Yvan Vandenplas is Professor of Pediatrics and Head of the Department of Pediatrics at the Academic Hospital of the Vrije Universiteit Brussel in Brussels, Belgium. He is a paediatric gastroenterologist speciality in gastro-oesophageal reflux with other interests including food allergy, pre- and probiotics. Professor Vandenplas is currently Scientific Secretary of the European Society of Paediatric Gastroenterology, Hepatology and Nutrition. He has published over 200 papers. a report by Yvan Vandenplas , Thierry Devreker and Bruno Hauser Department of Paediatrics, Academic Hospital of Vrije Universiteit Brussel Gastro-oesophageal reflux (GOR) is the involuntary passage of gastric contents into the oesophagus. GOR is a physiological event occurring in every individual several times during the day, particularly after meals. Most reflux episodes are asymptomatic, brief and limited to the distal oesophagus. However, severe GOR disease (GORD) may cause minor symptoms and minor GOR may cause severe symptoms. GOR may be a primary gastro-intestinal motility disorder, but may as well be secondary to other conditions such as cow’s milk protein allergy. According to the literature, cow’s milk protein allergy is a frequent cause of GOR during infancy. This review will discuss the advantages and disadvantages of pH and impedance techniques to measure GOR, independent of the cause of reflux. Impedance – The Technique The development of oesophageal pH-monitoring in the 1980s changed the work-up of GOR substantially. It took many years to discover the advantages and pitfalls of pH monitoring. The basic principle of impedance is identical to pH-monitoring: registration of oesophageal events with a probe placed transnasally and connected to a recorder. Whenever a new technique is developed to replace an older one, the newer is in general considerably more expensive. Both the device and the electrodes for impedance are currently considerably more expensive than those used for pH-monitoring. Impedance, that is electrical resistance, is determined by the quantity and flux of ions into the tissue. Impedance allows the detection of the frequency, the oesophageal height and duration of reflux episodes, independent of the pH of the refluxate. Experience with pH-monitoring has shown the pitfalls of an arbitrary cut-off limit such as pH 4.0. Indeed, it is likely that a pH of 3.9, 4.0 or 4.1 has exactly the same clinical relevance. A similar comment can be made for impedance: the automatic analysis considers only a drop of impedance of 50% or more as a reflux episode. However, it is likely that a drop of 49% can also be attributed to a reflux episode. Although impedance allows or better necessitates a manual analysis, the relevant question that remains is: what is the decrease in impedance needed to be considered as a reflux episode? The drop in impedance is not related to the volume of the refluxate. If pH-monitoring were performed with a probe with multiple pH sensors, it would also be possible to determine the height of the refluxate. The major difference between both techniques is restricted to the detection of non-acid reflux. As a consequence, another fundamental question arises: what is the clinical relevance of non-acid, weakly- acid and alkaline reflux? The reproducibility of MII-pH recording on two consecutive days is rather poor, especially for non- acid reflux.1 The limits of agreements for the number of acid and non-acid reflux episodes with a second recording performed two days after a first recording have been shown to have a high variation: 0.2–5.3 and 0.04–8.6 times the value obtained at day one, respectively. 1 However, reproducibility of pH- monitoring on two consecutive days was previously reported to have high Pearson correlation coefficients, ranging from 0.88 to 0.98. 2 Over 95% of the reflux events detected by the automatic MII-pH analysis were confirmed by two independent investigators, although they added about 33% acid, weakly acid and non-acid reflux episodes. 3 The agreement between both investigators for the reflux episodes detected by manual reading of the 24-hour MII-pH tracing was only about 50%. 3 Inter-observer variability was reportedly much better in impedance recordings in neonates during a period of six hours. 4 Non-acid Reflux Up to now there has been general consensus that investigations measuring reflux during the postprandial period (ultrasound, radiology, scintigraphy) are of limited value in the diagnosis of GORD because of the high prevalence of GOR in the postprandial period. The pH of reflux during a postprandial period is mostly above pH 4 (so commonly regarded as non-acid based on pH- monitoring criteria). If a naso-gastric tube passes the cardia, impedance shows an increase in postprandial reflux (from 72 to 122 episodes) in 16 pre-term Gastro-oesophageal Reflux Disease – Oesophageal Impedance Versus pH-monitoring Gastro-oesophageal Reflux Disease EUROPEAN GASTROENTEROLOGY REVIEW 2006 29