Process Challenge Devices, Bowie-Dick and Helix Tests: Complete Guide

Process challenge devices (PCDs), the Bowie-Dick test, and the Helix test are the three principal challenge testing tools used to verify autoclave performance. Each addresses a different failure mode: PCDs define the general framework for worst-case load simulation; the Bowie-Dick test targets air removal in the pre-vacuum phase; and the Helix test targets steam penetration into narrow hollow instrument lumens. They are not interchangeable — a passing Bowie-Dick result does not confirm adequate sterilization of hollow instruments, and a passing Helix test does not substitute for a daily Bowie-Dick run. This guide covers all three: definitions, when to use each, how to run them, and how they fit together in a routine monitoring and validation program.

Author: Sterilization Specialist (credentials TBD)

What Are Process Challenge Devices (PCDs)?

A process challenge device is a test system designed to simulate the most difficult-to-sterilize conditions a sterilizer will encounter in routine use. By placing the hardest physical barrier between the sterilant and the test indicator, a PCD creates a worst-case challenge: if sterilization conditions are adequate inside the device, they are adequate throughout the rest of the load.

PCDs are used across all sterilization modalities — steam, vaporized hydrogen peroxide, hydrogen peroxide gas plasma — but the focus here is steam autoclave PCDs. Two standardized steam PCDs dominate routine practice: the Bowie-Dick test pack for porous and wrapped loads, and the Helix device for hollow lumens. Facilities can also assemble custom PCDs for specific load configurations, though commercial devices are preferred for consistency and regulatory traceability.

Regulatory basis

• EN ISO 11140-1: Defines chemical indicator types used inside PCDs
• EN ISO 11140-5: Governs Bowie-Dick test packs specifically
• EN 13060: Covers Class B and Class S autoclave performance requirements, including Helix test use
• AAMI ST79: Primary U.S. reference for steam sterilization practice, covering PCD use for routine monitoring and validation

Chemical indicator types inside PCDs

The indicator type inside a PCD determines what the result actually confirms:

Type 2 is the required standard for Bowie-Dick test packs. For internal in-load monitoring, Types 5 and 6 are more informative. For a full explanation of all indicator types and how they fit into daily monitoring, see the sterilization indicators and monitoring guide.

The Bowie-Dick Test

What the Bowie-Dick Test Is

The Bowie-Dick test is a standardized PCD that verifies air removal and steam penetration in pre-vacuum autoclaves. Named after Dr. J.H. Bowie and J. Dick, it was developed to detect residual air in the pre-vacuum phase — the primary mechanism of sterilization failure in pre-vacuum steam systems.

Where air is present in the chamber, it mixes with steam and disrupts the saturated steam pressure–temperature relationship: the actual temperature at that location is lower than the gauge pressure predicts, even if the pressure reading looks correct. For the underlying physics, see Autoclave Temperature and Pressure: Saturated Steam Explained.

The test pack consists of a standardized porous load simulating wrapped textiles or instrument trays, with a Type 2 CI sheet at its center. When the pre-vacuum phase functions correctly, steam displaces all air and contacts the CI uniformly, producing a uniform color change across the entire sheet. Incomplete or uneven color change indicates residual air.

The Bowie-Dick test does not assess microbial kill. It is a physical test of the vacuum mechanism, not a sterility test.

Regulatory basis: AAMI ST79, EN 285, EN ISO 11140-5.

When to Run a Bowie-Dick Test

• Daily, as the first cycle of the day, in a fully empty chamber (test pack only)
• After any autoclave installation, relocation, or repair
• After any maintenance that involved the vacuum system, door seal, or steam supply

The empty chamber is essential — a load in the chamber alters the airflow resistance and thermal mass, invalidating the test result.

Autoclave class applicability:

• Class B (pre-vacuum): Required daily — these autoclaves are designed for porous and hollow loads where air removal is critical
• Class S: Required if the specified load types include porous or hollow items that rely on pre-vacuum air removal
• Class N (gravity displacement): Not applicable — Class N autoclaves have no pre-vacuum mechanism

How to Run a Bowie-Dick Test

1. Confirm the chamber is empty except for the test pack.
2. Place the test pack on the lowest shelf, near the chamber drain — this position most challenges air removal.
3. Select the dedicated Bowie-Dick cycle (132–135°C, 3.5–4 minutes, with 3–4 pre-vacuum pulses). Do not substitute a standard sterilization cycle.
4. After the cycle completes, remove the test pack and allow it to cool briefly.
5. Inspect the CI sheet immediately. Compare against the pass/fail reference supplied with the test pack.

