Some form of electronic hematology analyzer is found at the center of most clinical laboratories. A blood cell count is usually the first screening test used to assess general health, rule out anemia or infection, and monitor cancer patients during the course of chemotherapy. It is difficult to imagine medical care without this workhorse device, and surprising to learn its origins.
Early testing of sonar, developed to detect submarines during World War II, revealed odd underwater echoes. To determine the cause, an electrical engineer was tasked with counting plankton in a large number of seawater samples. He devised a dual chamber container whose two sides were separated by a thin membrane fashioned from a cellophane cigarette carton wrapper. Having set up an ohm meter to measure impedance across the orifice, he partially drained one chamber. As seawater flowed through the orifice, each plankton cell caused a momentary drop in impedance. In this way, the concentration of plankton particles could be measured. The odd echoes were attributed to plankton, the principle was patented, and the practice of hematology was revolutionized.
All modern hematology analyzers, whether they are top-of-the-line models or the simple hematology analyzers used in physician's office laboratories, depend upon electrical impedance to classify and count red and white blood cells based on their size. Improvements to the application of electrical impedance include pulse editing, which corrects for cells passing through the aperture at various angles or at the same instant.
Cell counters must be sophisticated in the complexities of liquid handling; their fluidics systems must deliver exquisitely precise volumes of sample, diluent, and reagents by means of tubing and valves. Early instruments measured volumes using a mercury manometer. Fluidics were improved in later models by introd