Is the 90 mm Petri dish required by USP?

USP does not explicitly mandate size, but all validated methods assume 90 mm plates.

Why are settle plates usually 90 mm?

Settle plate exposure studies and CFU/m² calculations were standardized using 90 mm plates.

Is 90 mm Petri dish size mandatory in GMP?

It is not explicitly mandatory, but it is the globally accepted validated standard. Any deviation requires scientific justification and validation.

Does Petri dish size affect CFU results?

Yes. Plate size affects agar depth, colony distribution, recovery efficiency, and statistical accuracy.

Are cleanroom microbial limits based on 90 mm plates?

Yes. Cleanroom environmental monitoring limits were established using 90 mm settle and air sampling plates.

Why Do We Use a 90 mm Petri Dish in Microbiology? Scientific, Practical & Regulatory Reasons

Q: Why is a 90 mm Petri dish used in microbiology?

A 90 mm Petri dish provides optimal surface area, validated agar depth, accurate CFU counting, and global regulatory acceptance.

Q: Can other plate sizes be used?

Yes, but only with full method validation and regulatory justification.

Why Do We Use a 90 mm Petri Dish in Microbiology? Scientific, Practical & Regulatory Reasons

In microbiology laboratories worldwide—from academic research labs to pharmaceutical GMP cleanrooms—the 90 mm Petri dish has become the undisputed standard for microbial cultivation, enumeration, and environmental monitoring. But have you ever asked:

Why not 80 mm or 100 mm?
Why do USP, EP, and PDA methods implicitly assume 90 mm plates?
Is the 90 mm dish scientifically superior or just historical?

This article answers those questions in depth, covering scientific reasoning, statistical reliability, practical workflow advantages, and global regulatory expectations.

1. What Is a Petri Dish?

A Petri dish is a shallow, circular, transparent laboratory container traditionally made of glass or polystyrene plastic. It is designed to hold solid or semi-solid culture media (such as agar) used to grow microorganisms under controlled conditions.

The most common sizes include:

55–60 mm (small research plates)
70 mm (limited specialty use)
90 mm (global standard)
100–150 mm (large-format research plates)

Among these, the 90 mm Petri dish dominates pharmaceutical, food, water, and environmental microbiology.

2. Why Exactly 90 mm? – The Core Scientific Rationale

2.1 Optimal Surface Area for Microbial Growth

A 90 mm Petri dish provides a surface area of approximately:

~63.6 cm²

This surface area is scientifically ideal because:

It supports uniform microbial distribution
It prevents colony overcrowding
It allows clear isolation of colonies
It ensures sufficient oxygen diffusion

Smaller plates (<60 mm) lead to colony overlap and poor counting accuracy. Larger plates (>100 mm) reduce practicality and increase contamination risk.

2.2 Statistically Reliable CFU Enumeration

In microbiology, accuracy in colony forming unit (CFU) counting is critical. The 90 mm plate allows:

Reliable counting in the range of 30–300 CFU
Lower probability of merged colonies
Better statistical confidence

This is why most validation studies, recovery efficiency calculations, and method verifications assume 90 mm plates.

3. Agar Volume Compatibility (20–25 mL Rule)

Agar depth plays a crucial role in microbial growth, oxygen diffusion, and moisture retention.

Petri Dish Size	Recommended Agar Volume	Practical Outcome
60 mm	8–10 mL	Dries quickly
90 mm	20–25 mL	Ideal depth & moisture
150 mm	50–60 mL	Excessive media usage

Most pharmacopeial methods are validated assuming 20–25 mL agar poured into a 90 mm plate.

4. Regulatory & Compendial Expectations

4.1 United States Pharmacopeia (USP)

USP chapters such as:

<61> Microbial Enumeration Tests
<62> Tests for Specified Microorganisms
<1116> Microbiological Control and Monitoring

implicitly assume:

Standard agar depth
Standard colony spacing
Standard CFU recovery efficiency

All these assumptions align with 90 mm Petri dishes.

