Producing Accurate Results Starts with Effective Calibration

4 Nov 2014
Sonia Nicholas
Managing Editor and Clinical Lead

By Sarah Kee, BSc
QC Scientific Consultant, Randox Laboratories

Do you see instrument calibration as a vital first step in producing accurate and reliable patient results, or just one more job in the daily routine of clinical laboratory testing?

Effective instrument calibration should be viewed as the foundation of all laboratory patient testing. It is the first step to producing reliable patient results and assuring instrument performance. As such it’s something that laboratories should be paying closer attention to.

Calibration is used to establish a reference point that assists instruments to produce accurate results. All instruments of measurement - whatever they measure - require calibration. Even everyday items, such as kitchen scales for food preparation, require calibration. Standard weights of known mass are placed on the scales and, if they display an inaccurate reading, they are adjusted until they read accurately.

The same principle applies to laboratory analyzers, but instead of using weight, labs run calibrators or ‘standards’ with a known concentration. During calibration, the lab programs the instrument with the concentration of each analyte according to the information provided on the kit insert or instructions for use supplied with the calibrator. The instrument then measures the calibrator and adjusts the reading to match the given values.

This process ensures the instrument is reading as accurately as possible. All subsequent patient and control samples analyzed will be calculated from the calibration, which is why this process is so fundamentally important in laboratory testing.

There are two vital components to ensuring accurate calibration: choosing the right calibration materials and knowing how frequently to recalibrate. Let’s look at each in turn.

To ensure a strong foundation for accurate patient results, choose a calibrator with:

• A similar matrix to the patient sample. Some labs use aqueous standards rather than serum calibrators. Serum is much more viscous than aqueous material and this difference in viscosity can cause problems for the instrument pipetting system, resulting in measurement inaccuracies. The color and translucency of the material are also important. Standards are often colorless, translucent materials unlike patient samples. Differences in translucency can cause problems, particularly with turbidimetric methods.

• Accurately assigned values. Target values are assigned from data obtained from laboratory tests. The greater the number of laboratories used to assign values, the more statistically valid and therefore accurate the assigned values will be.

• A long shelf-life and multi-analytes. Multi-analyte calibrators enable labs to cover the vast majority of routine chemistry tests in one calibrator, reducing the need to use numerous single calibrators, saving the lab time and money. Choose calibrators that have a long shelf-life, thereby minimizing laboratory wastage.

• Traceability to international reference methods to ensure comparable and accurate test results.

The frequency of calibration will vary depending on the test and the instrument in use, but the following guidelines will help you determine what’s right for your test system:

• Follow the instrument or reagent manufacturer’s instructions for calibration. If not available, choose a frequency that is dictated by your internal QC frequency.

• Assess the required frequency of calibration when validating assays. Depending on the stability of the assay, it may be necessary to recalibrate more/less frequently.

• Re-calibrate every time a reagent batch is changed, unless you can demonstrate that changing reagent lot numbers does not adversely affect control values and patient results.

• Re-calibrate when your QC results are showing a systematic bias - calibration can eliminate trends or small analytical bias.

• Re-calibrate after major instrument maintenance, such as lamp changes, which can cause shifts in QC values.

By following these guidelines you can be assured your laboratory equipment is properly calibrated and the foundations for accurate patient results are in place.

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