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- What to consider when performing method comparisons on blood gas analyzers
When performing method comparisons, it is important to address a number of preparatory and preanalytical issues to ensure that comparisons solely reflect the analytical differences between the two methods in question.
This article provides preparatory and preanalytical checklists that can be used when comparing two or more blood gas analyzers.
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The purpose of method comparison is to determine the agreement between two or more methods or analyzers measuring the same analyte.
The experiment is preferably performed on split samples measured on both methods, and any difference found between the two methods should be interpreted as analytical difference. A method comparison is recommended whenever a new method or analyzer is introduced into a healthcare institution as part of a method validation.
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In the US, for instance, there is a legal requirement to perform method comparisons twice a year to validate this. As the purpose is to determine if there is an analytical difference between the two methods, it is extremely important to eliminate inconsistent contributions from the preanalytical phase, which by experience is a major source of error during method validation of blood gas analyzers.
Checklist with general preparatory considerations when performing method comparisons:.
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Prior to a method comparison it is necessary to determine the allowable difference or bias if the comparative method is a reference method on the analytes which is acceptable to the laboratory. In order to determine the allowable difference, it is necessary to take the known performance specifications of each of the two methods into account.
Performance specifications such as inaccuracy and imprecision are often determined by the manufacturer and should include questionable results from calibration solutions, electrodes, etc. Analytical differences caused by a matrix effect  should be identified. An example is the measurement of electrolytes on direct or indirect ISE.
It is also important to investigate whether the manufacturer has identified any interfering substances , which could influence the method performance. The aim and the requirements of the study should be defined prior to starting the test. A detailed description of the test is needed for all participants.
All personnel participating in the test should be familiar with methodologies, maintenance, etc. The sample material should reflect the patient population from which the routine samples will come from.
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Within the given patient population it is important to have a sufficient range of results, representing also the extremes.
If the samples are only used for method comparison, extended storage time is one way to gain values in the outer range. The sample material used for the test should reflect the material used for routine testing. It is preferable to analyze on split samples, if possible. If capillaries are used for neonatal testing, it is important that the samples are drawn at the same time and from the same site.
It is debatable what the adequate number of samples is, and it may vary due to the nature of the analyte. Prior to the test, it is important to ensure that the analyzers are in control by doing QC and calibrations according to the manufacturers' recommendations.
If the samples used for the method comparison are also used for patient evaluation, storage time before measurement should be according to the manufacturers' or the laboratory's procedures! As a blood sample is living material, storage between measuring on the two methods will change the values of most parameters and should, therefore, be minimized.
Preanalytical considerations The preanalytical issues listed in the table below are the most important considerations when performing blood gas analysis. If the samples used for the method comparison are also used for patient evaluation, all of the listed preanalytical considerations are important and even more variables may be considered. However, if the results are only used for method comparison, some of the issues are less important, e.
By far the most important preanalytical consideration in connection with a method comparison is to ensure uniform quality of the sample that is introduced into the two methods. For instance, minimizing storage time between the two measurements is critical. In order to avoid non-analytical errors, the best approach is to perform measurements on routine patient samples with the analyzers side by side.
Detailed description of preanalytical considerations when performing method comparisons:. Air bubbles will have an effect on the p O 2 results. Air bubbles must be removed prior to each measurement. Storage time 1 between the measurements will affect p O 2 and eventually p CO 2 and pH. The maximum time between measurements on two methods should be min. Avoid cooling a sample directly on ice or mixing it vigorously between measuring on the two methods. The maximum time between measurements on the two methods should be min.
Evaporation is another consequence of storage and can occur from open tubes or microcups. It is important to ensure an even distribution of anticoagulant prior to the first analysis. Storage temperature1 is less important when the sample is only used for a method comparison.
Metabolites c Glu, c Lac, and c tBil. Avoiding storage time 1 between measurements is one of the most essential considerations, especially for c Glu and c Lac.
Hemolysis affects c Glu and c Lac on some enzymatic measuring methods and should be avoided . Settled samples are the most common cause of errors on especially c tHb. Mixing the sample very thoroughly prior to the first measurement and between measurements is essential. As air bubbles affect the p O 2 result, the s O 2 will also be affected. The maximum storage time 1 between measurements on the two methods should be min.
Printed from acutecaretesting. January What to consider when performing method comparisons on blood gas analyzers. The following table is a checklist with general issues to consider in the preparatory phase of the method comparison. The next table is a more detailed description of preanalytical considerations for each of the parameter groups typically measured on blood gas analyzers.
Expected outcome Prior to a method comparison it is necessary to determine the allowable difference or bias if the comparative method is a reference method on the analytes which is acceptable to the laboratory.
Matrix effect Analytical differences caused by a matrix effect  should be identified. Test protocol The aim and the requirements of the study should be defined prior to starting the test. Storage of samples before measurements If the samples used for the method comparison are also used for patient evaluation, storage time before measurement should be according to the manufacturers' or the laboratory's procedures!
TABLE I Preanalytical considerations The preanalytical issues listed in the table below are the most important considerations when performing blood gas analysis. Air bubbles Air bubbles will have an effect on the p O 2 results. Evaporation Evaporation is another consequence of storage and can occur from open tubes or microcups.
Anticoagulant It is important to ensure an even distribution of anticoagulant prior to the first analysis. Metabolites c Glu, c Lac, and c tBil 9. Light Light degradation of bilirubin may affect the result . Mixing Settled samples are the most common cause of errors on especially c tHb. Air bubbles As air bubbles affect the p O 2 result, the s O 2 will also be affected.
What to consider when performing method comparisons on blood gas analyzers
Air bubbles must be removed prior to each measurement Storage time The maximum storage time 1 between measurements on the two methods should be min. Use at least 40 patient samples. Split the sample between the two analyzers. Alternate the sample sequence between the two methods. Perform the test over five operating days. Analyze each patient sample in duplicate on both methods. This will help validate patient samples outside normal reference values.
After the comparison, NCCLS recommends that you analyze the data for outliers and plot the data in a scatter plot and a bias plot. The data analysis can be performed by using a statistical tool, such as the EP evaluator. All documentation from the method comparison needs to be saved for the regulatory inspection and presented during the site inspection.
Other recommendations are to…. Remember always to expel a few drops of blood from the blood gas syringe prior to the measurement and to wipe off the inlet to avoid contamination of the successive sample.
When mixing, small air bubbles may develop. Be careful not to inject or aspirate them into the analyzer. If you are comparing an injection analyzer to an aspirating analyzer, use the injection analyzer first to avoid air bubbles contaminating the sample. Conclusion The objective of this article was to provide checklists with preparatory and preanalytical issues to be considered to ensure that a method comparison only reflects the analytical difference between two or more methods.
Misinterpretation of the results caused by preanalytical errors may lead to a lot of extra work, or, in the worst case, erroneous acceptance of a failing analyzer. Evaluation of matrix effects. Interference testing in clinical chemistry. Villanova, Pa. Method comparison and bias estimation using patient samples. Young DS.
Effects of preanalytical variables on clinical laboratory tests. Skurup A. Guide to evaluating bilirubin measurements - designing the optimal test. Radiometer Publication Bulletin no.
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