Limit of detection: 80% B/B₀: 7 pg/ml · Sensitivity: 50% B/B₀: 35 pg/ml · Thromboxane A₂ (TXA₂) is produced from arachidonic acid by many cells and causes irreversible platelet aggregation and vascular and bronchial smooth muscle contraction.^1,2,3 TXA₂ is rapidly hydrolyzed non-enzymatically to form TXB₂. Although it is common to estimate TXA₂ levels by measuring TXB₂, most of the TXB₂ measured is due to ex vivo platelet activation or intra-renal production.⁴ Measurement errors are compounded by the fact that normal concentrations of circulating TXB₂ are extremely low (1-2 pg/ml), and highly transient (t1/2 = 5-7 minutes).^5,6 To circumvent this problem, it is necessary to measure a metabolite that cannot be formed by platelets or by the kidney. TXB₂ may be metabolized by 11-hydroxy thromboxane dehydrogenase to form 11-dehydro TXB₂, or by β-oxidation to form 2,3-dinor TXB₂.⁷ Infusion studies using TXB₂ have shown that both metabolites are formed equally, but that the circulating half-life of 2,3-dinor TXB₂ is shorter (t1/2 = 15 minutes). Therefore, measurement of 2,3-dinor TXB₂ will give a more episodic indication of TXA₂ production.⁶ Because plasma concentrations of 2,3-dinor TXB₂ are low (and artifactual TXB₂ concentrations may be high), it may be necessary to purify and concentrate plasma samples. It is also recommended to purify urine samples to remove possible crossreacting metabolites and TXB₂ from the sample.⁸

1 Hamberg, M., Svensson, J., and Samuelsson, B. Thromboxanes: A new group of biologically active compounds derived from prostaglandin endoperoxides. Proc Natl Acad Sci USA 72 2994-2998 (1975).

2 Ellis, E.F., Oelz, O., Roberts, L.J., et al. Coronary arterial smooth muscle contraction by a substance released from platelets: Evidence that it is thromboxane A₂. Science 193 1135-1137 (1976).

3 Salzman, P.M., Salmon, J.A., and Moncada, S. Prostacyclin and thromboxane A₂ synthesis by rabbit pulmonary artery. J Pharmacol Exp Ther 215 240-247 (1980).

4 Samuelsson, B., Granström, E., Green, K., et al. Prostaglandins. Annu Rev Biochem 44 669-695 (1975).

5 Patrono, C., Ciabattoni, G., Pugliese, F., et al. Estimated rate of thromboxane secretion into the circulation of normal humans. J Clin Invest 77 590-594 (1986).

6 Lawson, J.A., Patrono, C., Ciabattoni, G., et al. Long-lived enzymatic metabolites of thromboxane B₂ in the human circulation. Anal Biochem 155 198-205 (1986).

7 Roberts, L.J., Sweetman, B.J., and Oates, J.A. Metabolism of thromboxane B₂ in man. J Biol Chem 256(16) 8384-8393 (1981).

8 Lellouche, F., Fradin, A., FitzGerald, G., et al. Enzyme immunoassay measurement of the urinary metabolites of thromboxane A₂ and prostacyclin. Prostaglandins 40 297-310 (1990).

Samuelsson, B., Granström, E., Green, K., et al. Prostaglandins. Annu Rev Biochem 44 669-695 (1975).

Hamberg, M., Svensson, J., and Samuelsson, B. Thromboxanes: A new group of biologically active compounds derived from prostaglandin endoperoxides. Proc Natl Acad Sci USA 72 2994-2998 (1975).

Lawson, J.A., Patrono, C., Ciabattoni, G., et al. Long-lived enzymatic metabolites of thromboxane B₂ in the human circulation. Anal Biochem 155 198-205 (1986).

Patrono, C., Ciabattoni, G., Pugliese, F., et al. Estimated rate of thromboxane secretion into the circulation of normal humans. J Clin Invest 77 590-594 (1986).

Wennmalm Å, Benthin, G., Granström, E.F., et al. 2,3-Dinor metabolites of thromboxane A₂ and prostacyclin in urine from healthy human subjects: Diurnal variation and relation to 24h excretion. Clin Sci 83 461-465 (1992).

Roberts, L.J., Sweetman, B.J., and Oates, J.A. Metabolism of thromboxane B₂ in man. J Biol Chem 256(16) 8384-8393 (1981).

Foegh, M.L., Zhao, Y., Madren, L., et al. Urinary thromboxane A₂ metabolites in patients presenting in the emergency room with acute chest pain. J Intern Med 235 153-161 (1995).

Ciabattoni, G., Pugliese, F., Davi, G., et al. Fractional conversion of thromboxane B₂ to urinary 11-dehydrothromboxane B₂ in man. Biochim Biophys Acta 992 66-70 (1989).

Maxey, K.M., Maddipati, K.R., and Birkmeier, J. Interference in enzyme immunoassays. J Clin Immunoassay 15 116-120 (1992).

Pradelles, P., Grassi, J., and Maclouf, J.A. Enzyme immunoassays of eicosanoids using acetylcholinesterase as label: An alternative to radioimmunoassay. Anal Chem 57 1170-1173 (1985).

Ellis, E.F., Oelz, O., Roberts, L.J., et al. Coronary arterial smooth muscle contraction by a substance released from platelets: Evidence that it is thromboxane A₂. Science 193 1135-1137 (1976).

Salzman, P.M., Salmon, J.A., and Moncada, S. Prostacyclin and thromboxane A₂ synthesis by rabbit pulmonary artery. J Pharmacol Exp Ther 215 240-247 (1980).

Lellouche, F., Fradin, A., FitzGerald, G., et al. Enzyme immunoassay measurement of the urinary metabolites of thromboxane A₂ and prostacyclin. Prostaglandins 40 297-310 (1990).

Maclouf, J., Grassi, J., and Pradelles, P. Development of enzyme-immunoassay techniques for the measurement of eicosanoids. 355-364 (1987).

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