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16 DIAGNOSTIC TESTS The utilization of diagnostic tests is seldom indicated in the workup of children with functional defecation disorders. However, in selected cases diagnostic testing can be useful, e.g. if an organic cause is suspected or to help differentiate between FC and FNRFI if the diagnosis is uncertain. Laboratory testing Laboratory testing (e.g. for hypothyroidism, celiac disease or hypercalcemia) in children with defecation disorders is only indicated when there is a suspicion for an underlying organic disease, it does not belong in the routine workup of children with functional defecation disorders. Radiology Abdominal radiography A plain abdominal X-ray is not an appropriate tool to diagnose constipation. The sensitivity and specificity rates are unsatisfactory, and low inter- and intra-observer reliability have been reported for the different scoring systems (Barr, Leech, Blethyn) that are used to evaluate fecal load based on abdominal X-rays. 33,42,43 Colonic transit time In the diagnostic workup of children with functional defecation disorders, measurement of the colonic transit time (CTT) is sometimes helpful, especially in children with severe and persisting symptoms or in cases where the diagnosis is unclear. Currently, the most widely used method to determine CTT in children is the radiopaque marker test, which is cost effective and simple to perform. 44 Radiopaque markers are ingested orally and the amount of intra-abdominal markers is then determined using an abdominal X-ray. 16,45,46 A CTT <62 hours is usually considered to be normal, while a CTT >62 hours suggests slow transit constipation. 46,47 An extremely prolonged CTT of more than 100 hours indicates a severe form of constipation. 46 Another method to determine CTT is radionuclide scintigraphy; after ingestion of radioactive isotopes, colonic transit is measured with a large-field-view gamma camera. Scintigraphy is a more novel technique than the radiopaque marker test and its use in children is less widespread. 48–50 In children with functional fecal incontinence without signs of constipation in whom the diagnosis is unclear, a colonic transit study can be useful to discriminate between FC and FNRF; in these children a normal CTT suggests FNRFI. 33,39