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By means of a haemonetics 30 cell separator platelets were collected for transfusion purposes from 18 different blood donors. A median yield of 5.7 X 10(11) platelets was obtained. The yield of platelets was found to be linearly related to the donors' initial platelet counts. The procedures were standardized by using one size of bowl (225 ml), performing 10 cycles per procedure, and performing all cycles as uniformly as possible. The median time per procedure was 118 min. Mild donor reactions were seen in 8 donors (44%) consisting of circumoral tingling. The decrement in the donors' platelet count was 86 X 10(9)/1, 37% of the initial platelet counts. The choice of the bowl size of 225 ml is motivated. The absurdity of the expression of yield pe h of processing, and of yield per 1 of blood processed is discussed. Furthermore, it is shown, that neither the fall in haematocrit nor the fall in the platelet count during the procedure are linear. The formulas commonly used for calculating the blood volume and platelet extraction efficiency, which are based on these assumptions, cannot therefore, in our opinion, be used.

Neuromuscular transmission and the staircase phenomenon in the adductor pollicis muscle were studied in six patients with myasthenia gravis before and 24 hours after single plasma exchanges given in short series over one to four weeks. An improvement in neuromuscular transmission was observed in all patients but one within 24 hours, suggesting that an immunological block of receptor sites is reversible or that acetylcholine receptors are rapidly resynthesized. In addition to a neuromuscular transmission defect, two of the patients also showed evidence of impaired excitation-contraction coupling with a negative staircase phenomenon and reduced posttetanic twitch potentiation. This condition became normal after plasma exchange, which may indicate that a reversible immunological impairment of the sarcoplasmic reticulum was present. Improvement lasted up to several months. The anti-acetylcholine receptor antibody titers were lowered in response to single plasma exchanges, but actual titer levels showed poor correlation with clinical and electrophysiological changes.