing Record specifies:
Target LOD: 1.5% to 2.5% w/w
After the specified initial drying time, the operator collects samples according to the approved sampling procedure.
The first LOD result is:
3.8%
This means the granules still contain more volatile matter than the specified acceptance range.
Drying is continued.
After additional drying, another representative sample is collected.
The second result is:
2.3%
Now the result is within the specified range.
The drying process may be stopped according to the approved manufacturing instructions, and the material can proceed to the next stage after completing the required checks.
The important point is that the operator should not decide the drying endpoint only by looking at the granules or by following a fixed drying time.
The approved process parameters and LOD acceptance criteria must be followed.
Principle of Loss on Drying
The principle of LOD is based on measuring the reduction in sample weight after drying under specified conditions.
First, the initial weight of the sample is recorded.
The sample is then heated at the specified temperature.
During heating, moisture and other volatile substances are removed.
After drying, the final weight of the sample is measured.
The difference between the initial and final weight represents the material lost during drying.
This weight loss is calculated as a percentage of the original sample weight.
LOD Calculation Formula
The general formula is:
LOD (%) = [(Initial Weight − Final Weight) ÷ Initial Weight] × 100
Example
Initial sample weight = 5.000 g
Final sample weight after drying = 4.875 g
Weight loss:
5.000 − 4.875 = 0.125 g
Therefore:
LOD (%) = (0.125 ÷ 5.000) × 100
LOD = 2.5%
If the approved specification is 1.5% to 3.0%, the sample complies with the specified LOD requirement.
Methods Used for LOD Testing
Different methods may be used to determine Loss on Drying depending on the material, product, pharmacopoeial requirement, validated analytical method, and company procedure.
The most common methods include:
1. Moisture Analyzer Method
A moisture analyzer is commonly used for rapid LOD testing during pharmaceutical manufacturing.
The instrument contains:
- Weighing balance
- Heating unit
- Sample pan
- Temperature control system
- Display and calculation system
The instrument continuously measures the sample weight during heating and calculates the percentage weight loss.
This method is fast and convenient for in-process testing.
However, the correct test parameters must be used.
These may include:
- Sample quantity
- Drying temperature
- Heating program
- Endpoint criteria
- Test duration
Using incorrect parameters may produce misleading results.
2. Hot Air Oven Method
In this method, the sample is weighed and dried in an oven under specified conditions.
After drying, the sample is cooled in a desiccator and weighed again.
Drying and weighing may be repeated until the required constant weight condition is achieved, depending on the approved method.
The percentage weight loss is then calculated.
The oven method is generally slower than a moisture analyzer but may be required by a pharmacopoeial monograph, specification, or approved analytical procedure.
3. Vacuum Oven Method
Some materials may be sensitive to high temperatures or may require drying under reduced pressure.
In such cases, a vacuum oven may be used according to the specified test method.
Reduced pressure can support the removal of moisture or volatile substances at lower temperatures.
The exact test conditions must always follow the approved analytical procedure.
Step-by-Step Procedure for LOD Testing Using a Moisture Analyzer
The exact procedure varies between instruments and manufacturing sites. The following is a general GMP-based workflow.
Step 1: Verify the Instrument Status
Before starting the test, check that:
- The instrument is clean.
- The calibration status is valid.
- The instrument is within the preventive maintenance period.
- The equipment identification number is correct.
- No previous sample residue is present on the sample pan or heating chamber.
Never use an instrument with an expired calibration status unless permitted through an approved quality system procedure.
Step 2: Verify the Test Method
Check the approved specification, SOP, BMR, or analytical method.
Confirm:
- Required sample quantity
- Drying temperature
- Drying program
- Endpoint criteria
- Acceptance limit
Do not use parameters from memory.
Different products may require different LOD test conditions.
Step 3: Collect a Representative Sample
Sampling is one of the most important parts of LOD testing.
The sample should represent the actual moisture condition of the batch.
During FBD drying, moisture may not always be uniform throughout the product bowl.
Depending on the equipment design and validated sampling procedure, samples may be collected from different locations.
For example:
- Top
- Middle
- Bottom
The exact sampling locations and number of samples should follow the approved procedure.
