?If you have been researching autoclaves for your hospital or dental clinic, you have almost certainly encountered the terms 'Class B', 'Class N', and 'Class S'. These classifications define what types of loads an autoclave can reliably sterilize — and getting this wrong can compromise patient safety and NABH compliance.
Class B is the gold standard in steam sterilization for hospital CSSDs. In this article, we explain exactly what Class B sterilization is, why it matters, what cycles it includes, and what you should look for when buying a Class B autoclave in India.
The European Standard Behind the Classification
The Class B, S, and N classification system comes from European Standard EN 13060, which defines sterilization performance requirements for small steam sterilizers. India's medical standards have adopted these classifications, and NABH accreditation assessors evaluate autoclaves against them.
Class N — simplest, most limited
Class N (N for 'Naked') sterilizers are suitable only for solid, unwrapped instruments. They use a single gravity air displacement cycle and cannot reliably sterilize hollow instruments, wrapped loads, or porous items. A simple steam pot used in older clinics is essentially a Class N device. These are not suitable for hospital CSSD use.
Class S — intermediate
Class S (S for 'Special') sterilizers are designed for specific load types as specified by the manufacturer. They offer more versatility than Class N but are not universally suitable for all CSSD loads.
Class B — the hospital standard
Class B (B for 'Big Small Sterilizers' — a term from the original standard) is the most capable and demanding classification. Class B autoclaves can sterilize any type of load: solid instruments, wrapped packages, hollow instruments (including lumened devices like endoscopes in some configurations), porous materials, and textiles.
What Makes Class B Different: The Pre-Vacuum Cycle
The key technical feature of a Class B autoclave is the pre-vacuum (also called fractionated vacuum or pulsed vacuum) cycle. Instead of simply letting steam push air out of the chamber by gravity (as Class N does), a Class B autoclave uses a vacuum pump to actively remove air from the chamber in multiple pulses before and during the steam injection phase.
This is critical because air is a poor conductor of heat and an effective barrier between steam and instrument surfaces. If any air remains trapped — inside a lumen, inside a wrapped package, inside a hinged forceps — the steam cannot reach that surface. That instrument will not be sterilized, no matter how long the cycle runs.
Pre-vacuum cycles eliminate this risk by physically removing air to below 50 mbar before steam injection. This guarantees that steam penetrates every surface of every item in the load.
Class B Cycle Types
Universal cycle (134°C, 3–18 minutes)
The standard Class B cycle for solid, hollow, wrapped, and porous loads. Most hospital instrument sets are processed on this cycle. Steam temperature reaches 134°C with pre-vacuum to ensure complete air removal and penetration.
Textile/linen cycle
Used for sterilizing surgical drapes, gowns, and other textile loads. Requires longer holding times and specific drying parameters to ensure thoroughly dried, sterile textiles.
Prion cycle
A specialised high-temperature, extended-time cycle for instruments used in neurosurgery where prion contamination is a concern. Not standard on all Class B autoclaves — must be specifically requested.
Quick cycle (flash sterilization)
Used for emergency sterilization of unwrapped instruments needed urgently in the OT. Not intended as a routine cycle for wrapped sterile inventory.
Class B vs Class N: Why It Matters for Patient Safety
Consider a laparoscopic instrument with a narrow lumen. In a Class N autoclave, gravity steam may not penetrate the lumen reliably — especially with a wrapped load. The outer surfaces may be sterile. The inner surfaces may not.
In a Class B autoclave with pre-vacuum cycle, the air is removed before steam injection. Steam penetrates the lumen under pressure differential. Every surface of the instrument is reliably sterilized.
For hospitals performing laparoscopic surgery, robotic surgery, or any procedure with hollow instruments, Class B sterilization is not optional — it is a patient safety imperative.
NABH Requirements for Class B Autoclaves
NABH CSSD standards require that sterilization processes be validated and documented. For hollow and wrapped instrument loads (which are the majority of hospital CSSD loads), Class B sterilization is the appropriate validated process.
NABH assessors look for Bowie-Dick tests (air removal test), biological indicators, chemical indicators, and cycle printouts as evidence of validated Class B sterilization performance. Cistron's rectangular and tabletop autoclaves are equipped to support all of these documentation requirements.
How to Verify Class B Performance: Validation Tests
Bowie-Dick test
The Bowie-Dick test is a standard Class B performance test that verifies adequate air removal and steam penetration in a full porous load. It should be run every day before the first sterilization cycle in any Class B autoclave. A passing Bowie-Dick test result must be recorded in the CSSD log.
Biological indicator
A biological indicator contains spores of Geobacillus stearothermophilus — a heat-resistant organism that represents the worst-case sterilization challenge. After a sterilization cycle, the biological indicator is incubated. If no growth is observed, the cycle achieved sterilization. Biological indicators should be run weekly in hospital CSSDs.
Chemical indicators
Class 5 and Class 6 chemical indicators are placed inside every packaged load. They change colour only when all critical sterilization parameters (temperature, time, steam) are met. They provide load-level evidence of sterilization process success.
Cistron's Class B Autoclaves for Indian Hospitals
Cistron manufactures both tabletop (rapid flash) and large horizontal Class B autoclaves for hospitals. Our rectangular steam sterilizers — available in Auto Door, Full Closure, and Console Type configurations — all feature pre-vacuum technology, PLC-based cycle control, and automatic printout of every cycle parameter.
Every Cistron autoclave is ISI marked, ISO 13485 certified, and NABH-validated. Our service engineers conduct Bowie-Dick tests during installation commissioning and train CSSD staff on daily validation procedures.
Conclusion
Class B sterilization is the non-negotiable standard for hospital CSSDs in India. The pre-vacuum cycle technology ensures reliable steam penetration into every load type — wrapped, hollow, porous, and solid. Without Class B capability, hospitals cannot reliably sterilize the full range of surgical instruments used in modern procedures.
If your hospital is still using Class N or Class S autoclaves for routine CSSD sterilization, it is time to upgrade. Cistron's Class B autoclave range starts at compact tabletop models for small clinics and scales to full-room rectangular sterilizers for large hospital CSSDs.