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 fully standardised at international level under the UNE-EN-ISO 17665 standards (for the validation and routine control of moist heat sterilisation) and EN 285 / EN 13060.
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 under the EN 13060 standard and large sterilisers under EN 285). It is neither necessary nor applicable in gravity displacement autoclaves (Class N or S), 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.
- Position the test pack: Place the B&D test kit on its own, horizontally in the centre of the lower tray, directly above 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 always be carried out using standardised, approved commercial kits. The chemical composition of the reagent and indicator guarantees reproducibility in the face of inspections. The use of "home-made" or non-standard packs invalidates the result before 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.
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 for porous loads 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. This means that penetration was good, but the steam was not adequately saturated (excess moisture or the presence of gases).
The usual causes of a failed test include vacuum pump faults, leaks in the door seals or poor quality of the steam supply. After two consecutive negative results, the unit must be declared out of service until it has been technically inspected.
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 performing the preliminary vacuum test 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 UNE-EN-ISO 17665 standard establishes that the results of each test must be recorded and filed together with the batch number and date, forming the mandatory documentary evidence in quality audits.
The Bowie-Dick test within the autoclave control system
The Bowie-Dick test is one of the four tests that make up the comprehensive control protocol for 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 | Daily, before the B&D | EN 13060, EN 285 |
| Bowie-Dick test | Air removal and steam penetration in porous loads | Daily, after the vacuum test | ISO 17665, EN 285 |
| Helix test | Steam penetration in long, narrow cavities (tubular instruments) | Periodic or according to protocol | ISO 11140-4 |
| Spore test (biological) | Effective destruction of resistant microorganisms | Periodic (variable frequency) | ISO 11138 |
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 ISO 17025 calibration, which makes it easier to keep the unit within the parameters required by the UNE-EN-ISO 17665 standard.