What is the Bowie-Dick test and how to read results?
— Equipo JP Selecta
The Bowie-Dick test is one of the most important quality control tests in the field of steam sterilisation. Developed in 1963 by Dr J.H. Bowie and Dr J. Dick within the British public health service, it came about to solve a recurring critical problem: the presence of air pockets trapped inside containers or porous loads, which prevented saturated steam from coming into direct contact with the surfaces and compromised sterilisation.
Today, this test is standardised at international level: EN 285 describes it for large sterilisers and EN ISO 11140-3 specifies the indicators for the standard test pack; EN 13060 covers small steam sterilisers.
What is the Bowie-Dick test for and what does it assess?
The main aim of the B&D test is to examine the physical behaviour of the sterilisation system before processing working material. Specifically, it assesses two essential aspects:
- The effectiveness of the pre-vacuum system: It checks that the fractionated vacuum pump removes the air from the chamber completely at the start of the cycle.
- Homogeneous steam penetration: It verifies that saturated steam penetrates uniformly to the centre of a dense porous load (such as textiles or wrapped material).
What the test does NOT measure: It is essential to clarify that the Bowie-Dick test is not a sterilisation test of the actual load, nor does it detect the presence or destruction of microorganisms or spores. Its function is exclusively to validate the correct thermodynamic and air removal performance of the chamber.
In which autoclaves is the B&D test mandatory?
The Bowie-Dick test only applies to pre-vacuum autoclaves: Class B units under the EN 13060 standard, Class S units for which the manufacturer specifies this test, and large sterilisers under EN 285. It is neither necessary nor applicable in gravity displacement autoclaves (Class N), since these do not have vacuum pumps for the fractionated removal of air.
How is the Bowie-Dick test carried out?
To guarantee a reliable result and compliance with quality audits, the procedure must be carried out following a strict routine:
- Check that the chamber is empty: The unit must not contain any other material or basket with samples.
- Warm up the chamber: First run the warm-up cycle specified by the autoclave manufacturer.
- Position the test pack: Place the B&D test pack on its own, horizontally, at the point indicated by the autoclave and kit instructions (usually on the lower tray, near the chamber drain point).
- Select the specific programme: Set the cycle exclusively for Bowie-Dick in the unit's menu (134 °C, 2.1 bar, with a sterilisation phase of 3.5 minutes).
- Start the cycle: Run the programme and keep the chamber closed throughout the process.
- Cooling and reading: When it finishes, remove the pack carefully and wait for it to cool before unfolding the reactive indicator sheet.
- Compare the result: Assess the colour change against the reference chart from the kit manufacturer.
Technical note: The test must be carried out with a standardised test pack: the EN 285 standard textile pack with an indicator conforming to EN ISO 11140-3, or an alternative device conforming to EN ISO 11140-4. An improvised pack that does not follow the standard invalidates the result in any audit.
Interpreting the test results
Reading the thermochromic indicator makes it possible to identify the state of the unit visually:
Correct result / uniform colour change
The indicator shows a uniform, complete colour change across the whole sheet, from the centre to the outer edges. This confirms total air removal and correct penetration of saturated steam. The autoclave can be used as normal; the test does not on its own demonstrate that the load is sterile.
Incorrect result / irregular or incomplete colour change
Patches or lighter-coloured areas appear in the centre of the paper. This indicates the presence of trapped residual air or pockets of non-condensable gas that prevented contact with the steam. The unit must not be used in the affected pre-vacuum cycles until the anomaly has been corrected.
Incorrect result / uniform colour change but incorrect shade
The sheet changes colour homogeneously but does not reach the shade or intensity specified by the manufacturer. The result is not acceptable: it must be treated as a failure and investigated following the indicator manufacturer's instructions.
The usual causes of a failed test include vacuum pump faults, leaks in the door seals or poor quality of the steam supply. A single incorrect result is enough to suspend the affected pre-vacuum cycles: the failure must be recorded and reported to the person in charge and, once the cause has been corrected, the planned checks must be repeated before the unit is authorised to return to service.
When should the Bowie-Dick test be carried out?
It is advisable to carry out this test at the following times:
- At the start of each working day, after the vacuum test, if the facility's procedure includes it, and before processing material.
- After maintenance work or technical repairs to the autoclave.
- After prolonged periods of inactivity of the unit.
- If anomalies are detected or unusual pressure drops appear in the cycle charts.
The results of each test must be recorded (date, unit, cycle, indicator batch and person responsible) and filed in accordance with the facility's quality system: they are the documentary evidence reviewed in audits.
The Bowie-Dick test within the autoclave control system
The Bowie-Dick test is one of the routine tests that make up the control of a pre-vacuum autoclave. Each test assesses a different aspect of the sterilisation process and none replaces the others:
| Test | What it assesses | When it is carried out | Standard |
|---|---|---|---|
| Vacuum test (leak test) | Tightness of the vacuum system | According to the facility's procedure; if daily, before the B&D | EN 13060, EN 285 |
| Bowie-Dick test | Air removal and steam penetration in porous loads | Usually at the start of the working day, after the vacuum test | EN 285, EN ISO 11140-3 |
| Helix test | Steam penetration in long, narrow cavities (tubular instruments) | According to the facility's procedure (often at the start of the working day) | EN ISO 11140-6 (small sterilisers only, EN 13060) |
| Spore test (biological) | Effective destruction of resistant microorganisms | Periodic (variable frequency) | ISO 11138 |
Besides the standard test pack, there are alternative devices for the air removal test, such as the DART test, whose indicator is usually cited with EN ISO 11140-4.
A correct Bowie-Dick test result depends as much on the procedure as on the autoclave itself: only correctly calibrated pre-vacuum units guarantee the air removal needed to pass it. At J.P. Selecta we offer pre-vacuum laboratory autoclaves together with technical service and calibration with reference standards certified by an ENAC-certified company, which makes it easier to keep the unit within the parameters of its validation.