Analisis tamoksifen, 4-hidroksitamoksifen dan endoksifen dalam dried blood spot pasien kanker payudara menggunakan kromatografi cair kinerja ultra tinggi-tandem spektrometri massa = Analysis of tamoxifen, 4-hydroxytamoxifen and endoxifen in dried blood spot of breast cancer patients by ultra high performance liquid chromatography tandem mass spectrometry