During an aging test of a high temperature proton exchange membrane fuel cell (HT-PEMFC) without exhaust pressure control loop, the anode and cathode pressures are dependent on variations in gas flow and atmospheric pressure. The voltage degradation rate is then impacted by atmospheric pressure variations. To extract the aging rate independently of the pressure, a possible solution is to estimate the voltage variation at a given pressure change. In this work, this voltage/pressure sensitivity was estimated in post-processing, by fitting regression planes (on measured voltage data) in the three dimensions: voltage, pressure, and time. It was applied on an aging test of 2753 h at 160 °C, at ambient pressure and at different constant currents (0.2, 0.4, and 0.6 A/cm2) which was carried out on an Advent Technologies Inc. PBI MEA of 45 cm2 active surface. The impact of varying atmospheric pressure on the calculation of degradation rates is then discussed and compared with another method developed in a previous work. It was found that a variation of 6 mV (2.4 % of initial voltage) at 1 A/cm2 during an aging test can be attributed to pressure variation alone, and not to cell degradation. Furthermore, it was observed that the voltage/pressure sensitivity is different depending on the period analyzed, at identical operating conditions (which would indicate that the dependence on pressure varies during aging). Indeed, at 0.2 A/cm2, the voltage response to a variation in pressure was quantified at approximately 50 μV/mbar and 80 μV/mbar at the start and end of life respectively. This method could therefore be used as an on-line diagnostic tool to monitor the fuel cell state of health.