Fluid loss from circulating plasma volume is one of the contributors of oedema formation. Measurement of colloid osmotic pressure (COP) both in plasma and in interstitial fluid can be utilised in planing of fluid therapy in patients with oedema. Different methods of sampling interstitial fluid are well validated in animal models, but not in human. Sampling of interstitial fluid by subcutaneous sewn wicks is a well-known method and can be painful. Local anaesthetics can reduce pain. By comparing interstitial fluid taken from areas with or without application of local anaesthetics, a possible influence in COP values can be detected.