Blood Control Standard

This invention relates to a blood control standard. More particularly, this invention is concerned with a stable blood control standard and method for the quality control of the measurement of blood pH and gases in the clinical laboratory.

Blood serum is a complex biological fluid containing various components of substantial physiological importance. In the normal or average healthy person the concentrations of these components fall within certain reasonably well-defined limits. When any of these components is found upon analysis to be outside of its normal range, a pathological condition may be indicated which requires medical attention.

The determination of blood gases, electrolytes and the acid-base balance is an important aspect of this blood analysis. Thus, abnormalities in pulmonary function may be indicated by the concentrations of oxygen and carbon dioxide in the blood. The importance of oxygen transport by the blood in respiration and the respiratory regulation of cation-anion balance is well known. By the process of homeostasis, the body tends to preserve a state of equilibrium which is manifested in three ways as applied to water and electrolyte metabolism:

1. Preservation of pH or acid-base balance.

2. Preservation of ionic composition.

3. Preservation of osmolality.

The buffer systems of the intra- and extra-cellular spaces preserve the pH within narrow limits. Of the various physiological buffers, only the bicarbonate system contains a component, carbon dioxide, which is volatile at body temperatures and, therefore, can be regulated by the lungs. Thus, an analysis of the bicarbonate buffer system enables a direct estimation of the respiratory acid-base balance. Dissolved carbon dioxide is present in plasma according to the following equation:

carbon dioxide also is present in the red cells in the dissolved state, or combined with hemoglobin to form carbamino-CO.sub.2, or in a complex based on the action of the enzyme, carbonic anhydrase, which is present in the erythrocytes.

The interrelation between total CO.sub.2, bicarbonate, carbonic acid, pCO.sub.2 and pH in blood can be shown by the well-known Henderson-Hasselbalch equation: ##EQU1## in which PH is the pH measured in arterial blood

PK is the log of the reciprocal of the dissociation constant of the bicarbonate system

Hco.sub.3 .sup.- is the true bicarbonate concentration in mmol/liter

H.sub.2 co.sub.3 is the carbonic acid concentration in mmol/liter

The values for pH, total CO.sub.2 and pCO.sub.2 can be determined experimentally and their mathematical relationships can then be illustrated by application of the Henderson-Hasselbalch equation.

Various instruments have been developed for the determination of the parameters which comprise the blood gases and acid-base balance. These instruments generally are capable of measuring blood pH, pCO.sub.2 and pO.sub.2. Illustrative of such instruments are those described in U.S. Pat. Nos. 3,658,478 and 3,652,843. Instruments of this type are commercially available from Instrumentation Laboratory Inc. as the IL 113 pH/Blood Gas Analyzer. Another such instrument is the Corning pH/Blood Gas Analyzer described in U.S. Pat. No. 3,763,422. Still another commercially available instrument for measuring blood pH, pO.sub.2 and pCO.sub.2 is the BMS3 Mk2 Blood Micro System and Digital Acid-Base Analyzer from The London Company, Radiometer A/S. Instruments of the latter type are described in U.S. Pat. Nos. 3,654,445 and 3,874,850.

The use of the foregoing and other such instruments for the determination of blood gases in the clinical laboratory presents unique problems of quality control. The instruments must, of course, be properly calibrated in the first instance. Calibration of such instruments can be accomplished by metering standardized gases through the instrument or by application of a calibration fluid such as an aqueous bicarbonate solution as described, for example, in U.S. Pat. No. 3,681,255. However, calibration of the instrument is only one of the problems of blood gas clinical instrumentation. To ensure high quality patient care, the instrumentation system must be tested frequently and laboratory personnel must be promptly responsive to any system malfunction. For the latter purpose, control standards have been developed which can be applied to the instrument periodically at predetermined intervals to ensure adequate quality control. One such type of control standard is a freeze dried human serum which is reconstituted with a liquid diluent prior to use. Examples of this type of control standard are described in U.S. Pat. Nos. 3,466,249 and 3,629,142. These materials, however, are not fully useful for control purposes when the blood gas analysis includes determination of oxygen because the reconstituted serum does not contain the desired level of dissolved oxygen. Rather, they are adapted to control of other biological values such as are determined on a Technicon SMA/12 Auto Analyzer.