4.2 PDA (Parenteral Drug Association)

PDA Technical Reports (TR 13, TR 33, TR 34) on:

Environmental monitoring
Cleanroom microbiology
Settle plate exposure studies

use 90 mm plates as the reference standard for:

4-hour settle plate exposure
CFU/m² calculations
Trend analysis

4.3 European & International Alignment

European Pharmacopoeia (EP), WHO GMP, PIC/S, and ISO 14698 all harmonize around the same assumptions—making 90 mm plates a global expectation.

5. Environmental Monitoring & Cleanroom Science

5.1 Settle Plate Calculations

A 90 mm plate area (~63.6 cm²) allows standardized conversion to:

CFU per m² per hour

This is critical for:

Grade A/B cleanroom monitoring
Trend analysis
Regulatory inspections

Changing plate size invalidates historical data trends.

5.2 Active Air Sampling Correlation

Air sampler heads are designed to impact microorganisms evenly across a 90 mm agar surface, ensuring:

Uniform impaction
Minimal rebound
Comparable recovery rates

6. Practical Laboratory Advantages

Compatible with automated plate pourers
Fits standard incubators
Works with colony counters
Lower media wastage
Reduced condensation risk

7. Why Not Use Other Sizes?

7.1 Smaller Plates (60 mm)

Overcrowding
Poor statistical confidence
Faster drying

7.2 Larger Plates (150 mm)

Difficult handling
Higher contamination risk
Not regulator-validated

8. Common Audit & Inspection Observations

Regulatory inspectors often question:

Use of non-standard plate sizes
Lack of validation for alternate sizes
Inconsistent historical trending

Using 90 mm plates avoids unnecessary regulatory scrutiny.

9. Frequently Asked Questions (FAQ)

Q1. Is 90 mm Petri dish mandatory?

Not legally mandatory, but it is the globally accepted validated standard.

Q2. Can I use 100 mm plates instead?

Only with full method validation and regulatory justification.

Q3. Why do settle plates specify 4 hours?

Because sedimentation studies were standardized using 90 mm plates.

Q4. Does plate size affect CFU results?

Yes. Plate size directly affects colony distribution and recovery.

10. Conclusion

The dominance of the 90 mm Petri dish is not accidental. It is the result of decades of scientific optimization, statistical validation, regulatory harmonization, and practical laboratory efficiency.

In pharmaceutical microbiology, changing plate size without justification is not innovation—it is risk.

Standardization protects data integrity, regulatory compliance, and patient safety.

11. Statistical & Mathematical Basis for the 90 mm Petri Dish

Beyond historical and regulatory acceptance, the dominance of the 90 mm Petri dish is deeply rooted in statistics, probability theory, and microbial spatial distribution models.

11.1 Colony Distribution and Poisson Statistics

Microbial colony formation on agar surfaces follows a Poisson distribution, especially when microorganisms are randomly dispersed in low concentrations.

The probability of observing n colonies on a plate depends on:

Surface area of the plate
Inoculum volume
Microbial concentration
Uniform spreading efficiency

A 90 mm plate provides the optimal surface-to-volume ratio that minimizes:

Colony overlap
Edge effects
Localized clustering

Smaller plates artificially increase colony density per cm², violating Poisson assumptions and reducing count accuracy.

11.2 Why the “30–300 CFU Rule” Fits a 90 mm Plate

The universally accepted microbiological counting rule states that:

Only plates with 30–300 CFU should be used for enumeration

This rule was empirically derived using 90 mm Petri dishes. On a 90 mm plate:

<30 CFU → high statistical uncertainty
30–300 CFU → optimal accuracy
>300 CFU → overcrowding and merging

If a different plate size is used, the acceptable CFU range must be recalculated and revalidated—something rarely done in routine labs.