A poor sample can produce a correct laboratory result that does not represent the actual batch.
Step 4: Prevent Moisture Loss During Sample Handling
After sample collection, perform the test without unnecessary delay.
Do not leave the sample exposed to the environment for a long period.
A warm sample may lose moisture during transportation and handling.
Hygroscopic materials may absorb moisture from the environment.
Both situations can affect the final result.
Use the approved sample container and follow the specified sample handling procedure.
Step 5: Clean and Place the Sample Pan
Check that the sample pan is clean and dry.
Place it correctly inside the instrument.
Tare the instrument according to the operating procedure.
Step 6: Add the Required Sample Quantity
Transfer the specified quantity of sample to the pan.
Spread the sample evenly.
Do not keep the complete sample in one thick heap unless specifically required by the method.
Uneven sample distribution can cause non-uniform heating and may affect the result.
Step 7: Start the Test
Select or verify the approved method.
Start the test.
The instrument heats the sample and continuously monitors the reduction in weight.
Do not disturb the instrument during testing.
Step 8: Record the Result
After completion of the test, record the result according to the approved documentation procedure.
The record may include:
- Product name
- Batch number
- Stage of processing
- Sample location
- Date
- Time
- Instrument ID
- Test method
- Temperature
- Sample weight
- LOD result
- Name or signature of the person performing the test
Electronic records should be handled according to applicable data integrity requirements.
Step 9: Compare the Result With the Specification
Compare the result with the approved acceptance criteria.
Three situations are possible.
LOD above the specified range:
The granules may require additional drying according to the approved manufacturing instructions.
LOD within the specified range:
The drying endpoint has been achieved, subject to completion of all other required process checks.
LOD below the specified range:
The granules may be over-dried.
Do not make an unapproved process adjustment.
Inform the responsible production and quality personnel and follow the applicable SOP.
Step 10: Clean the Instrument
After completion of testing:
- Remove the used sample.
- Clean the sample pan.
- Clean any spilled material.
- Complete the required equipment usage record.
- Keep the instrument ready for the next test.
How Is LOD Sampling Done During FBD Drying?
This is a practical area where mistakes can easily occur.
A single sample may not always represent the moisture distribution of the complete batch.
During drying, some portions of the product bed may dry faster than others.
This may happen due to:
- Airflow distribution
- Product load
- Granule size variation
- Filter condition
- Equipment design
- Inlet air temperature
- Sampling technique
Therefore, the sampling procedure should be established during process development and validation.
Where the approved procedure requires samples from multiple locations, the operator should follow those exact locations.
For example, samples may be collected from the top, middle, and bottom portions of the product container.
The results may be tested individually or as a composite sample depending on the approved procedure.
Never change the sampling plan simply to obtain a passing result.
What Is the Acceptable LOD Limit in Pharmaceuticals?
There is no single universal LOD limit for all pharmaceutical products.
The acceptable limit depends on:
- Drug substance characteristics
- Excipients
- Formulation
- Manufacturing process
- Product stability
- Granule properties
- Compression behavior
- Approved product specification
- Process validation data
- Pharmacopoeial requirements, where applicable
For one product, the required LOD may be 1.0% to 2.0%.
For another product, it may be 2.0% to 3.5%.
Some materials may require a maximum limit rather than a range.
Therefore, always follow the approved specification, BMR, SOP, or validated analytical procedure.
What Happens If LOD Is Too High?
High LOD means the sample has more volatile matter than the specified requirement under the test conditions.
In wet granulation, this usually indicates insufficient drying.
Possible consequences include:
- Granules sticking during milling
- Poor powder flow
- Material sticking to equipment surfaces
- Picking and sticking during compression
- Weight variation
- Changes in compressibility
- Stability problems
- Increased microbial risk in moisture-sensitive products
- Difficulty in achieving consistent tablet quality
Practical Manufacturing Example
Suppose wet granules are transferred for milling before achieving the required LOD.
During milling, the granules may stick to the screen.
The mill output may reduce.
Material may accumulate inside the equipment.
After blending, the blend may show poor flow.
