Mirror Images: Why We Need Them (and How to Get Them)
by Kim Przekop, MBA MLS(ASCP)CM, author, educator, quality champion
Commutability– What is it? Should I be concerned about it?
In our scientific laboratory world, we strive to know which assays we can trust. Any assay labeled gold standard or reference method gives us confidence. These methods are robust, tried and true, with proven accuracy and precision. Assays that are based on reference methods also give us confidence. Without that trust, we don’t know for sure if we are reporting accurate results. The same logic holds true for QC (quality control) samples. If you have ever wondered why a QC sample shows a pattern that is not mirrored in the patient samples, commutability may be the issue. In a report by Franzini and Ceriotti 82% of quality control materials were noncommutable for ten common analytes.1
Commutability is the term we use to describe how a material (QC sample) behaves in relation to a native sample, in this case, a patient’s blood sample or body fluid sample. The closer the QC sample performs to the native sample, the more commutable it is. Interpreting QC results from uncommutable QC materials is risky since we are not sure if the QC material mirror our patients’ samples. Creating highly commutable QC samples is no small feat- there are intricacies in the molecules we want to measure, and the matrix chosen to hold the molecules must not interfere with the testing.
Manufacturers may or may not state that their quality control materials are commutable to native samples. I checked a major chemistry QC supplier and the package insert I reviewed did not mention commutability (there were dozens of analytes in this material). The point of knowing commutability is to trust that the controls will react as if they were patients. If they don’t, why are we running controls in the first place?
There are standards we can refer to when reviewing our own QC materials for commutability. CLSI (Clinical & Laboratory Standards Institute) has three guidelines:
- C24-ED4 “Statistical Quality Control for Quantitative Measurement Procedures”
- EP30-A “Characterization and Qualification of Commutable Reference Materials for Laboratory Medicine”
- EP14-A3 “Evaluation of Matrix Effects”
To compare your QC material to patient samples, experiments must be performed and statistical calculations applied. However, simple visual graphs help to identify those QC samples that are not near the patient sample population. Don’t forget to ask the manufacturer if their QC materials are commutable.
- Franzini C, Ceriotti F. Impact of reference materials on accuracy in clinical chemistry. Clin Biochem 1998;31:449-57.
- CLSI Guideline C24-ED4 “Statistical Quality Control for Quantitative Measurement Procedures” 2017.
- CLSI Guideline EP30-A “Characterization and Qualification of Commutable Reference Materials for Laboratory Medicine” 2010.
- CLSI Guideline EP14-A3 “Evaluation of Matrix Effects” 2014.
- Vesper H, Miller WG, Myers GL. Reference materials and commutability. Clin Biochem Rev 2007;28:139-148
Meet me at AACC in San Diego! I have a poster with Zoe Brooks, George Sweeney, and John Hopkins “Managing Risk with Acceptable Risk Criteria and Mathematically-Optimized Risk Evaluation”. To book Zoe’s Brown Bag on “Acceptability of Patient Risk at Monthly/Regular QC Review” go to: