Representation of standard curves obtained with a conventional p24 ELISA and with the ultrasensitive p24 assay. The conventional p24 standard curve (ZeptoMetrix Co., NY, USA) is represented by the black line, with values indicated on the right y axis; the range is from 7.8 to 125 pg/ml. The standard curve of the ultrasensitive p24 assay is represented by the red line, with values indicated on the left y axis; the range is from 0.017 to 37.8 pg/ml. OD, optical density; AEB, average amount of enzyme per bead.