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Calibration

Calibration is the foundation of all clinical laboratory testing that insures the accurate reporting of patient results. Calibration is the process that links the analytical signal with the concentration of analyte present in serum, urine or other body fluid.

Calibration uses a series of at least five calibrators containing known concentrations of an analyte. Before beginning calibration a medical laboratory scientist programs the instrument with the concentration of each analyte according to the information provided on the package insert supplied with the calibrator kit. The instrument then measures the calibrator and adjusts the signal to match the given values. Depending on the method, this signal might be potentiometric, photometric, fluorometric, chemiluminescent, nephelometric or turbidimetric. Plotting signal on the Y-axis versus analyte concentration on the X-axis creates a calibration curve. The purpose of a calibration curve is to establish the relationship between the concentration of an analyte and the magnitude of the signal given by the measuring device. The relationship can be linear or nonlinear.

A calibration curve shows the signal rising linearly with increasing concentration of analyte from the limit of detection (LOD) to the limit of linearity (LOL). This is the analytical measurement range (AMR). Beyond LOL, the line is no longer linear and the signal is no longer linearly related to analyte concentration. Once the calibration curve is established, the signal from a patient sample can be compared to the calibration curve to determine the concentration of analyte in the patient sample.

Calibration materials should have the same matrix as patient samples. Serum based calibrators should be used when testing patient plasma or serum, while urine based calibrators should be used for urine chemistry tests. Calibrators should be traceable to standard reference materials to insure comparable and accurate results.

Calibration should be repeated when:

  • The instrument or reagent manufacturer’s instructions says it is necessary
  • Every time a reagent lot is changed
  • Whenever quality control results show a systematic bias
  • After major instrument maintenance that can cause shifts in quality control
  • When reagents have poor stability

Instruments or reagents that require frequent calibration have a much higher cost per test because of the added labor and expense of calibration kits.

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