Colloid osmotic pressure of human subcutaneous interstitial fluid sampled by nylon wicks: evaluation of the method

Interstitial fluid was collected from human subcutaneous tissue by implantation of multifilamentous nylon wicks on the side of thorax. The skin was anaesthetized by a small injection of lidocaine. Direct measurement of colloid osmotic pressure in a control group of 20 healthy volunteers gave a mean of 15.8 (SD 2.3) mmHg in wick fluid and 26.9 (SD 4.1) mmHg in serum samples. Standard deviation of the method for wick fluid samples was 1.2 mmHg. Significant blood contamination occurred in less than 20% of the wicks. Fluid content in wicks varied within narrow limits independent of its colloid osmotic pressure. The dynamics of wick fluid protein uptake was studied by uptake of 131I albumin from plasma, and by measurement of colloid osmotic pressure after 30, 60, 120 and 180 min of implantation. The results showed an initial rapid uptake from plasma of a major part of the wick fluid proteins within 30 min. After 120 min wick fluid colloid osmotic pressure reached the same level whether the wicks were 'primed' with serum proteins or not. Altogether these observations support the view that wick fluid reflects interstitial tissue fluid with respect to colloid osmotic pressure when the wicks are implanted for 60 min.