The international protocols of electron dosimetry published by the Nordic Association of Clinical Physics and the Sociedad Española de Física Médica recommend the use of a total perturbation correction factor pu, pp which is constant for all plane parallel chambers. In this work we have compared the dose measured with the PTW-Markus and NACP plane parallel chambers in respect to a cylindrical one. The obtained results indicate that for the ionization chamber NACP the value of pu, pp = 1.000 +/- 0.005 is adequate while for the ionization chamber PTW the observed difference is explained by considering an individual total perturbation correction factor variable with the mean energy at depth z, Ez, as the fluence perturbation correction factor pf is usually given for cylindrical chambers.