Radioimmunoassay of beta-thromboglobulin. False high values from frozen plasma samples

Aliquots of plasma samples assayed after storage at -26 degrees C can show a significantly higher level of measured beta-thromboglobulin (beta-TG) than the corresponding unstored aliquots. Blood spun at the recommended speed of 1,800 X g still contains platelets that will release beta-TG on freezing to yield false results--a point that has not been stressed in the literature. Aliquots of blood plasma spun at either 1,800 X g (low speed [LS]) or 20,000 X g (high speed [HS]) were assayed by radioimmunoassay fresh and after storage at 4 degrees C or -26 degrees C. Storage at -26 degrees C increased mean beta-TG values of fresh or 4 degrees C stored LS samples from 0.78 to 1.94 nmol/L (28-70 ng/mL), whereas all HS mean values were +/- 0.51 nmol/L (between 18 and 19 ng/mL). The authors believe that in addition to the accepted precautions regarding blood sampling and handling, it is essential to stress that for accurate measurement of beta-TG in plasma, samples should be centrifuged at HS or, if this is not feasible, that they be assayed as soon as possible without prior freezing.