In recent years, the utilization of health care databases has been increasing worldwide. It is expected that Real World Data (RWD) will soon be effectively used for clinical research in Japan. On the other hand, database studies that use accumulated existing data such as electronic medical records, Diagnosis Procedure Combinations (DPCs), and health insurance claims, require extremely high loads of data preprocessing before statistical analysis is possible. So far, there is insufficient literature that describes the challenges of RWD preprocessing from an academic point of view. In this review paper, the challenges of database study are classified into three categories：（1）data content,（2）data structure, and（3）large-volume data handling. We then investigated existing preprocessing research and systematically introduced them. Most data preprocessing research targeted the improvement and reliability of the database itself through supplementing data contents required for each clinical research. There is very little research with the primary purpose of solving problems related to data structures and large-volume data processing. As the use of RWD for clinical research increases, the importance of the data preprocessing field will be recognized. In the future, we expect to see more research focused on RWD, which can enable the growth of clinical researches using RWD.

In recent years, the number of patients with non-tuberculosis mycobacteria (NTM) has rapidly increasing.According to the nationwide survey conducted in 2014, the number of patients with NTM was reported to increase 9.7 times compared to the survey in 1980. Among them, the patients with Mycobacterium avium complex (MAC) account for about 88.8% of them. It is the main cause of the rapid increase of NTM patients mainly in middle-aged and elderly woman. To treat patients with MAC, it is common to do chemotherapy over one year after the bacteria becomes negative. Among experts of NTM, it is recommended to do chemotherapy preventing generation of resistant bacteria by using clarithromycin (CAM) and rifampicin and ethambutol (EB) in combination. Meanwhile, a monotherapy of CAM and high-dose EB administration over a long period are not currently recommended due to side effects. However, it has not been clarified so far how many such drug prescriptions had existed. Therefore, in this study, we investigated the actual drug prescription of 571 patients who were presumed to be NTM in health insurance data collected from 2015 to 2016. As a result, about 5.1% (29 cases) of CAM monotherapy and 4.4% (15 cases) of EB high-dose prescription over 3 months were observed. In general, because NTM is a case where a long-term antibiotic treatment is required, it increases the possibility of any disadvantages exerting on patients. Hence, we consider it is an important and urgent matter to inform the correct information widely to clinical workers and sites.