10 commonly used methods for laboratory quality control and their applicable scopes

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1 Monitoring with reference materials 1.1 Quality control process The usual approach is for the laboratory to use appropriate certified reference materials or internal standard samples as monitoring samples. These monitoring samples are tested periodically or ad hoc, either as comparison samples or control samples, using the same procedures and methods as those applied to the actual samples. Once the testing is complete, the results are submitted to the relevant quality control personnel; alternatively, the testers can themselves include reference materials during the sample testing in order to verify the accuracy of the results. 1.2 Scope of application It can generally be used for: controlling the status of instruments, controlling the sample testing process, performing instrument comparisons within a laboratory, comparing personnel, comparing methods, as well as conducting comparisons between laboratories. This method is characterized by high reliability, but at a high cost. 2 Personnel Comparison 2.1 Quality Control Process: Laboratory technicians carry out testing on the same sample, using the same methods and on the same testing instruments, within a reasonable time frame. They compare the results obtained to determine the consistency of the technicians’ performance and assess the reliability of their operational skills. The laboratory conducts personnel comparisons, choosing comparison tasks with as complex testing processes as possible, especially those involving numerous manual operation steps. The operations of the inspectors should be independent of each other to avoid interference. Generally, laboratories conduct more frequent supervision of newly hired staff than of regular employees, and when comparing results, it is best to always use the values reported by experienced and reliable testers within the laboratory as a reference. 2.2 Scope of Application Internal personnel comparisons conducted in the laboratory are primarily aimed at assessing whether the testing personnel possess the skills and qualifications required to perform their duties or to take on new roles. Therefore, they are mainly used to evaluate the testing skills of newly recruited staff and those who have recently completed training, as well as to monitor the testing skills of staff already in position. 3 Comparison of Methods 3.1 Quality Control Process Method comparison involves comparative tests between different analytical methods; it entails the use of distinct testing methods by the same analyst on the same sample to measure the same parameter, with the aim of comparing the accuracy of the results and determining their comparability in order to verify the reliability of the methods. The objects of assessment in method comparison are testing methods, with the main purpose being to evaluate whether there are significant differences in the test results obtained using different methods. During comparison, the test results obtained using standard methods are usually used as reference values, and the results from other testing methods are compared against them. The differences in test results between the various methods should meet the specified evaluation criteria; otherwise, it indicates that the non-standard method is not suitable, or it requires further modification and optimization. 3.2 Scope of Application Method comparison is primarily used to examine the systematic errors existing among different testing methods and to monitor the validity of test results; it is also used for the validation of non-standard methods employed in the laboratory. The overall testing methods generally include sample pretreatment methods and instrumental methods; as long as the pretreatment methods differ, regardless of whether the instrumental methods are the same or not, it is classified as method comparison. However, if the sample pretreatment method is the same across different testing methods and only the testing instruments differ, it is generally classified as instrument comparison. 4 Instrument Comparison 4.1 Quality Control Process Instrument comparison refers to the practice where the same tester uses different instruments and equipment – whether of the same type or not – to test identical samples using the same testing methods, in order to compare the accuracy of the results obtained and determine the comparability of the instruments’ performance. The objects of assessment in instrument comparison are testing instruments, with the main purpose being to evaluate the performance differences among various testing instruments (such as sensitivity, precision, interference resistance, etc.), determine the degree of consistency of the test results, and identify any existing issues. The selected testing items and methods should be suitable for and fully reflect the performance of the instruments participating in the comparison. 4.2 Scope of Application Instrument calibration is typically used in laboratories to verify and control the performance of newly acquired or repaired instruments and equipment; it can also be employed to assess the degree of differences in the test results obtained from different instruments and equipment. When performing instrument calibration, it is particularly important to maintain consistency in all conditions other than those related to the instrument itself throughout the calibration process, to ensure that any differences in the results accurately reflect the performance of the instrument. 5 Retest of retained samples 5.1 Quality control process Retest of retained samples refers to the re-testing of the same sample at different times (or within reasonable time intervals). By comparing the consistency of the results obtained from these two tests, it is possible to determine whether there are any issues with the testing process, as well as to verify the reliability and stability of the test data. If the results of the two tests meet the evaluation requirements, it indicates that the laboratory’s testing capability for this parameter remains valid ； If it does not meet the requirements, the reasons should be analyzed, corrective actions taken, and previous test results reviewed if necessary. In fact, retesting retained samples can be considered a special type of internal laboratory comparison, that is, a comparison across different times. When conducting retesting on retained samples, attention should be paid to the stability of the performance parameters of the samples used; in other words, there should be sufficient data or expert evaluations indicating that the values assigned to the retained samples remain stable. 5.2 Scope of Application As a means of internal quality control, repeat testing of retained samples is mainly applicable to: samples with a certain amount of testing data or positive samples, samples whose test parameters are relatively stable, as well as in situations where it is necessary to monitor the characteristics of the retained samples or verify the reproducibility of the test results. Conducting retests on retained samples helps to monitor the ongoing stability of the test results for this project and to observe its trends over time ； It can also encourage inspectors to take each inspection task seriously, thereby improving their professional competence and technical skills. It should be noted, however, that retesting with retained samples can only control the reproducibility of the test results, and cannot determine whether there are systematic errors in those results. 