The working mechanism of implanted wicks has been carefully evaluated by Wiig et al where true COPi is best obtained by the crossover method, and optimal implantation time for wicks is believed to be between 90 and 120 minutes in rat models [5]. We found a slow, but statistically non-significant increase in COPi for IF collected from both dry and wet wicks between 75 and 120 min. after insertion which may be ascribed to local inflammation due to mechanical tissue trauma, similar to what has been described in earlier studies on healthy humans [11]. Implantation for longer than 120 min. seems to increase protein concentration in wick fluid from local inflammation with extravasation of plasma proteins [3]. In normally hydrated subcutaneous tissue COPi in wick fluid is found in the range 12.3–18.8 mmHg on the thorax and reduced to 6.5–13.0 below waist level (ankle) [12] [24]. Prolonged horizontal positioning before implantation results in a modest increase in COPi. The finding of a higher mean COPi in all our subjects compared to other studies of healthy humans may be due to longer transfer time of wicks from subcutaneous tissue too mineral oil filed tubes. However, this seems unlikely because implantation of wicks was done according to protocol in all subjects as in previous studies in our lab. The present study has some limitations. In order to optimize evaluation of topical anaesthesia, the subjects as well as the investigator should ideally have been blinded to which arm EMLA cream was applied. Blinding of the investigator would have been References 1. Starling EH (1896) On the Absorption of Fluids from the Connective Tissue Spaces. J Physiol (Lond) 19: 312–326. 2. Fadnes HO, Reed RK, Aukland K (1978) Mechanisms regulating interstitial fluid volume. Lymphology 11: 165–169. 3. Aukland K, Fadnes HO (1973) Protein concentration of interstitial fluid collected from rat skin by a wick method. Acta Physiol Scand 88: 350–358. 4. Fadnes H, Aukland K (1977) Protein concentration and colloid osmotic pressure of interstitial fluid collected by the wick technicue: Analysis and evaluation of the method. Microvascular Research 14: 11–25. 5. Wiig H, Heir S, Aukland K (1988) Colloid osmotic pressure of interstitial fluid in rat subcutis and skeletal muscle: comparison of various wick sampling techniques. Acta Physiol Scand 133: 167–175. 6. Kramer GC, Sibley L, Aukland K, Renkin EM (1986) Wick sampling of interstitial fluid in rat skin: further analysis and modifications of the method. Microvascular Research 32: 39–49. 7. Heltne JK, Husby P, Koller ME, Lund T (1998) Sampling of interstitial fluid and measurement of colloid osmotic pressure (COPi) in pigs: evaluation of the wick method. Lab Anim 32: 439–445. 8. Tollan A, Oian P, Fadnes HO, Maltau JM (1993) Evidence for altered transcapillary fluid balance in women with the premenstrual syndrome. Acta Obstet Gynecol Scand 72: 238–242. 9. Noddeland H, Riisnes SM, Fadnes HO (1982) Interstitial fluid colloid osmotic and hydrostatic pressures in subcutaneous tissue of patients with nephrotic syndrome. Scand J Clin Lab Invest 42: 139–146. 10. Khiabani HZ, Anvar MD, Kroese AJ, Stranden E (2000) Transcapillary forces and the development of oedema in the lower limb of patients with chronic critical limb ischaemia (CLI). Eur J Vasc Endovasc Surg 19: 598–604. 11. Noddeland H (1982) Colloid osmotic pressure of human subcutaneous interstitial fluid sampled by nylon wicks: evaluation of the method. Scand J Clin Lab Invest 42: 123–130. 12. Noddeland H (1982) Influence of body posture on transcapillary pressures in human subcutaneous tissue. Scand J Clin Lab Invest 42: 131–138. 13. European Union (2008) Ethical considerations for clinical trials on medicinal products conducted with the paediatric population. Eur J Health Law 15: 223–250. 14. Aukland K, Johnsen HM (1974) A colloid osmometer for small fluid samples. Acta Physiol Scand 90: 485–490. 15. Wiig H, Halleland EG, Fjaertoft M, Aukland K (1988) Measurement of colloid osmotic pressure in submicrolitre samples. Acta Physiol Scand 132: 445–452. 16. Oian P, Maltau JM, Noddeland H, Fadnes HO (1986) Transcapillary fluid balance in pre-eclampsia. Br J Obstet Gynaecol 93: 235–239. 17. Oian P, Maltau JM, Noddeland H, Fadnes HO (1985) Oedema-preventing mechanisms in subcutaneous tissue of normal pregnant women. Br J Obstet Gynaecol 92: 1113–1119. 