12. Agar Depth, Oxygen Diffusion & Moisture Control

12.1 Agar Depth Standardization

Agar depth directly influences:

Oxygen diffusion
Nutrient availability
Colony morphology
Moisture retention

When 20–25 mL of agar is poured into a 90 mm plate, the resulting depth is approximately:

3.5–4.0 mm

This depth has been shown to be ideal for:

12.2 Problems with Incorrect Agar Depth

Agar Condition	Impact
Too shallow	Rapid drying, stressed colonies
Too deep	Reduced oxygen diffusion
Uneven depth	Non-uniform colony size

Using non-90 mm plates often disrupts this balance.

13. Edge Effect & Condensation Control

13.1 What Is the Edge Effect?

The edge effect refers to abnormal colony growth near the rim of a Petri dish caused by:

Temperature gradients
Condensation accumulation
Uneven agar cooling

The geometry of a 90 mm plate minimizes these effects by balancing:

Surface area
Wall height
Lid clearance

13.2 Condensation Management

Condensation droplets can:

Spread colonies
Cause false high counts
Invalidate test results

90 mm plates are designed to:

Support inverted incubation
Maintain stable humidity
Reduce lid dripping

14. Validation & Method Transfer Considerations

14.1 Method Validation Assumptions

When microbiological methods are validated, they assume:

Specific plate surface area
Defined agar volume
Standard colony spacing

Changing plate size without revalidation compromises:

Accuracy
Precision
Reproducibility

14.2 Method Transfer Failures Linked to Plate Size

In inter-laboratory transfers, deviations often occur due to:

Use of 100 mm plates instead of 90 mm
Different agar fill volumes
Inconsistent spreading techniques

Regulators frequently cite this as a root cause of OOS microbial data.

15. Cleanroom Environmental Monitoring Applications

15.1 Settle Plates

Settle plates rely on gravitational settling of microorganisms. The 90 mm plate:

Provides validated exposure area
Supports 2–4 hour exposure studies
Allows historical trend comparison

Using a different size disrupts long-term contamination trending.

15.2 Surface Monitoring (Contact Plates vs 90 mm Plates)

Although contact plates are used for surfaces, 90 mm plates are preferred when:

Swab eluates are plated
Irregular surfaces are sampled
Higher recovery is required

16. Compatibility with Automated & Digital Systems

16.1 Colony Counters

Most automated colony counters are calibrated for:

90 mm plate diameter
Standard grid overlays
Edge exclusion zones

Non-standard plate sizes reduce counting accuracy.

16.2 Robotic Plate Pourers & Stackers

Automation systems in QC labs are optimized for:

90 mm stacking height
Standard lid geometry
Uniform agar cooling

17. Regulatory Inspection Expectations

17.1 Common Auditor Questions

Why is a non-standard plate size used?
Where is the validation data?
How do you compare historical trends?

Using 90 mm plates avoids unnecessary justifications.

17.2 Data Integrity Perspective

Consistency of tools is a core data integrity principle. Plate size variation introduces:

Hidden bias
Unexplained variability
Regulatory risk

18. Practical Case Study

Case: Environmental Monitoring Failure

A sterile manufacturing site switched from 90 mm to 100 mm plates to “improve recovery.” Result:

Sudden increase in CFU counts
False contamination trend
Regulatory observation during inspection

Root cause: Increased surface area without recalculating limits.

19. Summary of Key Advantages of 90 mm Petri Dishes

Scientifically optimized surface area
Validated agar depth
Accurate CFU enumeration
Global regulatory acceptance
Automation compatibility
Data integrity protection

20. Lead-in to Advanced Topics (Part 3)

In Part 3, we will cover:

Why air samplers

21. Why Are Active Air Samplers Designed for 90 mm Petri Dishes?

One of the strongest scientific validations of the 90 mm Petri dish is its universal adoption in active air sampling systems used in pharmaceutical cleanrooms.

21.1 Impact Principle & Agar Surface Geometry

Most microbial air samplers operate on the impaction principle, where air is drawn through a perforated head and microorganisms are impacted directly onto the agar surface.