6 Blank Test 6.1 Quality Control Process A blank test, also known as a blank experiment, is a process in which quantitative analysis is carried out using the same methods and procedures as those employed for analyzing the sample under test, but without adding the sample itself (in special cases, a blank sample that contains no components to be analyzed but has a matrix similar to that of the sample can be used instead), in order to obtain analytical results. The result obtained from the blank test is called the blank test value, or simply the blank value. The blank value generally reflects the background of the testing system, encompassing the combined effects on the sample caused by factors such as noise from the testing instruments, impurities in the reagents, as well as contamination from the environment and the testing process. It has a direct impact on the accuracy of the final test results, and can be deducted from the analysis results of the sample. This kind of deduction can effectively reduce systematic errors caused by reagent impurities or reagent interference. 6.2 Scope of Application By conducting blank tests, the laboratory can effectively evaluate and correct errors caused by impurities introduced by reagents, experimental water, equipment, and environmental factors ； On the other hand, while ensuring effective monitoring of blank values, it is also possible to identify the differences among different analysis methods and testers. Furthermore, conducting blank tests also enables the accurate evaluation of technical parameters such as the detection limit and quantitation limit of this testing method. 7 Repeat Testing 7.1 Quality Control Process Repeat testing, also known as repeatability testing or parallel sample testing, refers to two or more tests conducted under conditions of repeatability. Reproducibility conditions refer to testing conditions in which, within the same laboratory, the same tester uses the same equipment and follows the same testing methods to conduct independent tests on the same test subject over a short period of time. 7.2 Scope of Application Repeated testing can be widely used in laboratories to monitor and evaluate aspects such as the uniformity of sample preparation, the stability of testing equipment or instruments, the precision of testing methods, the technical proficiency of testers, and the analysis interval between duplicate samples. It should be noted that, depending on the concentration level of the component being tested, the factors that can have a significant impact on the testing precision may vary greatly. 8 Recovery Test 8.1 Quality Control Process The recovery test, also known as the \"spiked recovery test,\" involves adding a substance of known mass or concentration to the sample to be analyzed; it is then measured using a specified method. The results obtained are compared with the known mass or concentration, and calculations are performed to determine the percentage by which the analysis result for the substance exceeds the amount added. The percentage of this calculation is referred to as the method’s “spiked recovery rate” for that substance, abbreviated as “recovery rate”. Generally, the higher the recovery rate is, the greater the accuracy of the quantitative analysis results; therefore, the value of the recovery rate can be used to assess the accuracy of such results. 8.2 Scope of Application The recovery rate test is characterized by its simple operation and low cost; it can take into account errors caused by various factors, and thus plays a very important role in the daily quality control of testing laboratories. Its main areas of application include: various types of chemical analysis, such as the control of detection results for low levels of heavy metals and organic compounds in various products and materials, the verification of the accuracy and reliability of chemical testing methods, as well as the assessment of the effectiveness of sample preprocessing or instrumental measurements in chemical testing. 9 Verification of calibration curves 9.1 Overview The calibration curve is used to describe the quantitative relationship between the concentration or amount of the substance to be measured and the corresponding value or indication from the testing instrument. The calibration curves obtained by using standard solutions and performing simplified or exactly identical analytical procedures as those for normal samples are respectively referred to as standard curves and working curves. To ensure that the calibration curve maintains good precision and accuracy at all times, appropriate methods need to be employed for verification. To verify precision, three concentration points—low, medium, and high—are typically selected from the calibration curve for validation. The verification of accuracy is usually controlled by using the spiked recovery test method. 9.2 Scope of Application The calibration curve method is a commonly used approach in laboratory instrumental analysis, typically for samples where the concentration of the component to be analyzed varies significantly and the volume of samples is large. The precision and accuracy of the calibration curve used during the testing process are influenced by various factors such as the laboratory’s testing conditions, the response characteristics of the testing instruments, and the skill level of the personnel performing the tests. Regular verification can, on the one hand, assess the response performance of the instruments and the consistency of the operating procedures followed by the testers, and on the other hand, provide information on the stability of the standard solutions used for curve plotting. 10 Quality Control Charts 10.1 Quality Control Process To control the precision and accuracy of test results, it is usually necessary to use control samples continuously during the testing process for monitoring purposes. Statistical analysis is performed on the accumulated monitoring data; by calculating statistical measures such as averages, ranges, and standard deviations, and following the procedures for creating control charts, the center line, upper and lower control limits, as well as upper and lower auxiliary lines and upper and lower warning lines are determined, thereby producing a control chart for analysis. After determining, through the analysis control chart, that the measurement process is in a stable or controlled state, the analysis control chart can be converted into a control control chart, and the control data from routine measurements can be plotted on it to determine whether there is any systematic variation or trend. 10.2 Scope of Application Quality control charts are applicable in the following situations: ① When it is necessary to predict the range of variation in the output of a process ； ②When determining whether a process is in a statistically controlled state ； ③When analyzing whether the source of variation in the process is random or non-random ； ④When deciding how to carry out a quality improvement project, to prevent the occurrence of specific problems or to make fundamental changes to the process ； ⑤When it is necessary to control the current process, in order to detect problems when they arise and take corrective actions. Quality control charts are undoubtedly an important evaluation method in quality control activities. It should be noted, however, that the conclusions drawn from this method are based on the test data of other quality control samples; quality control is achieved through the statistical analysis of such quality control data. Therefore, compared to other quality control methods, it tends to function more as a tool for evaluating quality control data, which is what sets it apart from the other internal laboratory quality control methods.