Another such control standard contains blood which is reconstituted by the addition of a liquid containing fluoride and an iodoacetate or a fluoroacetate to stabilize the blood as disclosed in U.S. Pat. No. 3,859,049. However, this material similarily does not provide the desired levels of oxygen for the control of instruments which include the determination of blood oxygen.

Although it is known that blood cells van be stabilized for various purposes such as by heat treatment or by various chemical fixatives as described in U.S. Pat. Nos. 3,574,137 and 3,640,896 or by thoroughly washing to separate from other blood constituents as described in U.S. Pat. No. 3,558,522, these materials are useful only for blood counting and similar such purposes and do not provide the necessary conditions for the control of blood pH and gases.

It is also known that blood cells can be stabilized for hemagglutination purposes or to serve as affinity absorbents for antigens or antibodies by rigorous treatment of the cells with aldehydes. Such treatment for use in diagnostic tests is described in U.S. Pat. Nos. 3,096,250, 3,426,123, 3,708,572, 3,714,345, and 3,925,541 and U.S. application Ser. No. 362,308, filed May 21, 1973, now U.S. Pat. No. 3,914,400. Again, these materials are useful for the disclosed diagnostic purposes but do not provide the herein-described conditions for control of blood pH and gases.

In accordance with the present invention, a stable blood control standard and method is provided for the quality control of the measurement of blood pH and gases in the clinical laboratory. The blood control standard comprises a sealed receptacle containing specially treated red cells and a gaseous head space of at least equal to about the volume of the treated blood cells. This special treatment comprises thorough washing and separating of the red cells from the plasma components and mild treatment with aldehyde and retention in a buffered solution. The head space consists of a gas or mixture of gases comprising from 0 to 15% CO.sub.2, 0 to 25% O.sub.2 and the balance substantially N.sub.2 and/or inert gas.

The principal fractions of blood are the plasma, red cells or erythrocytes, platelets and white cells. In the average adult human body which contains about 5 liters of blood, the red cells represent about 2.2 liters. In accordance with the present invention, these red cells are first separated from the other blood components and then treated as defined herein.

In order to prevent clotting in storage, whole blood is normally collected in an anticoagulant solution such as heparin, EDTA, ACD or CPD solution. Whole blood collected in this manner can normally be stored for up to 21 to 28 days without seriously affecting the viability of the residual red cells. Since the end of World War II, the collection and storage of blood has undergone considerable change, and various advances in the separation of the blood components have given rise to the practice of modern blood component therapy. By these procedures, the red cells can be separated from whole blood by sedimentation and centrifugation. The separated red cells can be glycerolized and then stored in a frozen state for subsequent use. Similarly, the separated plasma can also be frozen and stored, or its separated fractions can be frozen and stored for latter use.

In accordance with the present invention, any of the above sources of red cells, fresh or outdated cells (cells stored in excess of 21 to 28 days) can be employed. These cells can be derived from human or other mammalian sources including, for example, equine, bovine, porcine, and sheep species.

In order to ensure complete separation from the other blood components, the red cells are sedimented or centrifuged and thoroughly washed. Sedimentation is facilitated by spinning in a conventional blood centrifuge. Centrifuges for such blood cell sedimentation are well-known, and a continuous flow type centrifuge such as is commercially available from the Haemonetics Corp. is preferred. Centrifuges of this type are described, for example, in U.S. Pat. No. 3,706,412. In this type of centrifuge, the bowl has two parts, one that rotates and another that is stationary. As the blood or previously separated red cells enter the spinning bowl, the cells are distributed to the periphery and as the bowl fills, the supernatant separates from the red cells. The red cells are held in suspension by centrifugal force while the supernatant is expelled through an effluent port into a waste collection receptacle.