The impaction head is engineered specifically for:

90 mm agar surface diameter
Uniform particle dispersion
Controlled jet velocity

Changing the plate size disrupts:

Jet-to-surface distance
Particle impaction angle
Microbial recovery efficiency

21.2 Regulatory Expectation During Air Sampler Qualification

During qualification (IQ/OQ/PQ) of air samplers, regulators expect:

Use of manufacturer-recommended 90 mm plates
Recovery studies using standard agar depth
Correlation with historical cleanroom data

Using non-standard plates without requalification is considered a data integrity risk.

22. Mathematical Basis of CFU/m³ & CFU/m² Calculations

22.1 CFU per Cubic Meter (Air Sampling)

CFU/m³ calculations assume a known agar collection surface. The formula:

CFU/m³ = (Number of Colonies × 1000) / Volume of Air Sampled (L)

This formula assumes:

Standard impaction efficiency
Standard agar surface (90 mm)
Validated recovery correction factors

Changing plate size alters recovery probability and invalidates the calculation.

22.2 CFU per Square Meter (Settle Plates)

Settle plate data is often normalized to:

CFU/m²/hour

Since a 90 mm plate has an area of approximately 0.00636 m², calculations are standardized.

Any deviation in plate size requires recalculation of contamination limits—rarely accepted by inspectors.

23. Why Are Results Often Reported as “<1 CFU”?

Microbiological results are frequently reported as <1 CFU instead of zero. This practice is directly linked to:

Sampling statistics
Detection limits
Plate surface area

A 90 mm plate represents a defined sampled area or air volume. “No colonies observed” does not mean absolute absence—only that contamination is below the detection limit.

Thus, <1 CFU is scientifically correct and regulator-accepted.

24. Cleanroom Classification & 90 mm Plates

24.1 Grade A / ISO 5 Expectations

In Grade A environments:

Expected CFU = 0
Detection sensitivity must be maximized
Data trending must be ultra-consistent

Using 90 mm plates ensures:

Maximum validated sensitivity
Comparable historical trends
Inspection defensibility

24.2 Grade B, C & D Areas

Microbial limits for these grades were developed using:

90 mm settle plates
90 mm contact plates (or equivalent area)
90 mm air sampler agar plates

This reinforces global harmonization.

25. Historical Evolution of the 90 mm Standard

Early microbiologists experimented with various plate sizes. Over decades:

90 mm provided the best balance
Standard glass molds were created
Regulatory bodies aligned on one size

What began as practicality evolved into a global scientific standard.

26. Risk Assessment: Deviating from 90 mm Plates

26.1 Technical Risks

Altered recovery efficiency
False high or low counts
Invalid trend analysis

26.2 Regulatory Risks

Audit observations
Method validity challenges
Data integrity concerns

27. Inspection-Ready Justification Statement

Example:

“The laboratory uses 90 mm Petri dishes for all microbiological enumeration and environmental monitoring activities in alignment with pharmacopeial expectations, historical validation studies, air sampler design, and industry best practices. This ensures data comparability, statistical reliability, and regulatory compliance.”

28. Advanced FAQs (Expert Level)

Q1. Can I use rectangular plates?

No. They lack validated recovery data.

Q2. Is agar volume more important than plate size?

Both are interdependent and validated together.

Q3. Why do colony counters assume 90 mm?

Algorithms are calibrated to known geometry.

Q4. Does plate size affect mold recovery?

Yes. Mold spread patterns are area-dependent.

Q5. Can limits be adjusted for 100 mm plates?

Only with full statistical revalidation.

29. Training & Knowledge Transfer Importance

Teaching microbiologists why 90 mm plates are used—not just that they are used—improves:

Regulatory confidence
Root cause analysis quality
Audit response maturity

30. Final Conclusion – Why the 90 mm Petri Dish Endures

The 90 mm Petri dish is not merely a laboratory accessory—it is a scientifically optimized, statistically validated, and regulator-endorsed measurement system.

Its continued use ensures:

Reliable microbiological data
Global harmonization
Patient safety
Regulatory confidence

In pharmaceutical microbiology, standardization is not a limitation—it is a safeguard.

