Settling microbe testing (settle plate testing) is a fundamental passive air-sampling method used to verify the microbiological cleanliness of cleanrooms and controlled environments in China's pharmaceutical, medical device, food, and healthcare industries. The core national method standard is GB/T 16293-2010, which works together with Chinese GMP (2010), GB 50591, GB 15982, and other national and ISO-aligned standards to define sampling procedures, exposure conditions, and acceptance limits. This article provides a technical overview of the Chinese national standard framework for settling microbe detection, covering principles, test items, methods, applications, and frequently asked questions.
Technical Overview
Settling microbe testing (also known as the settle plate method or falling bacteria test) is a passive, gravimetric air-sampling technique in which open culture dishes containing nutrient agar are exposed to the air of a controlled environment for a defined period. Viable airborne particles deposit onto the agar surface by gravity, and after incubation the resulting colonies are counted and expressed as colony-forming units (CFU) per plate per exposure period. Because it requires no powered sampler, the method is simple, low-cost, and directly reflects microbial contamination risk at working height, making it a mandatory monitoring item for most cleanroom classifications in China.
The primary Chinese national standard for this test is GB/T 16293-2010, Testing Method for Settling Microbes in Clean Rooms (Areas) of the Pharmaceutical Industry, which superseded GB/T 16293-1996. It is applied alongside companion standards GB/T 16294-2010 (airborne microbes by impaction) and GB/T 16292-2010 (airborne particles). Acceptance limits are established by product-sector regulations, most notably the Chinese Good Manufacturing Practice for Drugs (2010 revision) for sterile pharmaceutical production, which specifies settling microbe limits for cleanliness Grades A through D.
The main standards and regulatory documents forming the national framework for settling microbe testing include:
- GB/T 16293-2010 — Testing method for settling microbes in clean rooms (areas) of the pharmaceutical industry (core method standard)
- GB/T 16294-2010 — Testing method for airborne microbes in clean rooms (areas) of the pharmaceutical industry by impaction
- GB/T 16292-2010 — Testing method for airborne particles in clean rooms (areas) of the pharmaceutical industry
- Chinese Good Manufacturing Practice for Drugs (2010 revision), including Annex 1 — settling microbe limits for Grades A, B, C, and D
- GB/T 25915 series (equivalent to ISO 14644 series) — cleanroom classification and testing
- GB/T 25916.1 (equivalent to ISO 14698-1) — biocontamination control of cleanrooms
- GB 50457-2019 — Standard for design of pharmaceutical industry cleanrooms
- GB 50591-2010 — Code for construction and acceptance of cleanroom
- GB 15982-2012 — Hygienic standard for disinfection in hospitals (air hygiene by settle plate)
- GB 50333-2013 — Technical code for hospital clean operating departments
- GB 14881-2013 — General hygienic practice for food production (environmental microbial monitoring)
- YY 0033 — Good manufacturing practice specification for sterile medical devices (cleanliness class limits)
- GB 14925-2010 — Laboratory animal — Requirements of environment and housing facilities
- Chinese Pharmacopoeia (2020 edition), General Chapter 9205 — Guidance for microbiological monitoring and control of pharmaceutical clean laboratories
Typical acceptance criteria applied in Chinese practice are: for sterile pharmaceutical cleanrooms under Chinese GMP (2010), less than 1 CFU per 4-hour exposure for Grade A, less than 5 CFU/4 h for Grade B, less than 50 CFU/4 h for Grade C, and less than 100 CFU/4 h for Grade D, all with 90 mm plates. For tests conducted under the classical shorter exposure of approximately 30 minutes, commonly applied criteria (for example under YY 0033) are 1 CFU per plate at ISO 5 (Class 100), 3 CFU at ISO 7 (Class 10,000), 10 CFU at ISO 8 (Class 100,000), and 15 CFU at Class 300,000. Hospital environments under GB 15982-2012 are generally evaluated against a limit of about 4 CFU per 30-minute exposure, while GB 50333-2013 assigns grade-specific settling limits for clean operating rooms, with the highest-grade operating zones controlled at approximately 0.2 CFU per 30-minute exposure.
