The production of platelet controls for assays quantitating platelet- associated IgG

Abstract

A variety of assays are available for measuring platelet-associated IgG (PAlgG) but the complexity of these assays increases the potential for technical errors. These errors are difficult to detect and, if possible, known positive and negative control platelets should be included with each run. However, patient platelets with elevated levels of IgG are seldom available. A method is described for producing positive control platelets that can be labeled with differing amounts of IgG. Normal serum IgG (Cohn fraction II) was incubated with washed 2% formalin-fixed platelets for 60 min at 37.degree. C. The amount of IgG on the platelets was proportional to the concentration of soluble IgG and the number of incubations. Normal platelet IgG levels were 1.2 .+-. 0.1 fg/platelet (mean .+-. SEM [standard error of the mean], n = 34) and positive control platelets had 4.6 .+-. 0.2 (n = 12) or 8.4 .+-. 0.4 (n = 7). There was no change in the level of PAIgG when stored at 4.degree. C for 1 wk, although there was a 28% loss in recoverable platelets. When mixed 1:1 with 60% glycerol and stored at -70.degree. C, the level of PAIgG was stable for 3 mo., with less than 12% platelet loss on recovery (n = 12). These positive control platelets have proved useful for monitoring assay performance.