In the world of healthcare, even the slightest error can lead to unreliable patient results. One often overlooked sources of these inaccuracies is sample Carryover.
What is Carryover?
Carryover is defined as the unintentional transfer of analytes between test samples. When carryover occurs, this can lead to false-positive or incorrect results ultimately compromising the integrity of patient care.
Verification for Carryover is performed before a method can be used for routine testing to detect and quantify any contamination between samples, addressing any possible risks before impacting final results. Verification of carryover is important to detect particularly in instruments with reusable parts such as probes or cuvettes.
Carryover Experiment Design
Following Clinical & Laboratory Standards Institute (CLSI) EP10’s recommendation to assess carryover, the minimal experiment design includes:
- Two samples at different concentrations - high (H) and low (L)
- Three replicates
Carryover Experiment Procedure
Step 1 - Define the Acceptance Criteria
The first step in any carryover verification experiment is to establish clear acceptance criteria. This limit will signal whether the carryover result is significant before fully implementing the testing method.
- Manufacturer’s specified limits: Often manufacturers will provide carryover limits that are considered acceptable.
- Laboratory-defined limits: If the manufacturer’s limits are unavailable, laboratories can define their own limits suited to their needs.
Step 2 - Prepare the Samples the be Used
The following samples are needed for a carry-over experiment:
- High Sample (H): These should be samples near the upper limit of the Analytical Measurement Range (AMR).
- Low Sample (L): These should be placed near a clinical decision point at the lower end of the AMR. The goal is to check whether residual high-analyte concentrations can affect the low-concentration sample results.
Step 3 - Conduct the Experiment
The next step is to run the samples in a recommended sequence to maximize the potential for detecting carryover: H1-H2-H3-L1-L2-L3
Running the high-concentration samples in sequence ensures that the instrument is thoroughly exposed to high analyte concentrations. Low samples are then immediately tested to compare any possible carryover.
Step 4 - Calculate the Amount of Carryover
Calculate the difference between samples L1 and L3 to determine the variation in low results using the formula:
Compute for Carryover using the formula:
Step 5 - Determine Carryover Acceptability
If Carryover % is equal to or less than the manufacturer’s claim or laboratory’s allowable difference, then the experiment is considered as passed.
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Resources
Clinical and Laboratory Standards Institute. (2022). EP10-A3: Evaluation of precision performance of clinical chemistry devices (3rd ed.). Clinical and Laboratory Standards Institute. https://clsi.org/standards/products/ep10/
Clinical and Laboratory Standards Institute. (2022). EP15-A3: User verification of performance for precision and bias (3rd ed.). Clinical and Laboratory Standards Institute. https://clsi.org/standards/products/ep15/
Westgard, James. (2020). Basic Method Validation, 4th Edition. Wisconsin, Westgard QC, Inc.