Why ATP readings differ between meters
An ATP reading in relative light units (RLU) depends on the meter that took it. Different brands report very different numbers for the same amount of ATP, so a pass or fail threshold belongs to one device model. Set a threshold for each model you use, and record it with every reading.
What an RLU reading is
An ATP meter measures the light from a reaction with the ATP on a swab. It reports that light as relative light units. The word "relative" matters: each meter defines its own scale, so an RLU number only means something next to the meter that produced it.
Readings don't travel between brands
ATP readings in relative light units are not comparable across meter brands.
Different brands produce widely different log10 RLU values for the same fixed quantity of ATP, so an acceptance threshold belongs to one device model and cannot be carried across to another. Repeatability is also poor near the commonly quoted 100 RLU mark, and published cutoffs in the literature range from 100 to 500 RLU.
Set a threshold for each device model
- Your facility sets the threshold for each meter model you use, with your infection prevention team and the meter maker's guidance for that model.
- Write down which model each threshold belongs to, and the date it took effect.
- When you change meters, set a new threshold for the new model. Don't carry the old number across.
- Old readings keep the threshold that applied when they were taken, so they can still be read correctly after the change.
Decide in advance what a close reading means
Repeat readings vary, especially near low thresholds. Decide before testing starts what happens when a reading lands close to the line, such as cleaning the surface again and retesting, and apply it the same way every time. A reading just over the line is a reason to look again, not a verdict on a person.
What to record with every reading
- The surface tested
- The reading, with a photo of the meter display where you can
- The meter model
- The threshold your facility set for that model
- Whether the reading met your acceptance criteria
- Who tested, and when
- Which room's record the reading belongs to
Sources
- The comparative performance of three brands of portable ATP bioluminometer intended for use in hospital infection control. Healthcare Infection (CSIRO Publishing), 2012.
This guide is general information, not clinical advice. Your facility's protocol and your product labels govern how rooms are cleaned.
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