Interpreting Bowie-Dick Results

Pass: Uniform color change across the entire CI sheet, including the center.

Fail: Any of the following indicate residual air:

• Pale or uncolored center (air pocket in the core of the pack)
• Streaking or radial banding (incomplete vacuum pulses left air channels)
• Peripheral color with central blanking (air trapped at the most challenging point)
• Partial sheet coverage (significant air entrapment)

After a failure:

1. Repeat with a fresh test pack — a defective pack can produce a false failure. This is a diagnostic step, not a resolution.
2. If the second test also fails, take the autoclave out of service immediately.
3. Quarantine all loads processed since the last confirmed passing result.
4. Investigate probable causes: vacuum pump degradation, door gasket leak, non-condensable gases (NCG) exceeding 3.5% in the steam supply, or a failed steam trap.
5. Do not resume routine loads until root cause is identified, corrected, and a passing Bowie-Dick confirms resolution.

For the complete diagnostic sequence — including NCG testing, leak rate testing, and service escalation criteria — see the autoclave troubleshooting guide.

The Helix Test

What the Helix Test Is

The Helix test is a PCD designed to verify steam penetration into narrow hollow instrument lumens — the most challenging load geometry for steam sterilization. The device consists of a coiled, narrow-bore tube with a chemical indicator at the closed end. Steam must travel the full length of the narrow lumen to reach and change the indicator; residual air or inadequate vacuum performance leaves the indicator unchanged.

The Helix test is load-specific, not chamber-wide. It addresses a failure mode the Bowie-Dick test does not — the Bowie-Dick tests whether air was removed from the chamber globally; the Helix tests whether steam actively penetrated a specific hollow geometry representative of the instruments being processed.

The Helix CI may be Type 2, 5, or 6. For routine hollow load monitoring, a Type 5 or 6 indicator is more informative than Type 2, as it confirms that sterilization parameters were met inside the lumen, not merely that steam was present. An optional biological indicator (Geobacillus stearothermophilus) can be incorporated into the Helix device for additional sterility assurance, particularly during validation runs.

Regulatory basis: EN 13060.

When to Run a Helix Test

• Daily for any Class B autoclave used to process hollow instruments
• With every cycle that includes hollow loads at facilities where cycle-by-cycle PCD release is required
• During OQ and PQ phases of autoclave validation when hollow loads are in the validated load family

How to Run a Helix Test

1. Place the Helix device inside the actual load, at the position identified as most challenging — typically the geometric center of a densely packed instrument tray.
2. Run the standard pre-vacuum cycle for the load type (e.g., 134°C, 3–5 minutes).
3. After the cycle, remove the Helix device and read the CI. If a BI was included, incubate per the manufacturer's instructions.
4. A complete, uniform color change in the CI (and no BI growth, if used) confirms adequate steam penetration.

Unlike the Bowie-Dick test, the Helix runs with the actual load in the chamber — it challenges steam penetration under real processing conditions, not in an empty chamber.

Helix vs Bowie-Dick: Key Differences

Choosing the Right Challenge Test

For selecting the right cycle program for your load type and autoclave class, including monitoring tool requirements for each, see the cycle programs guide.

Challenge Testing as Part of Routine Validation

PCDs and challenge tests sit at the intersection of daily routine monitoring and formal validation. Understanding where each fits prevents both under-testing and the common mistake of treating routine monitoring as a substitute for qualification.

In daily practice, Bowie-Dick and Helix tests confirm that the autoclave is performing within its qualified parameters on that day. A passing result authorizes release of that day's loads; a failure triggers quarantine and investigation. Results must be logged with autoclave ID, cycle number, date, and operator name — the same documentation requirements as biological indicator results per AAMI ST79.

