The baseline PD is rather low, even for normal, (at around -10mV) and so is the subsequent response to amiloride (perfusion started at point a). The response to perfusion with a solution low in chloride ions is large (point b). This specific response is the best discriminator between non-CF and CF profiles (see later figure). The final perfusion change is the addition of ATP (point c). This is the most variable response in normal epithelia. Some subjects have a consistently large (though transient response). In most cases the response is small. This contrasts to CF subjects, in whom the response to ATP is generally large (although again transient).