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Therapeutic Drug Monitoring (TDM) is a vital tool for personalised medicine, and External Quality Assessment (EQA) is the 'gold standard' for evaluating TDM laboratories' testing capabilities. Today, we will discuss the TDM interlaboratory quality assessment organised by the Ministry of Health's Clinical Laboratory Centre, focusing on interpreting its grouping principles and acceptance criteria.
I. What is TDM interlaboratory quality assessment?
Interlaboratory quality assessment involves multiple laboratories analyzing the same batch of samples and an independent organisation statistically evaluating the testing capabilities of each laboratory. For TDM, the core objectives include:
1
evaluating laboratory capability
assessing the accuracy and precision of a laboratory’s blood drug concentration measurements
2
identifying issues
Detecting systematic errors in testing methods, instruments, or procedures
3
Promoting standardization
Enhancing the comparability of test results across different laboratories.
According to the 'Expert Consensus on Standard Operating Procedures for Therapeutic Drug Monitoring', TDM laboratories should 'participate in quality assessment activities organized by TDM professional organisations or government-authorized quality management agencies, and meet the established requirements'.
II.Grouping Principles: Why categorize samples?
The grouping principles for TDM interlaboratory proficiency testing are based on the 'three commonalities' principle: 'same method, same instrument, same reagents'. This is because:
Methodological differences: Different analytical methods (e.g. immunoassay, HPLC and LC-MS/MS) may produce different results for the same drug.
Instrument differences: Analytical instruments of different brands and models have different performance parameters.
Reagent differences: Reagents from different manufacturers may differ in antibody specificity and the assigned values of calibration standards.
Why group laboratories in this way?
Only by grouping laboratories that use the same testing system can their testing capabilities be objectively evaluated, thereby avoiding 'false non-conformities' caused by methodological differences.
III. Passing criteria: How to determine 'acceptability'?
The passing criteria for the TDM interlaboratory quality assessment usually cover the following areas:
1. Overall evaluation criteria
According to the National Health Commission's National Center for Clinical Laboratories, the passing criteria for TDM interlaboratory quality assessment are as follows:
2. Reference for Target Concentrations and Acceptable Ranges of Commonly Used Drugs
The table below lists the commonly used acceptable limits for interlaboratory proficiency testing of common TDM drugs in 2026:
Category
Drug
Acceptable Bias in Interlaboratory Proficiency Testing
Antiepileptics
Phenytoin
Target value ±20%
Valproic acid
Carbamazepine
Immunosuppressants
Tacrolimus
Cyclosporine A
Sirolimus
Cardiovascular drugs
Digoxin
Target value ±20% or ±0.2 μg/L (whichever is larger)
Antibiotics
Vancomycin
Target value ±25%
Antiasthmatics
Theophylline
Note: The acceptable range specified is subject to the interlaboratory proficiency testing programme guidelines for that year.
IV. Explanation of Scoring Calculation Rules
'Target value'-based scoring method
1. Exclude outliers: Remove results that fall outside the range of the mean ± three times the standard deviation within each group.
2. Calculate the target value. Take the median or robust mean of the remaining results.
3. Calculate the deviation: Calculate the relative deviation between the laboratory result and the target value.
4. Determine acceptability: If the relative deviation falls within the predefined acceptable range, the sample is considered acceptable.
The scoring formula for each batch of interlaboratory quality assessment samples is as follows:
Score of sample = (Number of passing results / Total number of results) × 100
The interlaboratory quality assessment meet the requirement if the score is ≥80.
V. Common causes of dissatisfaction and recommendations for improvement
Factor 1: Calibration issues
Phenomenon: All sample results are systematically higher or lower than expected.
Recommendations for improvement:
-Check the expiry dates and storage conditions of the calibration standards.
- Recalibrate the instrument.
- Replace the batch of calibration standards.
Factor 2: Unnoticed quality control issues
Phenomenon: High variability in results with no discernible pattern
- Strictly enforce internal quality control protocols.
- Use quality control materials immediately after reconstitution.
- analyze trends in quality control data to identify issues promptly
Factor 3: Operational errors
Phenomenon: abnormal results in individual samples
- Check the accuracy of the sample dispenser.
- Standardize the sample handling process.
- Enhance staff training.
Factor 4: Reagent/method issues
Phenomenon: Overall deviation from the method-specific group.
- Verify batch-to-batch variations in reagents.
- Verify the parameters specified in the reagent instructions.
- Change the testing method if necessary.
Conclusion:
TDM interlaboratory proficiency testing reflects a laboratory’s true capabilities and measures the quality of personalised medication. To truly achieve “precise monitoring to ensure medication safety”, we must understand the principles of grouping, thoroughly grasp the acceptance criteria, and continuously improve quality.
Reference
1. 中国药理学会治疗药物监测研究专业委员会. 治疗药物监测工作规范专家共识（2019版）[J]. 中国医院用药评价与分析, 2019, 19(8): 897-899.
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