The 90 mm Petri dish remains the gold standard.

31. Practical Laboratory Examples: Why 90 mm Plates Matter in Daily QC Work

Understanding the theory is important, but microbiologists truly appreciate the value of the 90 mm Petri dish when examining day-to-day laboratory scenarios.

31.1 Example 1: Product Bioburden Testing

In a routine bioburden test, 1 mL of sample is plated using the pour plate method.

Using a 90 mm plate ensures even dispersion
Colonies are well separated
CFU counting remains accurate and reproducible

If a smaller plate is used, colony crowding occurs. If a larger plate is used, colonies may spread unevenly, creating interpretation challenges.

31.2 Example 2: Water Testing (PW / WFI)

For pharmaceutical water testing using membrane filtration:

Filters are typically placed on 90 mm agar plates
The filter fits without curling
Uniform nutrient diffusion is achieved

This geometry ensures recovery of stressed or slow-growing organisms.

32. Deviation & OOS Investigations Related to Plate Size

32.1 Typical Deviation Scenario

A laboratory reports a sudden increase in environmental monitoring counts.

Investigation findings:

Plate supplier changed
Plate diameter increased slightly
Agar fill volume remained unchanged

This resulted in:

Reduced agar depth
Altered recovery
False contamination trend

Conclusion: Plate size standardization failure.

32.2 Corrective and Preventive Actions (CAPA)

Revert to qualified 90 mm plates
Requalify new suppliers
Update SOP to specify plate dimensions

33. Trending & Data Analysis: Why Consistency Is Critical

Microbial trending relies on long-term consistency. Plate size changes introduce hidden variables that:

Distort control charts
Trigger false alerts
Mask real contamination events

Using 90 mm plates ensures year-on-year comparability.

34. SOP Language: How Plate Size Should Be Defined

Example SOP Statement:

“All microbiological enumeration, environmental monitoring, and air sampling activities shall be performed using sterile 90 mm diameter Petri dishes containing 20–25 mL of qualified culture media, unless otherwise validated.”

Such clarity prevents ambiguity during audits.

35. Regulatory Inspection Questions & Model Answers

Q1. Why does your laboratory use only 90 mm Petri dishes?

Because historical validation, pharmacopeial methods, air sampler design, and microbial recovery studies are based on 90 mm plates, ensuring data integrity and regulatory compliance.

Q2. What happens if plate size changes?

Any change requires method revalidation, trend re-establishment, and risk assessment.

Q3. How do you ensure supplier consistency?

By qualifying suppliers and verifying plate dimensions during receipt.

36. Training Perspective: Teaching New Microbiologists

New analysts often treat plate size as trivial. Training should emphasize that:

Plate size is part of the measurement system
It affects detection sensitivity
It impacts regulatory confidence

Understanding this transforms technicians into scientists.

37. Interview & Exam Questions Based on 90 mm Plates

37.1 Short Answer Questions

Why is 90 mm considered the standard Petri dish size?
How does plate size affect CFU accuracy?
Why are agar volumes standardized?

37.2 Long Answer Questions

Discuss the scientific and regulatory rationale for using 90 mm Petri dishes in pharmaceutical microbiology.
Explain how plate size influences environmental monitoring trends.

38. Risk-Based Thinking (ICH Q9 Perspective)

From a quality risk management viewpoint:

Plate size deviation = process variability
Variability = increased risk
Risk impacts patient safety

Thus, standardization is a risk mitigation tool.

39. Myth vs Reality

Myth	Reality
Bigger plates give better recovery	Recovery must be validated, not assumed
Plate size is not critical	Plate size directly affects results
Regulators don’t check plate size	They do, especially during investigations

40. Final Practical Takeaways

90 mm plates are part of validated systems
They support statistical accuracy
They simplify audits and inspections
They protect data integrity
They are globally harmonized

41. Closing Statement for the Blog

In microbiology, precision is not achieved by complexity but by standardization. The continued use of the 90 mm Petri dish represents decades of scientific learning, regulatory convergence, and practical wisdom.

