Presentation on theme: "Electrical Impedance and Colorimetric Measurements Joanna Ellis, MLS(ASCP)"— Presentation transcript:
Electrical Impedance and Colorimetric Measurements Joanna Ellis, MLS(ASCP)
Objectives Cite the electrical impedance principle of cell counting. Identify and interpret microcytic and macrocytic RBC histograms Define coincidence Identify the cell populations represented on a 3-part differential WBC histogram
History You CAN patent a hole Prior to the 1950s blood cell counts were performed by manual methods: – Hemacytometer blood counts – Spun hematocrits – Spectrophotometrically determined hemoglobins – Peripheral blood cell evaluation for all differentials In 1953, Wallace Coulter patented the Coulter Principle in which particles are counted in fluid passed through a hole. The incredulous attorneys who had told him “You can’t patent a hole” were proven wrong. Hematology automation has since grown to include additional principles such as optical light scatter and flow cytometry.
The Coulter Principle The poorly conductive blood cells are suspended in a conductive diluent (liquid). The diluent is passed through an electric field created between two electrodes. The liquid passes through a small aperture (hole). The passage of each particle through the aperture momentarily increases the impedance (resistance) of the electrical path between the electrodes. The increase in impedance creates a pulse that can be measured. The number of pulses = blood cell count The amplitude (height) of the pulse = Volume of cell
Electrical Impedance (The Coulter Principle) Sweep Flow: Steady stream of diluent that flows behind each aperture to prevent cells from re-entering the aperture
Counting Chambers Most common chambers using impedance: RBC/Platelet chamber WBC chamber RBC/Platelet Chamber WBC Chamber Differential Chamber Reticulocyte Channel
RBC and Platelet Histograms The black line represents normal cell distribution. The red line on the RBC histogram graphically represents a microcytic red cell population.
Bimodal Histogram Bimodal peak can be seen in situations such Cold agglutinin disease Post-transfusion Post-treatment of IDA
Coincidence Coincidence: Multiple cells passed through the aperture at once.
WBC Chamber Lysing agent In some analyzers the WBC count is directly measured by electric impedance after the red cells have been destroyed by a lysing agent. The lysing agent also shrinks the leukocyte cell membrane and cytoplasm; therefore, the WBC count represents the measure of the cell volume not native cell size. Colorimetric measurements are used to determine hemoglobin. Aperture in electric field WBCs Hgb released
Colorimetric Measurements Hemoglobin is often determined by a colorimetric method. – Imidazole Non-cyanide reagent with color change and read at 540nm Instruments – Abbott CELL-DYN Sapphire – Sodium Lauryl Sulfate Non-cyanide reagent with color change and read spectrophotometrically Instruments – Sysmex XT and XE – Lysing agent converts free hemoglobin to cyanmethemoglobin and read spectrophotometrically at 540nm. Instruments – Advia 120 – Some Beckman Coulters