Figure 3. Distribution of macromolecules in interstitial fluid. High resolution size exclusion chromatography of plasma (black line) and interstitial fluid derived from: A; dry (red line) or wet (blue line) wicks, B; wet wicks with (red line) or without (blue line) topical anaesthetics. Wicks were collected after 60 min. of implantation. Ordinate: Absorption units at UV 210 normalized to the highest peak (HSA). Retention time for human serum albumin (HSA), IgG and alpha 2 macroglobulin (a2M) standards indicated. A small haemoglobin peak between 11 and 12 min. retention was usually seen in interstitial fluid. doi:10.1371/journal.pone.0031332.g003 Discussion The transmural colloid osmotic pressure plays major roles in the control of fluid transport across the capillary endothelium and provides important information when fluid is administrated to patients. COPi will significantly influence the transcapillary fluid flux under normal and diseased states [26]. However, the usefulness of measuring COP in clinical practice is still limited due to lack of simple and reliable methods, and discomfort during implantation and removal of wicks. In this study we show that COPi from subcutaneous tissue is easily obtained and that fluid harvesting is well tolerated when topical anaesthetic is used. Most importantly, the use of a topical anaesthetic did not alter the ease of IF harvesting or COPi values when using the wick method for sampling. To our knowledge, there are few studies of sampling IF without the use of injected local anaesthetics, and the use of topical lidocaine/prilocaine cream prior to wick insertion has not Local Anaesthetics and Colloid Osmotic Pressure previously been evaluated. In a study by Poulsen et al, one participant had 3 out of 19 wicks inserted without any anaesthesia, with equal results as the mean of all wicks in the same person [27]. EMLA cream, which contains lidocaine and prilocaine, is a low cost topical anaesthetic. It has been found to give equally effective dermal analgesia when assessed with VAS and verbal rating scales (VRS) for venous puncture in children [28] and in adults [29]. The analgesic effect is not affected by skin pigmentation [30]. EMLA cream can be applied from birth (gestational age .37 weeks) and has proven safe and effective in decreasing pain response to venous puncture in neonates in combination with oral sucrose [31]. Intradermal or subcutaneous injection of lidocaine will give almost instant local anaesthesia, but has the disadvantage of an extra uncomfortable skin puncture. In the present study the use of EMLA cream reduced the pain associated with the implantation of wicks as judged from a lower VAS-score. No adverse effects were observed, except that a mild and transient local paleness or redness was observed in some patients. Local injections of different concentrations of lidocaine and prilocaine have shown variable vasoactive responses, although prilocaine has vasodilator proper- ties [32]. Skin reflectance spectroscopy and laser Doppler blood flowmetry show a biphasic skin-blood-flow response with initial vasoconstriction followed by vasodilatation after 3 hours of EMLA cream occlusion [33]. Comparison of vein diameter by Doppler colour ultrasound using EMLA cream or amethocaine indicates a capillary constrictor response rather than venous dilatation [34]. Our results show that despite small variations in skin colour, COPi obtained from wet wicks throughout the implantation time was not significantly affected by the use of EMLA cream indicating that only minor changes in local microcirculation and transudation of fluid occurred across the capillary membrane. Although the manufacturer recommends application of EMLA cream on intact skin for at least 60 min. before procedures [35], and prolongation to 120 min. does not seem to have additional efficacy [30], longer application than 60 min. may contribute to a more pronounced vasoconstriction. Its effect on COPi will presumably be of minor importance since both vasoconstriction and vasodilatation is rapidly reversed after removal of EMLA cream. Sampling of IF by subcutaneous wicks may cause tissue and vessel damage with subsequent local inflammation and bleeding resulting in increasing capillary permeability to proteins and increased COPi. We experienced more frequent bleeding than reported in previous studies [11] [9] where injections of anaesthetic agents were used. One possible explanation may be that a small deposition of liquid can influence local haemostasis with less bleeding as a consequence. Blood stained wicks were discarded in accordance with the recommendations of Aukland & Fadnes [3]. In this study it was reported that a haemoglobin concentration less than 0.2 g/dl in clear and pink wicks [3], indicating a maximum increase in total wick fluid protein content of only 5% as long as the wick was not clearly blood stained. HPLC indicates a low haemoglobin contamination in sampled IF confirming a well operating colloid osmometer. Normal plasma COP in humans is 25–30 mmHg and our findings agree well with this [26]. Subcutaneous implantation of wicks in the upper arm region represents the approximate level of the heart, and variations in body position will only have limited influence on COPp and COPi [12]. All study persons participated in daily activity before and after implanting the wicks. As in previous studies we experienced fluctuations in COPp that were probably due to individual differences within the study population, but were also modestly related to low COPp and low COPi. This association was more pronounced after 90 and 120 min. of implantation. PLoS ONE | www.plosone.org 4 February 2012 | Volume 7 | Issue 2 | e31332