When you choose a 90 mm plate, you are not following tradition—you are applying science.

42. Global Regulatory Harmonization: Why 90 mm Plates Are Universally Accepted

One of the strongest reasons the 90 mm Petri dish has endured is global regulatory harmonization. While regulations may differ in wording, the scientific assumptions remain the same.

42.1 Pharmacopeial Alignment Overview

Authority	Application Area	Implicit Plate Assumption
USP	Bioburden, EM, Sterility Support	90 mm standard agar plate
European Pharmacopoeia (EP)	Microbial Enumeration	Standard plate geometry
WHO GMP	Environmental Monitoring	Validated settle plates (90 mm)
PIC/S	Cleanroom Monitoring	Harmonized plate assumptions
ISO 14698	Biocontamination Control	Standardized sampling surfaces

This alignment means data generated using 90 mm plates is globally portable and defensible.

43. PDA & USP Guidance: How Inspectors Interpret Plate Size

Neither USP nor PDA explicitly states “you must use 90 mm plates.” However, inspectors evaluate:

Historical validation data
Recovery efficiency assumptions
Consistency of environmental monitoring trends

Because all these were developed using 90 mm plates, any deviation triggers scrutiny.

44. Extended Case Studies (Real-World)

44.1 Case Study: Supplier Change Without Requalification

A QC lab switched to a lower-cost plate supplier.

Declared size: 90 mm
Actual size: 92–93 mm
Agar volume unchanged

Impact:

Reduced agar depth
Higher CFU recovery
False upward EM trend

The deviation was identified during an inspection due to unexplained trend shifts.

44.2 Case Study: Audit Observation Avoided

Another site explicitly specified:

Plate diameter tolerance (90 ± 1 mm)
Agar volume verification
Supplier qualification records

Result: Zero microbiology observations during inspection.

45. Inspection-Ready Documentation Checklist

Microbiology labs should maintain:

Approved plate specifications (size, material)
Supplier qualification documents
Agar fill volume SOPs
Environmental monitoring trend reports
Deviation handling procedures

This documentation demonstrates control of the measurement system.

46. Quality Risk Management (ICH Q9) Applied to Plate Size

From a QRM perspective:

Risk Element	Impact
Plate size variation	Data variability
Data variability	False OOS/OOT
False OOS	Regulatory action
Regulatory action	Patient risk

Standardizing on 90 mm plates effectively reduces this risk chain.

47. Role of 90 mm Plates in Data Integrity (ALCOA+)

Data integrity principles require that results be:

Attributable
Legible
Contemporaneous
Original
Accurate

Plate size consistency supports:

Accurate counts
Reproducibility
Traceable trends

Thus, plate geometry becomes part of ALCOA+ compliance.

48. Management Perspective: Cost vs Compliance

Some organizations attempt cost reduction by:

Using non-standard plates
Changing suppliers without validation
Reducing agar volumes

Short-term savings often result in:

Investigations
Batch delays
Regulatory findings

The 90 mm plate is cost-effective when viewed through a compliance lens.

49. Final Master Summary Table

Aspect	Why 90 mm Matters
Science	Optimal surface & diffusion
Statistics	Accurate CFU enumeration
Regulation	Implicit global standard
Operations	Automation compatibility
Audits	Defensible & inspection-ready
Training	Clear standard for staff

50. Final Authoritative Conclusion

The continued use of the 90 mm Petri dish is one of the best examples of how microbiology balances science, statistics, and regulation.

It is not simply a container—it is a calibrated measurement platform upon which microbial quality decisions are made.

Changing plate size without justification is equivalent to changing a balance without recalibration.

For accurate data, regulatory confidence, and patient safety, the 90 mm Petri dish remains non-negotiable.

52. Best Answer (Use as Highlight Box)

Why is a 90 mm Petri dish used in microbiology?
A 90 mm Petri dish is used because it provides an optimal surface area, validated agar depth, accurate CFU enumeration, compatibility with air samplers, and global regulatory acceptance. Most pharmacopeial and cleanroom microbiology methods were developed and validated using 90 mm plates, making them the scientific and regulatory standard.

