Many a slip?

" C ystic fibrosis (CF) is a common disease (up to 1 in 2,000 in some populations), and there is a simple, cheap and reliable diagnostic test which is within the compass of a hospital of any size. Yet even so, wrong diagnoses are not uncommon, because there is a long list of causes of a false-positive elevation in sweat electrolytes [1]. The most important problem is lack of experience and attention to detail by the operator. Furthermore, borderline cases may need sophisticated testing to establish the diagnosis, such as nasal potential differences or complete sequencing of the CF gene. The consequences of a wrong diagnosis of CF are potentially serious, including the prescription of inappropriate medications , and the generation of a great deal of anxiety. The situation with primary ciliary dyskinesia (PCD) is even more complex. The prevalence is debated, and although probably less than CF, PCD is still an important diagnosis to make. Diagnosis is delayed to a greater extent than CF [2], in part because many of the symptoms (cough and a runny nose) are common in normal children [3]. The diagnosis is important; treatment leads to stabilisation of lung function, and those diagnosed late have worse spirometry [4, 5]. Furthermore, PCD upper airway disease, specifically chronic secretory otitis media, has different implications than the same problem in otherwise normal children [6, 7]. However, the diagnostic difficulties, particularly in terms of false positives, are much greater in PCD than in CF. The diagnostic pathway for PCD will often start with the elimination of commoner causes of respiratory symptoms, for example CF and hypogammaglobulinaemia, unless there are clear pointers to the diagnosis, such as unexplained neonatal respiratory distress in a baby with mirror image arrangement [8]. A number of screening tests for PCD have been advocated. Ciliary function can be measured in vivo using the saccharine test [9], or radionuclide isotope scanning [10], and if normal, may obviate the need for further testing. However, if the patient sniffs during the saccharine test, a taste may be reported, and PCD eliminated incorrectly. Nasal nitric oxide (nNO) is almost invariably low in PCD [11], but this is not specific, and low nNO has been reported in CF [12] and diffuse panbronchiolitis [13]. Hence, these screening tests can never be used to make a positive diagnosis of PCD; definitive diagnostic testing is mandatory. Although newer diagnostic tests such as gene …