Have you ever wondered why a laboratory runs a sample more than once? In healthcare, repetition is not redundancy - it’s assurance. Each rerun not only tests the quality of the sample itself, but the reliability of the method. This is also referred to as Precision.
What is Precision?
Precision refers to the closeness of results obtained from multiple testing under the same conditions. In simple terms, precision describes how consistent a test result is when repeated. In an MV study, verifying precision is critical because this confirms that a method is stable and reproducible - regardless of who performs the test, when it is done, or which instrument is being used.
To assess precision, a Replication Experiment is performed by measuring the same sample under similar conditions. Variation in results is then assessed to reflect precision. This variation arises from combined effect of various random errors.
What is Random Error? Random Error refers to unpredictable fluctuations that naturally occur in any test. These may arise from the following:
- Temperature Fluctuations
- Differences in personnel technique
- Procedure or timing differences
While random errors cannot be avoided, these can be minimized by catching the source early and implementing corrective action to mitigate the risk.
In a Method Verification (MV) process, different types of precision are evaluated: Within-Run and Between-Run Precision.
Within-Run Precision
Other name: Repeatability
Within-Run Precision refers to the variation in test results when multiple measurements of the same sample are taken in a single run or a single assay. This assesses the consistency of results within the same run. Within-Run Precision is expected to produce minimal variations but only reflects the short-term consistency of the method.
Between-Run Precision
Other name: Intermediate Precision, Between-Day Precision, Total Precision
Between-Run Precision refers to the variation in test results when the same sample is tested across different runs. This incorporates variation in laboratory settings which may involve different operators, reagent lots, temperature fluctuations, helping to assess the consistency of the method over time.
Between-Run Precision expresses random errors within a laboratory over a longer period frequently lasting up to several months.
5-Step Precision Verification
Below are five key steps to following when conducting an MV study for Precision:
Step 1 - Define Acceptance Criteria
Start with creating a verification plan to avoid any preventable errors along the way.
- Duration: 5 days
- Samples: at least 2 levels of analyte concentration low and high
- Replication Scheme: space replications out to better represent normal analytical variation
- Total Measurements: minimum of 25 measurements
Step 2 - Conduct Testing
Evaluate the method under the laboratory’s routine testing conditions to catch any potential random errors. Ensure testing is performed with routine calibration, quality control, and maintenance procedures in place.
Step 3 - Record and Organize Results
Collect results in a table with the following recommended structure from Cualia:
Step 4 - Calculate Precision Estimates
Precision is estimated through statistical values such as Mean, SD and CV% for both Within-Run and Between-Run Precision.
Use the following formulas below to obtain precision estimates:
Mean:
SD:
CV%:
Step 5 - Evaluate Acceptability of Precision
If the calculated SD or CV% from Step 4 falls below the manufacturer claims for SD or CV% - the result is considered as passed.
Low CV means the spread in results is small in relation with the mean. In other words, the repeated measurements are close to each other reflecting good precision.
Precision for Qualitative Methods
Qualitative methods may not provide numerical concentrations, but verifying precision is equally as crucial before routine testing. Acceptance Criteria for Days, Replicates and Total Measurements is the same as quantitative testing.
Step 1 - Select Samples for Testing
Samples selected for testing should be just below and just above the method’s cut-off value. Prepare samples with the following concentrations at medical decision points 20% above and 20% below the cutoff concentration:
Sample Concentration | Expected Result |
High Negative (C5) | concentration where 95% negative ; 5% positive |
Low Positive (C95) | concentration where 5% negative ; 95% are positive |
- High Negative = 0.80-0.99 mg/dL
- Low Positive = 1.01-1.20 mg/dL
Step 2 - Conduct Testing in Replicates
Test the samples in replicates under routine conditions. Record data obtained in a table as recommended:
Step 3 - Analyze Data for Acceptability
Results for both C5 and C95 samples should be in at least 95% agreement. In other words, 19/20 should be correct for a method to be considered precise.
Ready to experience Precision made easy?
📄 Generated Tables
With Cualia, tables for results are tailored to your preferred acceptance criteria - whether it’s days, samples, replicates and more!
📏 Automated Calculations and Acceptability
All you need to do is enter results in - Cualia instantly handles the calculations and determines whether your MV has passed!
Try out Precision with Cualia here!
References
Westgard, James. (2020). Basic Method Validation, 4th Edition. Wisconsin, Westgard QC, Inc.
Westgard, James. (2008). The Replication Experiment. Basic Method Validation, 1st Edition. https://www.westgard.com/lesson22.htm
Clinical and Laboratory Standards Institute. (2008). CLSI EP12-A2: User Protocol for Evaluation of Qualitative Test Performance; Approved Guideline-Second Edition. Wayne, PA.
Clinical and Laboratory Standards Institute. (2005). CLSI EP15-A2: User Verification of Performance for Precision and Trueness; Approved Guideline—Second Edition. Wayne, PA.
Rebane, R. (2016). Repeatability, Intermediate Precision and Reproducibility. LC-MS Method Validation. https://sisu.ut.ee/lcms_method_validation/41-precision-trueness-accuracy
Naidis, I. , Turpeinen, S. (2009). Guidance for the Validation of Analytical Methodology and Calibration of Equipment used for Testing of Illicit Drugs in Seized Materials and Biological Specimens. New York, United Nations Publications.
National Association of Testing Authorities. (2012, June). Guidelines for the Validation and Verification of Quantitative and Qualitative Test Methods. Australia.