Hematocrit measurements by electrical conductivity

Abstract

Method and apparatus for determining the hematocrit of blood samples from the electrical conductance of whole blood by means of a portable, simple to use instrument with a digital readout. A dip-electrode-probe and/or a "fingerstick"-probe, requiring one drop of blood, have been developed to provide an easy to use and clean electrode system. Electronic improvements further provide for uniformity of readings between observers, automatic operator alert when poor sample contact or insufficient battery power, an exponential solution to the Fricke, Curtis and Maxwell equations and further eliminates electrode polarization by providing a separate DC path to conduct any DC component in the sine-wave signal. A provision is also made to estimate the hemoglobin value from the measured hematocrit.

We claim: 
     
       1. An apparatus for displaying in digital form the volume percentage of red cells in a conducting fluid sample comprising: a. 2 spaced electrodes cooperating with said fluid sample;   b. a sinusoid alternating current source having its constant current output connected to said electrodes;   c. a variable gain amplifier having its input terminals connected to said electrodes;   d. a differential amplifier having its input terminals connected between the output of said variable gain amplifier and the constant voltage output of said alternating current source;   e. rectifying means and DC amplifier connected to the output of said differential amplifier;   f. a dynamic attenuating resistive network, whose output voltage approximates the volume percentage of red red cells in said fluid sample, driven by the output of said DC amplifier;   g. means operable to render said attenuating network output substantially independent of temperature variations of said fluid sample; and   h. digital display means driven by the output of said attenuating network to provide direct readout of either the volume percentage of red cells in said fluid sample or the hemoglobin estimate of said fluid sample.   
     
     
       2. The apparatus as in claim 1 wherein filtering means is provided to cause the input signal of said variable gain amplifier to be substantially free of the capacitive reactance component produced by said electrodes. 
     
     
       3. The apparatus as in claim 1 wherein a variable resistance is connected between said constant voltage output of said alternating current source and one input terminal of said differential amplifier, to achieve a null output of said differential amplifier when said fluid sample contains no red cells. 
     
     
       4. The apparatus as in claim 1 wherein said rendering means comprises a negative temperature coefficient thermistor mounted in close proximity to said fluid sample, driven by a dynamically varying DC voltage source, achieving a thermistor current jointly proportional to the temperature and the volume percentage of red cells in said fluid sample and amplifying means to convert the thermistor current to a corresponding DC voltage linearly added to the output of said attenuating network. 
     
     
       5. The apparatus as in claim 1 wherein said digital display means achieves the said hemoglobin estimate readout by reducing the said volume percentage of red cells readout by the ratio 3 to 1. 
     
     
       6. The apparatus as in claim 1 wherein said spaced electrodes comprise first and second conductive areas each having a cylindrical shape, wherein the inner surface of the first conductive area is exposed to said fluid sample and the outer surface of the second conductive area, centered within the first conductive area and of substantially less diameter, is exposed to said fluid sample, and a fixed volume of said fluid sample is confined above said conductive areas by a planar glass cover slip normal to the axis of the cylinders and below by the top surface of an insulator separating the first and second conductive areas. .Iadd. 7. An apparatus for determining a conductance characteristic of whole blood, the apparatus comprising: base means of insulating material;   a region defined by said base means for receiving and holding a liquid sample of whole blood, said region having a volume substantially corresponding to one drop of whole blood;   first and second electrodes positioned in spaced-apart relation in said base means with the exposed portions of said electrodes being entirely within the boundaries of said region to be completely covered by and submerged in a one-drop blood sample when same is received in said region, said electrodes being configured so that the path of a current produced by the electric voltage applied between said electrodes is completely contained within the one-drop blood sample received in said region; and   means for electrically connecting said first and second electrodes to an electric circuit capable of creating and measuring a current between said electrodes to enable a characteristic of whole blood to be determined. .Iaddend..Iadd. 8. The apparatus of claim 7 further comprising a coverslip for enclosing said liquid blood sample in said region. .Iaddend..Iadd. 9. The apparatus of claim 7 further comprising temperature sensing means associated with said base and means for electrically connecting said temperature sensing means to an electrical circuit for compensating for temperature variation in said liquid blood sample. .Iaddend..Iadd. 10. The apparatus of claim 7, wherein said insulating material is an epoxy plastic. .Iaddend..Iadd. 11. The apparatus of claim 7, wherein each of said electrodes are made from stainless steel. .Iaddend.