What is RLU?
RLU stands for Relative Light Units and it is the standard read-out value given by luminometers.
RLU values must be converted to ATP concentrations prior to data interpretation using the UltraCheck Standard supplied with your kit. The guidelines for good control for your application can be found in the test kit instructions supplied with your kit.
Why does my RLU drop the longer I let an assay tube sit?
The ATP-luciferase interaction will be reduced in intensity as time goes on as the enzyme and ATP are consumed in the reaction. The light production is stable for ~30s after mixing; therefore you should always take the reading in the PhotonMaster within 5-15 seconds.
My UltraCheck calibration RLU's are too low. What is the cause and what should I do?
There are several possibilities for this situation listed below from most to least likely:
1. Luminase is too old. Over time, Luminase activity degrades. Obtain a new vial of Luminase.
2. Luminase may have been exposed to excessive heat for too long Exposure to excessive heat (>30°C) greatly accelerates loss of activity. Make sure to keep Luminase in the refrigerator whenever not in use. If it is a new bottle that won't be used for a while, keep it frozen. If the activity has already dropped to unacceptable levels because of heat exposure, obtain a new bottle.
3. There may be a problem with the luminometer. The photomultiplier tube in luminometers will sometimes become less effective over time. Obtain a Luminometer Standardization Kit and plot a linearity curve. A biennial linearity check should be completed as regular maintenance.
4. Luminase is not reaching the bottom of the tube. If Luminase is dropped against the side of the assay tube instead of directly into the bottom, only a fraction of it will contact the UltraCheck/diluted extract/diluted sample and therefore the resulting RLU will be lower. Make sure to hold the assay tube and dropper bottle/pipette tip vertically when adding liquid to an assay tube.
Can I keep using units of RLU? Why do I need to convert RLU to ATP concentrations?
RLUs alone are not an indicator of microbial activity - they are relative units that vary based on the activity of the enzyme. Our software automates calculating the required ATP values to harness the informational power of our test kits. LuminUltra does not recommend storing results in raw RLU's nor making process decisions on these values without their first being converted to an ATP concentration.