A washing solution is then made to follow the same path as the red cells. The washing solution is a saline solution which preferably is normal physiological saline containing about 0.9% NaCl but can also contain other substances such as, for example, the components of Alsever's solution. The geometry of the centrifuge keeps the cells circulating against the flow of fresh wash solution as the used wash solution is expelled through the effluent port. When the washing is complete, the centrifuge is stopped and the washed cells are siphoned into a separate collection vessel.

Another example of a conventional blood centrifuge that is suitable for use in the invention is the Celltrifuge separator which is commercially available from the American Instrument Company.

In the foregoing washing procedures, the red cells are preferably washed with from about 5 to about 30 volumes of the saline washing solution. In a preferred example, a unit of blood (one pint) is washed with about 3-4 liters of saline.

Following the saline washing, the red cells are ready for the mild treatment with aldehyde. It not treated immediately, it is preferred to temporarily store the cells in Alsever's solution. This solution can be prepared by admixing the following components in the stated amounts and diluting with water to a volume of three liters: Component Amount ______________________________________ Glucose (dextrose) 61.5 grams Sodium citrate 24.0 grams Sodium chloride 12.6 grams Citric acid (1% solution) 15.6 ml. Neomycin 300 mg Chloramphenicol 990 mg ______________________________________

The components should be mixed well and the pH adjusted to within a range of about 6.4 to 6.8. The washed red cells can be retained in the Alsever's solution for about 45 days at 2.degree. to 8.degree. C.

The washed red cells, when ready for the aldehyde treatment, are first re-suspended in saline solution in proportions of about one part by volume of cells to about 5 to 30 parts by volume of the saline. The mild treatment with aldehyde which follows comprises the relatively slow addition of a saline solution of the aldehyde to the cells and admixing at room temperature and generally within the range of about 20.degree. to about 26.degree. C. The aldehyde solution preferably ranges from about 0.1 to about 0.6 molar aldehyde in saline. Aldehyde substances which are employed in the aldehyde/saline solution generally are aliphatic aldehydes having from one to about six carbon atoms, such as, for example, formaldehyde, acetaldehyde, propionaldehyde, butyraldehyde, malonic aldehyde, succinaldehyde, glutaraldehyde, and pyruvic aldehyde. The saline preferably is normal physiological saline and the aldehyde preferably is a monoaldehyde, especially formaldehyde. The suspension of the red cells in the aldehyde/saline solution is mixed such as by stirring for about 15 minutes to about 4 hours, preferably about 60 minutes, during which time the cells take on a bright red appearance which resembles fresh arterial blood.

Following the mild treatment with aldehyde, the treated cells are sedimented such as by centrifugation and again washed with saline in about the same range of proportions as in the initial saline washing.

As before, the red cells can be used directly in the next step or the cells can be temporarily stored in Alsever's solution at 2.degree. to 8.degree. C.

After completion of the aldehyde treatment, the red cells are buffered and transferred into suitable receptacles which are capable of being sealed so as to be completely gas tight from the ambient atmosphere. These receptacles can be, for example, glass ampules, vials or bottles.

The buffering of the cells is such as to maintain a desired pH of from about 7 to about 7.7, depending upon whether the blood control standard is to be representative of the normal range, acidosis or alkalosis. The normal range is about 7.4 .+-. 0.1 while the acidosis is 7.0-7.3 and alkalosis is 7.6 .+-. 0.1. The molarity of the buffer preferably is from about 0.05 to about 0.2 molar. Conventional buffer materials such as, for example, phosphate and tris buffers can be used, but phosphate is generally useful only at pH below 7.5 while tris is generally useful only at pH above 7.5. Preferred buffer materials for maintaining the desired pH are N-tris(hydroxymethyl)methyl-2-aminoethanesulfonic acid (TES) and N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid (HEPES). These and other such suitable buffer materials are described by Good et al., Biochemistry, 5, 467-77 (1966).

Sufficient bicarbonate ion, for example, NaHCO.sub.3, also is added to the red cells to bring the pCO.sub.2 to a level of from about 20 to about 55 mm Hg.