During compression, sticking or picking may occur.
The original problem started at the drying stage, but the visible manufacturing problem appeared later during milling or compression.
This is why drying endpoint control is important.
What Happens If LOD Is Too Low?
A low LOD result may indicate that the material has been over-dried.
Over-drying is also undesirable.
Possible consequences include:
- Brittle granules
- Excessive fines generation
- Poor compressibility
- Weak tablets
- Increased friability
- Segregation risk
- Changes in dissolution behavior
- Static charge problems
- Reduced process yield due to dust generation
Practical Manufacturing Example
Suppose the target LOD is 1.5% to 2.5%, but drying is continued until the granules reach 0.7%.
The granules may become too dry and fragile.
During milling, excessive fines may be generated.
The final blend may have poor particle size distribution.
During compression, tablet hardness may become difficult to maintain.
This demonstrates an important manufacturing principle:
The lowest possible LOD is not necessarily the best LOD.
The correct target is the validated moisture range.
Common Causes of High LOD
If the LOD remains above the specified range, possible causes should be investigated.
Common causes include:
- Insufficient drying time
- Low inlet air temperature
- Poor airflow
- Excessive product load
- High initial moisture content
- Incorrect binder quantity
- Excessive binder addition
- Poor atomization or binder distribution
- Large wet lumps
- Improper granulation endpoint
- Blocked FBD filters
- Incorrect shaking cycle
- Air leakage
- High humidity of inlet air
- Incorrect LOD test parameters
- Non-representative sampling
The process should be evaluated systematically instead of simply extending drying time without understanding the cause.
Common Causes of Low LOD
Possible causes of unusually low LOD include:
- Excessive drying time
- High inlet air temperature
- Delayed sampling
- Sample exposed to open air
- Incorrect sample quantity
- Wrong instrument method
- Incorrect endpoint setting
- Instrument calibration problem
- Non-representative sample
- Process parameters outside the validated range
An unexpected low result should not automatically be accepted as a better result.
It should be evaluated against the approved process requirements.
Common Mistakes During LOD Testing
1. Taking the Sample From Only One Convenient Location
The easiest sampling point may not represent the complete batch.
Follow the approved sampling procedure.
2. Delaying the Test After Sampling
Moisture can be lost or absorbed during sample handling.
Test the sample within the specified period.
3. Using the Wrong Product Method
Different products may have different temperatures, sample quantities, and endpoint criteria.
Always verify the method before starting the test.
4. Placing the Sample in a Thick Layer
A thick sample layer may dry unevenly and affect the result.
Spread the sample as required by the approved procedure.
5. Testing an Incorrect Sample Quantity
Too much or too little sample can affect drying behavior and repeatability.
Use the specified sample quantity.
6. Continuing Drying Without Following the BMR
Operators should not independently change drying parameters or extend processing beyond approved instructions.
Any required action should follow the established procedure.
7. Repeating the Test Until a Passing Result Is Obtained
Repeated testing without a scientifically justified reason is not acceptable GMP practice.
Unexpected, atypical, or failing results should be handled according to the applicable SOP and quality system.
8. Ignoring Differences Between Sampling Locations
A passing average can sometimes hide non-uniform drying.
Where individual location results are required, each result should be evaluated according to the approved procedure.
9. Poor Cleaning of the Moisture Analyzer
Residue from a previous sample can contaminate the next sample or affect instrument performance.
10. Recording Results Later From Memory
LOD results should be documented at the time the activity is performed according to good documentation practices.
LOD and Drying Endpoint Detection
In routine manufacturing, LOD is often used as an important indicator for determining the drying endpoint.
However, drying endpoint control should be based on the validated process.
The following parameters may also be monitored:
- Inlet air temperature
- Outlet air temperature
- Product temperature
- Drying time
- Airflow
- Relative humidity
- Moisture trend
- Process analytical technology, where available
A fixed drying time alone may not always guarantee the same final moisture level.
Initial wet mass moisture, environmental conditions, batch size, airflow, filter condition, and raw material variability can influence drying behavior.
Therefore, a scientifically established endpoint strategy is more reliable than simply drying every batch for the same number of minutes.
