(A) Correlation between MDR1 mRNA expression by RT-PCR and Pgp expression by flow cytometry (r = .95, P < .0001). All patients negative by RT-PCR expressed a fluorescence ratio from 0.7 to 1.3 in the Pgp detection assay. Therefore, the threshold of positivity of 1.3 (horizontal dotted line) was used (with this cut-off of fluorescence ratio, 51% of patients expressed Pgp). (B) Correlation between the effect of CsA on calcein-AM uptake and MDR1 mRNA expression by RT-PCR in 40 patients (r = .63, P < .0001). (C) Correlation between the effect of CsA on calcein-AM uptake and Pgp expression by flow cytometry in 40 patients (r = .66,P < .0001). In some cases, the ratios of fluorescence in Pgp activity assay ranged from 0.61 to 1. This was probably caused by variations in cellular uptake of calcein-AM in the experiments. Conversely, the fluorescence ratios of up to 1.39 can also represent some experimental variability (area between the two vertical bold lines) (B and C). With these thresholds of positivity, 14 patients (35%) were Pgp+/activity+, 12 patients (30%) were Pgp−/activity−. However, as previously described,1,6 11discrepant cases were identified, including 13 samples (32.5%) Pgp+/activity− and 1 sample (2.5%) Pgp−/activity+.