<prompt> True about cystic fibrosisThe underlying defect in sodium channel functionThe immune reactive trypsin I may be checked on the Guthrie cardIn most cases the gene c defect is a loss of Valine at position 508 on chromosome 7A sweat test with a sodium content above 40 mmoll is diagnos c  <response>: The immune reactive trypsin I, used in diagnosis of those under 3 months can be checked on the Guthrie card. The sweat sodium concentrate on is the definitive test and this is diagnostic if on two tests the sodium is above 60mmoll. False positive sweat tests are seen in Ectodermal Dysplasia. 