Testing Samples
The object of a settling microbe test is not a physical product sample but the ambient air of a controlled environment, sampled indirectly through agar plates exposed at representative locations. Environments typically subject to national-standard settling microbe testing include:
Air in Pharmaceutical Cleanrooms and Controlled Areas
Cleanrooms and locally protected zones used for sterile drug preparation, aseptic filling, formulation, and primary packaging, classified from Grade A through Grade D under Chinese GMP, tested at commissioning, periodic requalification, and during routine environmental monitoring programs.
Air in Medical Device and Sterile Product Manufacturing Environments
Production areas for sterile medical devices, implants, dialysis consumables, and other sterile products, where cleanliness classes and corresponding settling microbe limits are specified by YY 0033 and related product standards.
Air in Hospital Environments
Clean operating rooms, protective isolation wards, ICU rooms, disinfection supply centers, and delivery rooms, evaluated under GB 50333-2013 and GB 15982-2012 for infection prevention and control purposes.
Air in Food, Cosmetic, and Laboratory Animal Facilities
Food processing workshops monitored under the hygiene principles of GB 14881-2013, cosmetics production areas under production licensing requirements, and barrier or isolation environments for laboratory animal housing under GB 14925-2010, all of which apply the settle plate method for airborne microbial hygiene assessment.
Testing Items
A settling microbe test program under the national standards typically comprises the following items:
- Total settling aerobic bacterial count, reported as CFU per 90 mm plate per exposure period (commonly CFU per plate per 30 minutes or per 4 hours)
- Settling mold and yeast count, using sabouraud dextrose agar (SDA) plates where fungal contamination is of concern
- Media sterility controls and blank dishes incubated alongside test plates to verify background contamination of the culture media
- Colony morphology recording and, where required for contamination investigation, identification of representative isolates to genus or species level
- Statistical evaluation of results across all sampling points, including average count per point, maximum single-plate count, and comparison against the applicable grade limits
- Optional trend analysis over time as part of an ongoing environmental monitoring program under Chinese GMP and Chinese Pharmacopoeia Chapter 9205
Testing Methods
Sampling Point Layout and Number
Sampling points are positioned at working height, generally 0.8 m to 1.5 m above floor level or directly on critical work surfaces, and distributed to represent the areas where product is exposed. Following the approach of GB/T 16293-2010, the minimum number of sampling points is calculated as the square root of the cleanroom or clean area floor area expressed in square meters, with a minimum of two plates, and additional plates placed at high-risk operational positions. Sampling should not be performed immediately after disinfectant or ultraviolet treatment, and personnel presence and activity should reflect the agreed test state.
Exposure and Culture Conditions
Standard 90 mm culture dishes are filled with sterile tryptic soy agar (soybean-casein digest agar) or nutrient agar to a uniform depth, pre-incubated to confirm media sterility, and transported in sealed containers. At each sampling point, the lid is opened and placed beside the dish with its inner surface facing downward, and the agar is exposed for the specified period, typically about 30 minutes under GB/T 16293-2010 static testing, or up to 4 hours during routine GMP monitoring of Grades A and B, provided the medium is validated to resist desiccation over the longer exposure.
Incubation, Counting, and Result Expression
After exposure, dishes are recovered, covered, inverted, and incubated. For total bacterial counts, incubation is at 30 to 35 degrees Celsius for 48 to 72 hours; for fungal detection with SDA, incubation is at 20 to 25 degrees Celsius for 5 to 7 days. Colonies are counted with a colony counter under adequate lighting, including pinpoint colonies. Results are reported as CFU per plate per exposure period for each sampling point, together with the average value, and are compared with the acceptance limits of the applicable cleanliness grade, sector regulation, or agreed specification. Media blank results must be reported and must meet the sterility requirement for the test to be valid.
