Abstract: Instruments for use by medical laboratories to measure oncotic (colloid osmotic) pressure of blood for determining the risk of pulmonary edema are improved by the provision in the sensing heads thereof of a sinuous, preferably spiral, elongate, sample chamber facing one side of the usual osmotic membrane, whose other side faces a similar reference chamber for saline solution. The membrane is preferably pre-mounted in a ring holder, which merely slips into place within a receiving recess of the sensing head, and a special waste collection system is provided for receiving liquid flushed from the sample chamber between individual uses of the device. Such waste collection system comprises a drainage receptacle which constantly maintains a constant depth pool of the drained liquid about the discharge end of the drain tube that leads from the sample chamber, so as to maintain a constant end effect for such drain tube.