LOD vs Water Content: What Is the Difference?
LOD and water content are related but are not identical.
LOD measures total weight lost during drying under specified conditions.
This may include water and other volatile substances.
Water content measures the amount of water specifically present in the sample.
Karl Fischer titration is commonly used when specific determination of water is required.
For materials containing residual solvents or other volatile substances, the LOD result may be higher than the actual water content.
Therefore, the correct analytical method should be selected based on the material characteristics and specification requirements.
LOD vs Moisture Content
In routine manufacturing, people often use the terms LOD and moisture content interchangeably.
Technically, this is not always correct.
LOD measures the total volatile weight loss under defined test conditions.
Moisture content refers specifically to moisture present in the material.
For water-based wet granulation processes where other volatile components are negligible, LOD may be used as a practical moisture indicator.
The analytical method and product specification should determine the correct terminology.
Role of Production Personnel in LOD Control
Production personnel should:
- Follow approved drying parameters.
- Collect samples according to the specified sampling plan.
- Prevent unnecessary sample exposure.
- Ensure correct identification of samples.
- Record processing parameters at the time of activity.
- Inform responsible personnel when results are outside the expected range.
- Avoid unauthorized process adjustments.
- Follow the BMR and applicable SOPs.
Production personnel should understand that LOD is not just a number entered into the batch record.
It is an important indicator of process performance.
Role of IPQA or QA Personnel
Depending on the pharmaceutical site’s quality system, QA or IPQA personnel may be responsible for:
- Verifying compliance with the approved procedure
- Reviewing sampling practices
- Checking instrument calibration status
- Reviewing LOD results
- Verifying entries in batch records
- Evaluating deviations
- Reviewing atypical process behavior
- Ensuring data integrity requirements are followed
- Supporting investigations when results are outside established limits
The exact responsibilities should be defined in approved procedures.
GMP and Data Integrity Requirements for LOD Testing
LOD testing generates GMP-relevant data.
Therefore, the result should be handled according to good documentation practices and applicable data integrity principles.
Important expectations include:
- Use only qualified and calibrated instruments.
- Use the correct approved analytical method.
- Record data at the time of testing.
- Do not overwrite original results.
- Do not delete electronic records without authorized procedures.
- Investigate unexplained repeat testing.
- Maintain audit trails where applicable.
- Ensure each test is attributable to the person who performed it.
- Retain records according to the approved retention procedure.
If the first result is unexpected, the result should not simply be discarded and replaced by another test.
The applicable investigation or repeat-testing procedure should be followed.
Troubleshooting Guide for LOD Problems
| Observation | Possible Cause | Recommended Check |
|---|---|---|
| LOD remains high | Insufficient drying | Review drying time and process parameters |
| LOD decreases very slowly | Poor airflow | Check filters, airflow, and equipment condition |
| Different results from different locations | Non-uniform drying | Review airflow distribution and sampling results |
| LOD suddenly becomes very low | Over-drying or sampling delay | Review drying time and sample handling |
| Repeated LOD variation | Non-representative sampling | Review sampling technique and locations |
| Moisture analyzer gives inconsistent results | Instrument or method issue | Check calibration, cleaning, sample quantity, and method |
| High LOD with large granules | Poor wet mass distribution | Review binder addition and granulation endpoint |
| Excessive fines after drying | Over-drying | Review endpoint and process parameters |
Best Practices for Reliable LOD Results
For consistent and reliable LOD testing:
- Follow the approved sampling plan.
- Use a calibrated and clean instrument.
- Verify the product-specific method before testing.
- Test the sample without unnecessary delay.
- Use the specified sample quantity.
- Spread the sample uniformly.
- Follow approved drying parameters.
- Document results immediately.
- Review trends instead of looking only at one number.
- Investigate unusual results.
- Never repeat testing only to obtain a passing value.
- Train operators on the manufacturing impact of both high and low LOD.
Practical Lessons From Pharmaceutical Manufacturing
Lesson 1: A Passing LOD Result Is Only as Good as the Sample
If the sample does not represent the batch, the test result cannot provide reliable information about the complete batch.
