Poison Spiked Pit Poison DC?

[Ataraxias]

Based on the poisoned spiked pit dealing damage with 1d4 spikes am I right in interpreting that each spike being landed upon will increase duration and DC of the poison as per the multiple dose rules?

[Darthslash]

Each spike would get its own save, with the DC increasing by +2 for each failed save.

Say you fell on 3 spikes and the poison DC was 15. (just picking a number, not sure what the actual save DC is)

Spike 1: Fort save DC 15 (fail)
Spike 2: Fort save DC 17 (fail)
Spike 3: Fort save DC 19 (save)

Ongoing poison saves to cure would be at DC 17, the highest DC you last failed at.