Measurement of transepithelial nasal potential difference (NPD) is an ancillary test that can be helpful when other tests are equivocal.
Technical factors:
(1) The reference electrode is inserted into the subcutaneous tissue of the forearm.
(2) Measurements should use standardized equipment and protocols.
(3) It is important to avoid squamous epithelium in the nose.
(4) Testing should be delayed if the patient has had recent rhinitis or hay fever.
Common measurements:
(1) sodium transport (baseline voltage is an index of sodium transport)
(2) chloride conductance
Findings may include:
(1) hyperpolarized baseline voltage readings
(2) excessive depolarization in response to a sodium channel inhibitor (amiloride, other)
(3) little or no chloride secretion in response to isoproterenol in a chloride-free solution (zero chloride plus beta agonist solution)
(4) greater potential difference in response to ATP (-10 mV +/- 5 vs -5 mV +/- 3 in normal patients
