A rapid assay for platelet thromboxane production and its use in assessing prior aspirin ingestion

In the laboratory evaluation of platelet disorders, there are a number of situations in which determining the capacity of platelets to synthesize thromboxane is of diagnostic value. An assay for thromboxane production is required for the diagnosis of cyclooxygenase and thromboxane synthetase defects and is useful in detecting prior aspirin ingestion. The authors describe an assay for platelet thromboxane production that is simple, rapid, inexpensive, and suitable for routine use in the clinical coagulation laboratory. It is based on platelet synthesis of ¹⁴C-thromboxane from ¹⁴C-arachidonate. ¹⁴C-thromboxane is isolated by thin-layer chromatography and its radioactivity quantitated by scintillation counting. The results are expressed as the thromboxane index: ¹⁴C-arachidonate converted to thromboxane/platelet count. The assay is linear with respect to platelet concentration and substrate concentration and is independent of recovery. Using this assay, the authors demonstrated that platelet thromboxane production, expressed as thromboxane index (mean ± standard deviation) was completely inhibited 12-16 hours after ingestion of a single aspirin dose of 650 mg (0.27 ± 0.14), 325 mg (0.29 ± 0.18), or 163 mg (0.13 ± 0.13), with partial inhibition by 81 mg (0.41 ± 0.46) or 41 mg (1.41 ± 0.75) (n = 3 for each dose). Thromboxane index for normal controls was 2.41 ± 0.77 (n = 25). The authors also determined the time for recovery of thromboxane synthetic capacity to normal levels in four subjects followed longitudinally after ingesting a single 650-mg dose of aspirin. Platelets from three of the four subjects recovered normal thromboxane synthetic capacity by the fifth day after aspirin ingestion, consistent with platelet half-life in the circulation. Thus, the authors have developed a rapid, inexpensive assay for assessing the function of cyclooxygenase and thromboxane synthetase in platelets, which can be especially useful as a screening test to detect ingestion of aspirin before performance of expensive and labor-intensive platelet function studies.