Which of the following is the last step in the 4-step ATP testing process?

In the context of the 4-step ATP testing process, interpreting the reading in relative light units (RLU) is indeed the final step. After a sterile swab has been wiped across the surface to collect any biological material, and that swab has been treated with a reagent chemical to react with the ATP present, the next logical step is to insert the swab into the luminometer. The luminometer then provides a reading, which is measured in RLUs.

However, interpreting this reading is crucial to assess the cleanliness of the tested surface and to make informed decisions based on the ATP levels detected. This interpretation process translates the quantitative data from the luminometer into actionable insights regarding sanitation and contamination levels. Without this final step of interpreting the RLU reading, the entire testing process would lack meaning, making it the essential concluding step.