Comparison of mass spectrometry and radioimmunoassay to measure medroxyprogesterone acetate in patients with endometrial cancer.

Serum medroxyprogesterone acetate (MPA) was measured by radioimmunoassay (RIA) and gas chromatography-mass spectrometry (GC-MS) in patients with endometrial cancer. Samples were obtained 3, 6 and 24 h after the oral administration of 100 or 200 mg MPA once a day. The levels obtained by GC-MS were lower (median 16-29%) than those obtained by RIA, which is probably attributable to the presence of metabolites interfering with the RIA. Two commercial MPA formulations gave different MPA serum levels by both RIA and GC-MS. The levels obtained by GC-MS were so low that frequently only partial saturation of the endometrial progesterone receptor may be achieved which may explain why high oral doses are needed to produce optimum therapeutic response.