In formal validation (IQ/OQ/PQ), challenge tests play a defined role at each stage:

• OQ: Bowie-Dick tests confirm the pre-vacuum mechanism meets EN 285 performance requirements across all relevant temperature settings. Multiple consecutive passing runs are required before advancing to PQ.
• PQ: Helix devices and PCDs with BIs are placed at mapped cold spots to confirm process lethality under worst-case load conditions. The number of consecutive passing PQ runs required is defined in the validation protocol.

For full scope and documentation requirements for each validation phase, see the autoclave validation guide (IQ/OQ/PQ).

Relationship to biological indicators: Challenge tests and BIs answer different questions. A Bowie-Dick or Helix result tells you whether the physical conditions for sterilization were present. A BI confirms whether those conditions achieved the required microbial kill. Both are required for a complete monitoring program. Neither result alone is sufficient to release implant loads. For a full breakdown of CI types, BI frequency requirements, and interaction between physical and biological monitoring, see the sterilization indicators and monitoring guide.

FAQ

What is a process challenge device?

A process challenge device (PCD) is a test system that simulates the hardest-to-sterilize conditions for a given sterilizer and load type. It contains a chemical or biological indicator inside a barrier — a porous pack, a narrow tube, or a simulated hollow instrument — that the sterilant must penetrate to register a passing result. A passing PCD confirms that sterilization conditions were adequate at the most challenging point in the load.

How often should I run a Bowie-Dick test?

Daily, as the first cycle of the day in an empty chamber, for all Class B and qualifying Class S pre-vacuum autoclaves. Also run after any installation, relocation, repair, or service that involved the vacuum system, door seal, or steam supply.

What does a failed Bowie-Dick test mean?

A non-uniform or incomplete color change on the Type 2 CI sheet means residual air prevented full steam contact with the test pack. First repeat with a fresh pack to rule out a defective test. If the second test also fails, take the autoclave out of service, quarantine loads processed since the last confirmed passing result, and investigate root cause before resuming. Common causes include vacuum pump wear, door gasket leak, and non-condensable gases exceeding 3.5% in the steam supply. See the autoclave troubleshooting guide for the full diagnostic sequence.

Is the Helix test the same as the Bowie-Dick test?

No. They target different failure modes and different load geometries. The Bowie-Dick test verifies chamber-wide air removal from the pre-vacuum phase and runs in an empty chamber with a porous test pack. The Helix test verifies steam penetration into narrow hollow instrument lumens and runs inside the actual load. A passing Bowie-Dick does not confirm adequate sterilization of hollow instruments, and vice versa. Both should be run on days where both porous and hollow loads are processed.

Can I use a PCD instead of a biological indicator?

For most routine loads, a chemical indicator PCD (Type 5 or 6) is accepted per AAMI ST79 as the daily monitoring method. However, biological indicators are required for every implant load without exception, regardless of CI results. BIs are also required during formal validation (PQ) and are recommended for routine weekly monitoring. A CI confirms physical parameters were present; a BI confirms microbial kill was achieved.

What do I do if my Bowie-Dick test fails?

Do not process any loads on that autoclave. Repeat the test with a new, unopened test pack — a defective pack can cause a false failure. If the second test also fails, take the autoclave out of service and quarantine all loads since the last confirmed passing result. Inspect the door gasket and check incoming steam supply pressure before calling for service. Document the failure, the investigation steps, the quarantine scope, and the corrective action per AAMI ST79. After repair or correction, run a passing Bowie-Dick before resuming routine loads.

Does a Class N autoclave need a Bowie-Dick test?

No. Class N (gravity displacement) autoclaves have no pre-vacuum mechanism, so the Bowie-Dick test does not apply. Class N autoclaves are monitored using BIs, Type 5 or 6 CIs inside loads, and physical cycle parameter records. For the correct cycle programs and monitoring approach for gravity displacement autoclaves, see the autoclave cycle programs guide.

Conclusion

PCDs, the Bowie-Dick test, and the Helix test each address a specific vulnerability in autoclave sterilization. PCDs define the framework; the Bowie-Dick test monitors pre-vacuum air removal daily; the Helix test monitors hollow instrument penetration daily or per cycle. None substitutes for the others. Integrate all three that apply to your autoclave class and load profile into a documented monitoring schedule, and connect the results directly to your validation records so that any performance deviation triggers the correct investigation and requalification response.