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Principles of Biochemistry and Clinical Chemistry Tests

Understanding the Measurement and Calibration Process

Introduction to Clinical Chemistry Tests

  • Biochemistry and clinical chemistry tests measure concentrations or activities of biological substances called analytes in various body fluids.
  • Common body fluids include whole blood, plasma, serum, urine, and cerebrospinal fluid.
  • These tests provide diagnostic and clinical meanings for the values obtained from the measurements.
  • The quantitation of routine chemistry analytes is typically based on photometry or potentiometry.

Measurement Principles in Clinical Chemistry

  • Measurement of light using photometry or spectrophotometry is one principle used for quantitation of analytes.
  • Measurement of electrochemical potential, also known as potentiometry, is another principle used.
  • The amount of light or electrical voltage is predictably related to the amount of analyte in the solution.
  • Calibrators are solutions of known concentration and establish the relationship between the signal and analyte concentration.

Calibration in Biochemistry Analysis

  • Calibration is the process of creating a standard curve using samples with known values.
  • This curve is used for analyzing unknown samples and determining their concentration.
  • Calibrators have a specified value traceable to the standard units, ensuring accuracy.
  • Both single point and multi-point calibrations can be used based on the assay requirements.

Quality Controls in Clinical Chemistry

  • Quality controls (QCs) are patient-like materials with known analyte concentrations.
  • QCs should be tested using the same procedure as patient samples to ensure the accuracy of the analytical system.
  • The absorbance values of QCs should lie within a specified range determined by the laboratory.
  • Regular calibration and QC testing ensure the analytical system is functioning properly.

Endpoint Reaction in Biochemistry Analysis

  • Endpoint reactions are used to estimate analytes that are completely consumed in the chemical reaction.
  • The absorbance increases over time until it reaches a stable value, which marks the endpoint of the reaction.
  • Absorbance is proportional to the analyte concentration and is measured at a specific wavelength.
  • Glucose measurement using the glucose oxidase or peroxidase method is an example of an endpoint reaction.

Fixed Time Method in Biochemistry Analysis

  • Fixed time methods assess the difference in absorbance between an initial and final value during a specified time interval.
  • These tests have a pre-incubation period to remove substances interfering with the reagent system.
  • The difference in absorbance is used to calculate the analyte concentration.
  • Creatinine measurement using the Jaffe's method is an example of a fixed time method.

Kinetic Method in Biochemistry Analysis

  • Kinetic methods measure the difference in absorbance between two points over a specified time period.
  • A constant amount of product is assumed to be produced during this time period.
  • The difference in absorbance is used to calculate the analyte concentration.
  • SGOT measurement using the FCC method without pyridoxal phosphate is an example of a kinetic method.

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