Test States and Precautions
Testing may be performed in the static (as-built or at-rest) state for cleanroom qualification and acceptance under GB 50591-2010, or in the operational (dynamic) state for routine monitoring under Chinese GMP. Precautions include disinfecting surfaces before testing and allowing residues to disperse, minimizing the number of personnel present, avoiding interruption of airflow patterns by the sampler, handling plates only by the sidewall, exposing and recovering plates in a consistent sequence, and recording environmental conditions such as temperature, relative humidity, and cleanliness class during the test. When counts exceed limits, the result triggers investigation under the site's deviation procedure, typically including root cause analysis, additional sampling, microbial identification, and corrective actions such as re-cleaning, filter checks, or revalidation.
Applications
Settling microbe testing in accordance with the national standards is applied across a wide range of regulated scenarios:
- Cleanroom qualification and acceptance testing for newly built or renovated pharmaceutical facilities under GB 50591-2010 and GB 50457-2019
- Routine and continuous environmental monitoring of aseptic processing areas under Chinese GMP (2010), supporting batch release and sterility assurance
- Periodic requalification of medical device cleanrooms and verification of YY 0033 cleanliness class compliance
- Hospital hygiene surveillance of operating rooms, wards, and disinfection supply centers under GB 50333-2013 and GB 15982-2012 for infection control programs
- Food and cosmetics factory hygiene audits and production licensing inspections referencing GB 14881-2013 and related hygienic requirements
- Laboratory animal barrier and isolation facility certification under GB 14925-2010
- Contamination investigation and validation of corrective actions following excursions in airborne microbial counts
- Independent verification and reporting by accredited third-party laboratories performing cleanroom performance testing and certification services
FAQ
What is the main national standard for settling microbe testing in China?
The core method standard is GB/T 16293-2010, which specifies the settle plate procedure for pharmaceutical cleanrooms, including sampling point selection, exposure conditions, culture media, incubation, and result reporting. Acceptance limits are defined by the applicable sector regulation, such as Chinese GMP (2010) for drugs, YY 0033 for sterile medical devices, GB 15982-2012 and GB 50333-2013 for hospitals, and GB 14881-2013 for food production.
What are the settling microbe limits under Chinese GMP (2010)?
For 90 mm plates exposed for up to 4 hours, the limits are less than 1 CFU for Grade A, less than 5 CFU for Grade B, less than 50 CFU for Grade C, and less than 100 CFU for Grade D. Shorter exposures are permitted for routine monitoring of Grades A and B when an appropriate conversion to the 4-hour basis is established.
How long should settle plates be exposed?
Under GB/T 16293-2010 static testing, plates are typically exposed for approximately 30 minutes. Under GMP monitoring programs, exposure may be extended to 4 hours for continuous assessment of critical areas, provided the agar medium is demonstrated to remain effective, that is, not dried out or nutritionally compromised, over the exposure period.
What is the difference between settling microbe and airborne microbe testing?
Settling microbe testing under GB/T 16293-2010 is a passive method that measures microbes deposited by gravity onto exposed plates, reported as CFU per plate. Airborne microbe testing under GB/T 16294-2010 is an active volumetric method that uses a slit or impaction sampler to draw a known volume of air onto the agar, reported as CFU per cubic meter. The two methods are complementary and are often performed together in cleanroom qualification.
Which culture media and incubation conditions are used?
Tryptic soy agar (or nutrient agar) is used for total bacterial counts with incubation at 30 to 35 degrees Celsius for 48 to 72 hours, and sabouraud dextrose agar is used for molds and yeasts with incubation at 20 to 25 degrees Celsius for 5 to 7 days. Media preparation and sterility verification should follow the requirements of the Chinese Pharmacopoeia and the test standard.
Should testing be performed at rest or during operation?
Both states are used. Static (at-rest) testing demonstrates the built capability of the facility and is applied during qualification and acceptance under GB 50591-2010, while dynamic (in-operation) testing reflects actual production contamination risk and is required for routine monitoring under Chinese GMP. The test state must always be recorded in the report, since acceptance criteria and expected counts differ significantly between states.