Lesson 2: More Drying Is Not Always Better
Excessive drying can damage granule properties and create downstream processing problems.
Lesson 3: Moisture Problems Often Appear at the Next Manufacturing Stage
Poor drying may first become visible as milling problems, flow issues, sticking, hardness variation, or excessive fines.
Lesson 4: Process Trends Are Valuable
Comparing drying time, inlet temperature, outlet temperature, product temperature, and LOD results across batches can help identify gradual process changes.
Lesson 5: Do Not Chase a Passing Result
Repeated testing without scientific justification creates serious GMP and data integrity concerns.
The process and the result should be evaluated according to approved procedures.
Frequently Asked Questions About LOD
What is the full form of LOD?
LOD stands for Loss on Drying.
What is LOD in pharmaceuticals?
LOD is the percentage of weight lost by a pharmaceutical sample after drying under specified conditions.
Why is LOD checked during wet granulation?
LOD is checked to determine whether wet granules have reached the required drying endpoint before further processing.
Is LOD the same as water content?
No. LOD may include water and other volatile substances, while water content specifically measures water.
Which instrument is used for LOD testing?
Common methods include moisture analyzers, hot air ovens, and vacuum ovens, depending on the approved test method.
What happens if LOD is high?
High LOD may cause poor flow, sticking, milling problems, compression defects, stability issues, and other manufacturing problems.
What happens if LOD is too low?
Very low LOD may indicate over-drying, which can cause brittle granules, excessive fines, poor compressibility, and tablet quality problems.
Can drying time be extended if LOD is high?
Any extension of drying time or change in process parameters should be performed according to the approved BMR, SOP, or authorized quality procedure.
Why are samples collected from different locations in an FBD?
Different areas of the product bed may have different moisture levels. Representative sampling helps detect non-uniform drying.
What is the ideal LOD value for granules?
There is no universal ideal value. The correct LOD range depends on the product, formulation, process development data, validation results, and approved specification.
LOD Interview Questions and Answers
1. What is Loss on Drying?
Loss on Drying is the percentage reduction in sample weight after drying under specified conditions.
2. Why is LOD important during wet granulation?
It helps determine whether granules have reached the required moisture range before milling, blending, and compression.
3. What is the formula for LOD?
LOD (%) = [(Initial Weight − Final Weight) ÷ Initial Weight] × 100.
4. What is the difference between LOD and water content?
LOD measures total volatile weight loss, while water content specifically measures water.
5. What should you do if LOD is above the limit?
Follow the approved manufacturing procedure, inform the responsible personnel, and continue or adjust the process only as permitted by the BMR or SOP.
6. What happens if granules are over-dried?
Over-dried granules may generate excessive fines, show poor compressibility, create static problems, and affect tablet quality.
7. Why is representative sampling important?
Because moisture may not be uniformly distributed throughout the batch.
8. Can you repeat an LOD test after obtaining a failing result?
Repeat testing should only be performed according to an approved procedure and with scientifically justified reasons. Testing should not be repeated simply to obtain a passing result.
9. What factors can affect LOD results?
Sampling technique, sample handling, sample quantity, drying temperature, instrument condition, test method, environmental exposure, and product characteristics can affect the result.
10. Is the lowest possible LOD always desirable?
No. The objective is to achieve the validated and approved moisture range required for consistent product quality.
Conclusion
Loss on Drying is much more than a routine in-process test.
It is an important link between the drying process and the performance of granules during milling, blending, compression, and subsequent product storage.
A high LOD result may indicate insufficient drying, while an unusually low LOD result may indicate over-drying. Both situations can affect manufacturing performance and product quality.
Reliable LOD control depends on representative sampling, correct test parameters, qualified instruments, proper sample handling, accurate documentation, and strict compliance with approved procedures.
The most important practical lesson is simple:
Do not dry a pharmaceutical product to the lowest possible moisture level. Dry it to the scientifically established and validated moisture range required for consistent processing and product quality.
When LOD is understood as a process control tool rather than just a number in the Batch Manufacturing Record, it becomes much easier to prevent downstream manufacturing problems and maintain consistent pharmaceutical quality.