18. Stranden E, Myhre HO (1983) Transcapillary forces in patients with lower limb ischemia. Scand J Clin Lab Invest 43: 233–239. Local Anaesthetics and Colloid Osmotic Pressure possible, but blinding of participants was difficult because EMLA cream is known to create a sensation of numbness. Since there is a tradition of using locally injected anaesthetics in wick studies, future studies should compare injection and topical administration of local anaesthesia before wick insertion. In conclusion, sampling of IF by wicks in adults was less painful after application of topical EMLA cream than without local anaesthesia. EMLA cream had no significant influence on COPi. The COP values obtained in the present study using wick technique were similar to reference data obtained with other sampling methods. Our results indicate an optimal wick implantation time between 75–90 min. Since topical anaesthesia did not affect COP measurements, it is possible to extend the usefulness of the wick method, especially in subjects who dislike technical procedures, including children. Acknowledgments We want to thank Trond Markestad for valuable and helpful comments on the manuscript. Author Contributions Conceived and designed the experiments: HJG AB HW. Performed the experiments: HJG OT AB HW. Analyzed the data: HJG TN OT AB. Contributed reagents/materials/analysis tools: HW OT HJG AB TN. Wrote the paper: HJG AB. 19. Noddeland H, Omvik P, Lund-Johansen P, Ofstad J, Aukland K (1984) Interstitial colloid osmotic and hydrostatic pressures in human subcutaneous tissue during early stages of heart failure. Clin Physiol 4: 283–297. 20. Koomans HA, Kortlandt W, Geers AB, Dorhout Mees EJ (1985) Lowered protein content of tissue fluid in patients with the nephrotic syndrome: observations during disease and recovery. Nephron 40: 391–395. 21. Haaverstad R, Romslo I, Larsen S, Myhre HO (1996) Protein concentration of subcutaneous interstitial fluid in the human leg. A comparison between the wick technique and the blister suction technique. Int J Microcirc Clin Exp 16: 111–117. 22. Semb KA, Aamdal S, Oian P (1998) Capillary protein leak syndrome appears to explain fluid retention in cancer patients who receive docetaxel treatment. J Clin Oncol 16: 3426–3432. 23. Heradstveit BE, Guttormsen AB, Langorgen J, Hammersborg SM, Wentzel- Larsen T, et al. (2010) Capillary leakage in post-cardiac arrest survivors during therapeutic hypothermia - a prospective, randomised study. Scand J Trauma Resusc Emerg Med 18: 29. 24. Bates DO, Levick JR, Mortimer PS (1994) Starling pressures in the human arm and their alteration in postmastectomy oedema. J Physiol 477(Pt 2): 355–363. 25. Bates DO, Levick JR, Mortimer PS (1993) Change in macromolecular composition of interstitial fluid from swollen arms after breast cancer treatment, and its implications. Clin Sci (Lond) 85: 737–746. 26. Aukland K, Reed RK (1993) Interstitial-lymphatic mechanisms in the control of extracellular fluid volume. Physiol Rev 73: 1–78. 27. Poulsen HL (1973) Subcutaneous interstitial fluid albumin concentration in long- term diabetes mellitus. Scand J Clin Lab Invest 32: 167–173. 28. Nilsson A, Boman I, Wallin B, Rotstein A (1994) The EMLA patch–a new type of local anaesthetic application for dermal analgesia in children. Anaesthesia 49: 70–72. 29. Shaikh FM, Naqvi SA, Grace PA (2009) The influence of a eutectic mixture of lidocaine and prilocaine on minor surgical procedures: a randomized controlled double-blind trial. Dermatol Surg 35: 948–951. 30. Riendeau LA, Bennett D, Black-Noller G, Fan L, Scavone JM (1999) Evaluation of the analgesic efficacy of EMLA cream in volunteers with differing skin pigmentation undergoing venipuncture. Reg Anesth Pain Med 24: 165–169. 31. Abad F, Diaz-Gomez NM, Domenech E, Gonzalez D, Robayna M, et al. (2001) Oral sucrose compares favourably with lidocaine-prilocaine cream for pain relief during venepuncture in neonates. Acta Paediatr 90: 160–165. 32. Willatts DG, Reynolds F (1985) Comparison of the vasoactivity of amide and ester local anaesthetics. An intradermal study. Br J Anaesth 57: 1006–1011. 33. Bjerring P, Andersen PH, Arendt-Nielsen L (1989) Vascular response of human skin after analgesia with EMLA cream. Br J Anaesth 63: 655–660. 34. Ashley EM, Quick DG, El-Behesey B, Bromley LM (1999) A comparison of the vasodilatation produced by two topical anaesthetics. Anaesthesia 54: 466–469. 35. AstraZeneca (2005) Emla Cream product information. Available: http://www1. astrazeneca-us.com/pi/EMLA.pdf. Accessed 2012 Jan 11. PLoS ONE | www.plosone.org 5 February 2012 | Volume 7 | Issue 2 | e31332