Two tests that sound similar and check different things
A Bowie-Dick test runs empty, once, at the start of the day, and asks whether the chamber removes air well enough for steam to reach every surface of a standardized test pack under ideal, controlled conditions. It says nothing about whatever gets loaded into the chamber for the next eight hours of real cases. A process challenge device (PCD) closes that gap: it’s a test pack — sometimes a simple folded-towel pack, sometimes a purpose-built reusable device — containing a chemical indicator (typically Class 5 or 6) placed inside an actual load, at the position within that load steam would have the hardest time reaching. Where the Bowie-Dick test validates the sterilizer in isolation, a PCD validates whether steam is actually getting where it needs to go in the load that’s really running.
Why “hardest to reach” is the point, not an inconvenience
A PCD is deliberately positioned to represent a worst-case location — often the center of a densely packed set, or the innermost layer of a wrapped tray — rather than wherever’s convenient to check. The logic mirrors why a Bowie-Dick test uses a standardized pack instead of a single loose indicator strip laid on top: a chemical indicator sitting in an open, exposed spot inside a load will pass even when a genuine air pocket exists two layers deeper. Placing the challenge at the point of greatest resistance means a pass is actually informative about the rest of the load, not just about the easiest part of it to sterilize.
Every cycle, not once a day
Because a PCD travels with a real load rather than running as a separate empty-chamber cycle, routine practice is to include one in every load, or at minimum in the first load of the day and any load without an implantable device already carrying its own internal indicator. That’s a materially different monitoring cadence than the once-daily Bowie-Dick test, and conflating the two — treating a clean morning Bowie-Dick as covering the day’s later loads — leaves every load after it unmonitored for actual steam penetration into that specific load’s contents.
What a PCD doesn’t replace
A PCD confirms steam penetration into a real load using a chemical indicator, which inherits everything covered elsewhere about what chemical indicators do and don’t prove: a Class 5 or 6 result correlates closely with lethality but still isn’t a direct measurement of spore death the way a biological indicator is. A PCD used with a biological indicator inside it — rather than a chemical one — turns the same worst-case-position logic into a load-level biological challenge test, which is the more rigorous version used for implant loads specifically.
In practice
Reusable PCDs exist as purpose-built devices sized to standard trays, but a correctly folded towel pack with an internal Class 5 or 6 indicator meets the same intent where a dedicated device isn’t in use — what matters for the result to mean anything is the challenging placement and the indicator class, not the specific hardware carrying it.