From MEpedia, a crowd-sourced encyclopedia of ME and CFS science and history

A PTLT (PEM Threshold Lowering Threshold) is the threshold of activity required to lower one's PEM threshold.

For example: Patient A has a PTLT of 10 minutes of walking and has a PEM threshold 5 minutes of walking. If patient A walks for 5 minutes he suffers PEM for 2 weeks then recovers back to baseline. If patient A walks for 12 minutes he has gone beyond his PTLT, he suffers 2 weeks PEM but from now onwards he suffers PEM after only 3 minutes of walking.