Figure 3 Nasal potential difference measurement in (A) a healthy person and (B) a person with classic CF. In a healthy person the basal PD is negative (−20 to −30 mV), rises moderately (towards zero) after application of amiloride, and decreases after application of chloride free solution and isoproterenol. In a patient with typical CF the baseline PD is more negative and the rise after application of amiloride is greater. There is no change in PD after application of chloride free solution and isoproterenol.