STORAGE OF PLATELETS FOR TRANSFUSION - CURRENT METHODS AND PROBLEMS TO BE SOLVED
Authors: Murphy S 
Published: August 2018

In the 1970's, when platelet concentrates (PC) were stored in "first generation" plastic containers at 22°C, many showed a fall in pH.In vivo viability was well maintained as pH fell from the starting level, 7.1, to 6.2 but was progressively lost below thatlevel. pH after a storage interval correlated with the platelet count in the PC, the higher the count, the lower the pH after storage. Since 30-50% of PC might have pH below 6.2 after 3 days of storage, shelflife was severely limited.

We now have "second generation" containers which allow storage for 7 days without significant fall in pH.Superior pH maintenance results from increased oxygen transport through the walls of the container thuspreventing platelet hypoxia within. Under all circumstances which we have examined, PC, pH correlates inversely with lactate concentration. The superiority of the "second generation" containers results from the lower rate of glycolysis within them due to the Pasteur effect. Like many cells, the platelet has a baseline rate of lactate production which is increased up to 8 times if oxidative metabolism is interrupted by hypoxia. For any container, there is an inverse relationship between PC pCL and platelet count since each platejlet |as a fixed oxygen demand. In "first generation" containers, the pO2 approached zero with platelet counts, 1.0-1.5 × 106 /mm3 , a range commonly seen in practice. At platelet counts above thatlevel, the platelets would have an increasing oxygendebt. The greater the oxygen debt is, the greater will be the rate of lactate production per platelet and, therefore, the rate of pH fall. An ideal "second generation" container will have oxygen permeability twice thgt 05 a "first generation" container such that platelets will begin to have an oxygen debt at platelet count, 2.4 × 106/mm3 , a level rarely seen in practice. In "second generation" containers, there is a continuing, linear production of lactate in spite of adequate oxygen supply. The major buffer in plasma for the hydrogen ion accompanying lactate production is bicarbonate. As lactate is produced, bicarbonate is consumed and pH remains stable until bicarbonate is completely depleted.Thereafter, pH falls precipitously.However, there is adequate bicarbonate in the 50 ml of plasma used as medium for the PC to buffer the baseline production of lactate for seven days.This is the major reason why storage for sevendays is now satisfactory.

With knowledge of the oxygen transport capability of a container and measurement of pO2 within the container, we can calculate the rate of oxygen consumptionduring storage, 1.1 nmol/min/109 platelets. Knowing the rates of oxygen consumption and lactate production and the predicted rate of ATP synthesis from ADP for each type of metabolism we calculated that 85% ofATP regeneration is from oxidative metabolism. When we contrasted the rate of glucose consumption with the rate of lactate production, we were surprised that the slope of the regression line was exactly 0.5 suggesting that two moles of lactate were produced for each mole of glucose utilized and that there must be a prominent substrate for oxidative metabolism other than glucose. Recent work in our laboratory suggests fatty acid as such a substrate.

There is much to be learned about storage in "secondgeneration" containers. At the onset of storage 70% of the platelets have a discoid appearance whereas after seven days only 30% do with most of the rest being spheres. Furthermore, 5-10% of cells are "leaf-shaped", elongated tubules, or ring forms. In addition, many studies have described reduced platelet function in vitro after storage. There is a 20-50% reduction in the maximal extent of aggregation with high ADP concentrations and the concentrations of single agonists which produce 50% of maximal response are increased after storage. However, we have been encouraged to find that the response to these agents was reduced much less when they were used a pairs. Finally, there is concern that trace bacterial contamination at the time of phlebotomy might allow enough proliferation to produce sepsis in recipients. Further research should improve our understanding inthese areas and the safety and efficacy of platelets after storage.