The buffered cells are placed into the receptacles to a level such as to leave a head space at least equal to about the volume of the red cells. Larger volumes of head space can be used, for example, up to 50 times the volume of red cells. This head space is then filled with a gas or a mixture of gases comprising 0-15% CO.sub.2, 0-25% O.sub.2 and the balance N.sub.2 and/or inert gas. As used herein, the term inert gas refers to any gas which is inert to the reactions which take place in the electrode systems of the blood pH and gas analyzer instruments. This includes the so-called inert gases which have a completed group of electrons in their outermost shells, for example, He, Ne, Ar, Kr, Xe and Rn. These gases can be added from separate gas sources or as a preadmixture of the desired gases.

The electrodes referred to above are the conventional pH, pCO.sub.2 and pO.sub.2 electrodes used in the blood pH and gas analysis instruments described hereinbefore. For example, the hydrogen ion concentration may be monitored with a pH responsive glass electrode in cooperation with a Ag/AgCl reference electrode, the partial pressure of carbon dioxide may be sensed in the circulating fluid by a CO.sub.2 electrode and the oxygen may be monitored with an oxygen-sensing electrode.

In order to ensure that the blood control standard will be saturated with the desired gas, the gas preferably is flushed into the receptacle in a volume of from about 10 to about 60 times the volume of the receptacle and the receptacle is then immediately sealed before any significant exchange with atmospheric gas can take place. The desired gas tight sealing can be achieved, for example, by using a glass ampule as the receptacle and melting the top of the ampule in a flame to provide a flame-sealed closure. In the case of vials or bottles, other types of conventional hermetic sealing can be employed.

The gases in the sealed receptacle will then come into equilibrium with the red cells to provide, in essence, a miniature tonometer. This final product remains stable and provides the desired pH, pO.sub.2 and pCO.sub.2 values for extended periods, for example, up to six months, when stored at about 2.degree. to 8.degree. C. The presence of the red cells also supplies hemoglobin to the blood control standard and, thereby, enables the determination of Base Excess, from which one can calculate O.sub.2 content and O.sub.2 saturation.

The following detailed example will further illustrate the invention although it will be appreciated that the invention is not limited to this specific example.

A unit (one pint) of fresh human blood collected in ACD or CPD anticoagulant solution is expressed into a Haemonetics Corp. continuous flow centrifuge. As the blood enters the centrifuge, the red cells are distributed to the periphery and the supernatant is expelled through the effluent port. While spinning, the cells are washed with 3 to 4 liters of a washing solution comprising an aqueous solution of 0.9% NaCl (normal physiological saline). The washed red cells are siphoned into a collection vessel and then transferred to a vessel containing five liters of normal physiological saline. To the red cells/saline mixture at 25.degree. C. is then added slowly over a brief time period of 5 to 10 minutes 500 ml of saline containing 40 ml of formaldehyde (37%) solution to thereby provide a 0.1 molar formaldehyde in 0.9% NaCl solution. The mixture is stirred at 25.degree. C. for 60 minutes, during which time the cells assume a bright red color resembling fresh arterial blood. The formaldehyde treated cell mixture is then transferred to the continuous flow centrifuge wherein the cells are further washed with 6 to 8 liters of 0.9% saline solution. The washed cells are then siphoned off into a collection vessel and buffered with an aqueous solution of 0.1 molar HEPES. Sufficient NaHCO.sub.3 solution is then added to adjust the pCO.sub.2 to 40 mm Hg. The pH is given a final adjustment to 7.4. The buffered red cells are then transferred in two ml. aliquots into glass ampules (Wheaton No. 1 glass), each having a capacity of 8 ml. A premixed gas containing 5% CO.sub.2, 12% O.sub.2 and 83% N.sub.2 is then flushed into the ampules at the rate of 600 ml. per 60 seconds, each amplule being flushed with 150 ml. of gas. ampule ampules are immediately sealed by rotating the top in a flame and pulling off the tip.

Various other examples will be apparent to the person skilled in the art after reading the foregoing description without departing from the spirit and scope of the invention and it is intended that all such examples be included within the scope of the appended claims.