Carlsson H, Hjorton K, Abujrais S, et al. - This study was intended to present a technique for the determination of hydroxychloroquine (HCQ) in human blood using liquid chromatography-high-resolution mass spectrometry (LC-HRMS) and compare the suitability of the three sample matrices. A technique for the determination of HCQ in human blood using LC-HRMS was established, validated, and used for the determination of HCQ levels in whole blood, serum, and plasma from 26 SLE patients. Researchers examined the reproducibility of the method, in the three matrices, applying quality control samples and repeated preparations and measurements of patient samples. For HCQ measurement in serum, the performance of the developed procedure was further analyzed by comparison with two previously reported extraction methods. They developed and validated a LC-HRMS method for the measurement of HCQ in human blood. It was shown that whole blood is superior sample matrix in terms of sample reproducibility. Therefore, for HCQ analysis, whole blood samples should be applied when patients are monitored for HCQ treatment impacts. It was indicated that the assay is in clinical use to monitor levels of HCQ in patients.