This block is optimized to win Google Featured Snippets.

53. People Also Ask (PAA) – SEO-Driven Q&A

Is 90 mm Petri dish compulsory in microbiology?

While not explicitly mandated, it is the globally accepted validated standard used in pharmacopeial methods and cleanroom monitoring.

What happens if a different size Petri dish is used?

CFU recovery, agar depth, and statistical assumptions change, requiring full method revalidation.

Why are agar volumes fixed for 90 mm plates?

Because agar depth affects oxygen diffusion, moisture retention, and colony morphology.

Are environmental monitoring limits based on 90 mm plates?

Yes. Cleanroom microbial limits were established using 90 mm settle and air sampling plates.

62. Ultimate FAQ Section – Practical, Regulatory & Interview Ready

Q1. Why is 90 mm considered the standard Petri dish size?

Because most microbiological methods, CFU calculations, environmental monitoring limits, and air sampler designs were validated using 90 mm plates.

Q2. Is using a different Petri dish size a GMP deviation?

It becomes a GMP concern if used without validation, risk assessment, and justification.

Q3. Does Petri dish size affect microbial recovery?

Yes. Surface area, agar depth, and moisture retention directly influence recovery efficiency.

Q4. Why do regulators focus on consistency of plate size?

Because microbiology is a comparative science, and inconsistent tools compromise data integrity.

Q5. Can I mix 90 mm and 100 mm plates in trending?

No. Mixing plate sizes invalidates trend analysis and may trigger false OOT or OOS results.

Q6. Are settle plate limits based on 90 mm plates?

Yes. Cleanroom settle plate limits were developed assuming a 90 mm surface area.

Q7. Why are agar volumes fixed for 90 mm plates?

To maintain validated agar depth (≈3.5–4 mm) for optimal microbial growth.

Q8. Is 90 mm plate size mentioned directly in USP?

USP does not explicitly mandate size, but all examples, validations, and industry practice assume 90 mm plates.

Q9. Can rapid microbiological methods replace 90 mm plates?

They may supplement testing, but culture-based confirmation using standard plates is still required.

Q10. What is the biggest audit risk related to plate size?

Unjustified deviation from validated plate dimensions without impact assessment.

63. Regulatory & Scientific Glossary (Quick Reference)

Term	Meaning
CFU	Colony Forming Unit
Settle Plate	Passive air monitoring agar plate
Active Air Sampling	Forced air impaction onto agar surface
OOT	Out of Trend
OOS	Out of Specification
ALCOA+	Data integrity principles
RMM	Rapid Microbiological Method
IQ/OQ/PQ	Installation / Operational / Performance Qualification

64. Ready-to-Use Microbiology Checklist (Audit Friendly)

64.1 Petri Dish Control Checklist

☑ Plate diameter specified as 90 mm ± tolerance
☑ Agar fill volume defined (20–25 mL)
☑ Supplier qualification completed
☑ Plate dimensions verified at receipt
☑ Single plate size used for trending

64.2 Environmental Monitoring Checklist

☑ Settle plates are 90 mm
☑ Air sampler plates are 90 mm
☑ Exposure times standardized
☑ Limits based on validated plate area

Results Reported as less than 1 CFU Instead of 0 CFU

Passive Air Sampling in Cleanrooms

Active Air Sampling in Cleanrooms

Plate Incubation Temperature and Inversion Incubation

💬 About the Author

Siva Sankar is a Pharmaceutical Microbiology Consultant and Auditor with extensive experience in sterility testing, validation, and GMP compliance. He provides consultancy, training, and documentation services for pharmaceutical microbiology and cleanroom practices.

📧 Contact: siva17092@gmail.com
Mobile: 09505626106

📱 Disclaimer: This article is for educational purposes and does not replace your laboratory’s SOPs or regulatory guidance. Always follow validated methods and manufacturer instructions.