RESUMO
A novel analytical method, using turbidometry, for reporting the time-dependent decay in the threshold concentration of adenosine diphosphate (ADP), required to elicit a secondary aggregation response in fresh human citrated platelet-rich plasma (PRP), is described. The phenomenon, called "transient aggregation resistance" (TAR) ends, usually within one hour after venepuncture, in a steady state or "baseline aggregation resistance" (BAR). Back extrapolation of the mathematically transformed data to t = 0, yields a maximal threshold concentration of ADP, representing the initial aggregation resistance (TARmax) at the time of blood withdrawal, which threshold is usually many orders of magnitude higher than the BAR-value. The exponential decay of TAR can be characterized by its half-life (t1/2). Mixing fresh, rapidly prepared, plasma with PRP, kept long enough to show only the stable low BAR-value, could restore the initial high (transient) aggregation resistance found in fresh PRP, suggesting that it concerns a natural, labile plasmatic factor. One hour old PRP, deliberately made refractory to ADP, did not show the TAR phenomenon again, but had a higher BAR-value. It is suggested that the level of clinical relevance of these early in vitro aggregation measurements is higher than that of classical, delayed aggregometry (e.g. BAR-values).