Strategies Insight: Why Can Laboratory Assay Test Results Differ Between Laboratories?
Why Is Assay Testing Important?
Assay testing is one of the most commonly performed analyses in laboratories to determine the amount or concentration of a specific compound in a sample. This type of testing is widely used for pharmaceutical products, cosmetics, raw materials, and dietary supplements to ensure that the active ingredient content complies with established specifications.
When receiving test results from two different laboratories, it is not uncommon to observe differences in assay values. For example, a sample may show an assay result of 98.8% in one laboratory and 99.2% in another.
This situation often raises questions regarding the accuracy of the results and the validity of the analytical methods used. However, in many cases, such differences are normal as long as they remain within an acceptable measurement uncertainty range.
Table of Charts:
- How Is Assay Testing Performed?
- Factors Affecting Assay Test Results
- Why Can Results Differ Between Laboratories?
- Are Different Test Results Still Valid?
How Is Assay Testing Performed?
In general, assay testing aims to measure the amount of a target compound present in a sample. Various analytical techniques may be used, including titration, high-performance liquid chromatography (HPLC), gas chromatography (GC), UV-Visible spectrophotometry, and other analytical instruments.
The basic principle of assay testing is to compare the response generated by the sample with that of a standard or reference material with a known concentration. Through specific calculations, this response is then converted into an assay value.
To ensure that the results are accurate and reliable, every stage of the analytical process must be carefully controlled, from sampling and sample preparation to instrument measurement and data processing.
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Factors Affecting Assay Test Results
Laboratory test results are influenced by numerous factors. Even when the same analytical method is used, small variations can still occur.
Some of the factors that may affect assay results include:
- Sample Collection and Homogeneity
Non-homogeneous samples may produce different assay values even when they originate from the same product. Therefore, sampling is considered one of the largest sources of variability in laboratory testing.
- Sample Preparation
Weighing, dissolution, extraction, filtration, and dilution steps can all contribute to result variability. Even minor errors during sample preparation may affect the final measured concentration.
- Analytical Instruments
Each analytical instrument has unique performance characteristics. Differences in detector sensitivity, chromatographic column conditions, pump systems, wavelength settings, and calibration status can influence measurement results.
- Standards and Reagents
The quality of reference standards, the purity of chemicals, and reagent storage conditions can also affect analytical outcomes.
- Operator Factors
Although many procedures are documented in standard operating procedures, differences in analyst experience and laboratory techniques may still introduce some degree of variability into the results.
Why Can Results Differ Between Laboratories?
Differences in results between laboratories do not necessarily indicate that one laboratory has made an error. Each laboratory may use different instruments, reference standards, environmental conditions, or even different analytical methods that have all been properly validated.
In addition, every analytical result is associated with measurement uncertainty. Measurement uncertainty represents the range of values that can be statistically accepted for a reported result. For example, if a laboratory reports an assay result of 99.0% ± 1.0%, the true value of the sample is expected to fall within that range.
As a result, two laboratories may report slightly different values while still being scientifically consistent with one another.
Are Different Test Results Still Valid?
In many cases, the answer is yes. The validity of a result is not determined by whether two laboratories obtain exactly the same numerical value. Rather, it depends on whether the testing was conducted using validated methods, calibrated instruments, and an appropriate laboratory quality system.
If the observed difference remains within the measurement uncertainty limits and meets the acceptance criteria of the analytical method, the results from both laboratories can be considered valid.
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Conclusion
Differences in assay results between laboratories are a common phenomenon that can arise from various factors, including sampling, sample preparation, analytical instruments, reference materials, and the measurement uncertainty inherent in every analytical process. A difference in reported values does not automatically indicate an error or invalidate the results.
As long as testing is performed using validated methods, calibrated instruments, and an appropriate quality management system, the results remain reliable and can be confidently used for decision-making purposes.
Author: Jihan
Editor: Lina
REFERENCES
Separovic, L., Simabukuro, R. S., Couto, A. R., Bertanha, M. L. G., Dias, F. R. S., Sano, A. Y., Caffaro, A.M., Lourenço, F. R. (2023). Measurement Uncertainty and Conformity Assessment Applied to Drug and Medicine Analyses – A Review. Critical Reviews in Analytical Chemistry, 53(1), 123–138. https://doi.org/10.1080/10408347.2021.1940086
Taverniers, I., De Loose, M., & Van Bockstaele, E. (2004). Trends in quality in the analytical laboratory. II. analytical method validation and Quality Assurance. TrAC Trends in Analytical Chemistry, 23(8), 535–552. https://doi.org/10.1016/j.trac.2004.04.001
