Humm, did you recalibrate the meter, or did it come down form the earlier 9ppm to 5ppm?
Anyway, assuming that your tap water is still around 148, your membrane efficiency is (148-5)/148*100=96%, which is excellent... no need to replace the membrane or any other cartridges, just replace the DI resin.
FWIW, I go through resin pretty quickly because the TDS of my tap water is around 250.
Nuno