The application of certified reference materials for clinical mass spectrometry

Abstract Background In the application process of clinical mass spectrometry, there are difficulties such as standardization, low degree of automation, complex instruments, and high requirements for operations, which will affect the accuracy and comparability of results. Reference materials are one of the major approaches to achieve measurement accuracy and metrological comparability. Methods Based on the problems of reference materials in clinical mass spectrometry, the precautions for the use of reference materials are summarized in the aspects of measurement method validation, calibrator usage, and quality control in this article. Additionally, combined with the previous experience of the author's laboratory, the operation mode and acceptance criteria of the new calibration solution lot replacement were formulated to ensure the continuous comparability of the measurement system. Conclusion Carefully understand and correctly regulate the use and management of reference materials to ensure the accuracy of test results, thereby improving the comparability and consistency of results between laboratories.


| The main problem when having a complete traceability chain
Due to the difficulty of obtaining human samples and the timeliness of diagnosis, clinical test aims to develop and apply on small samples, easy automation, 3,7 as well as focus on the application of supporting commercial reagents, such as new screenings and 25-hydroxyvitamin D 3 in mass spectrometry technology.
At the 2020 Clinical Mass Spectrometry Application Conference and Mass Spectrometry Reagent Exhibition, experts have pointed out that the 25-hydroxyvitamin D 3 screening in laboratory developed test (LDT) and the average coefficient of variation (CV%) of commercial reagents is 9.3% and 9.5%, respectively; the average CV% of LDT and commercial reagents of 25-hydroxyvitamin D 2 are 11.5% and 12.2%, respectively. It shows that the quality of current commercial reagents does not have substantial advantage compared with LDT. The reason could be that the calibrator indicators provided by the reagent manufacturers, such as the accuracy of 80%-120% and differences between lots of no more than 20%, are too wide to accept the standard, the preparation method of the calibration solution is inconsistent, and the IVD manufacturers' traceable certified reference materials or traceability are inconsistent, etc. 8,9

| The main problem when missing a complete traceability chain
Commercial reagents in clinical mass spectrometry technology have certain limitations in practical applications. 1 For example, CFDA shows that as of March 2020, only five types of kit products contain calibrators (Table 1). Obviously, with the increasing demand for personalized medicine and precision medicine, these kits cannot meet the testing needs. Therefore, clinical mass spectrometry laboratories often build their own detection methods based on the flexibility of mass spectrometry. First, since the supply of certified reference materials cannot fully meet the need of all aspect, 14 this has caused the laboratory' LDT method to be unable to carry out effective measurement method confirmation; Secondly, differences among different matrix from different calibration solutions, different sources of the calibrators lead to the lack of interoperability. In addition, the inability to obtain suitable quality control materials when carrying out indoor quality control results in the failure of reflecting system deviations during long-term quality control. The above factors lead to inaccurate results even when other aspects of the measurement method are implemented as required.

| PREC AUTI ON S FOR THE APPLI C ATI ON OF REFEREN CE MATERIAL S IN MA SS S PEC TROME TRY
Reference materials are widely applied in the confirmation, calibration, metrological traceability, quality control, etc., of the measurement methods by their manufacturers. In the field of clinical mass spectrometry, manufacturers and laboratories also involved in the application of reference materials. However, due to the lack of uniform standards, a lot of aspects still need to be paid attention to when in applications.

| Application of reference materials in the confirmation of measurement methods in clinical mass spectrometry
The

| Application of reference materials as calibrators in clinical mass spectrometry
At the beginning of each project, the laboratory combined with the "Clinical Application of Liquid Chromatography-Mass Spectrometry" 12 recommends, "when purchasing and using com- is compared with the theoretical value, and accept ±15% as acceptable. It is easy to cause overall deviation of the system due to large acceptance criteria. Since the calibration solution is also part of the measurement system, the measurement system changes when the calibration solution changes. Therefore, the author's laboratory has formulated the comparison of operation method and acceptance criteria after the lots change of calibration solution (Table 2). 1. Firstly, the slope a is recommended to be within the range of 0.9-1.1, which is when the total allowable error of the analyte is 30%, if the slope of the calibration solution meets the requirements when the lot number of the calibration solution is changed, the system offset will be <10%, and the overall detection result will be within 10% in subsequent applications.

| Application of reference materials as quality control materials in clinical mass spectrometry
In order to ensure the stable and reliable performance of the measurement method, the quality control methods that the laboratory can use include inter-laboratory comparison and intra-laboratory quality control methods, etc. , the other concentration points should be 85%-115%, 18 and the linear slope and intercept should be kept stable every day.

| Suggestions
At this stage, reference materials are receiving more and more atten-

CO N FLI C T S O F I NTE R E S T
The authors declared that they have no competing interests.

DATA AVA I L A B I L I T Y S TAT E M E N T
Data sharing is not applicable to this article as no new data were created or analyzed in this study.