Improvements in instrumentation have made the determination of blood pH, gas, electrolytes and CO-ox fractions relatively routine in clinical laboratories. Typically, pH and blood gas instruments measure blood pH, pCO.sub.2, and pO.sub.2. CO-oximeter instruments typically measure the total hemoglobin concentration (THb); the hemoglobin fractions such as oxyhemoglobin (O.sub.2 Hb); methemoglobin (MetHb); carboxyhemoglobin (COHb); and reduced hemoglobin (HHb) (collectively referred to herein as "CO-ox fractions"). Electrolyte instruments measure any number of blood electrolytes, including sodium, potassium, lithium, calcium, and so on. instrument systems currently available may combine the measurement of blood pH, gases, electrolytes, various metabolites, and CO-ox fractions in one instrument for a comprehensive testing of the properties of blood, as is particularly useful in respiratory and pulmonary ailments. Vigorous therapeutic treatment is often dictated by such test results.
Quantitative determination of the various CO-ox fractions in clinical settings is desirable as CO-ox fractions relate to the loading of oxygen onto the hemoglobin of red blood cells circulating through the pulmonary capillaries. The actual amount of oxygen loaded onto the hemoglobin is determined not only by the concentration of total hemoglobin (THb), but also by the amount of non-oxygen-binding derivatives of hemoglobin such as carboxyhemoglobin (COHb) and methemoglobin (MetHb). Reduced hemoglobin (HHb) is an unoxygenated form of normal hemoglobin and elevations in the arterial fractional HHb indicate that lesser amounts of oxygen have been bound as a result of ventilation and/or perfusion defects.
Blood, hemoglobin and hemoglobin CO-ox fractions absorb visible light. A normal blood spectrum has a main absorption peak at 578 nm and decreases rapidly close to zero at wavelengths greater than about 610 nm as shown in FIG. 1. The second absorption peak of blood is at 542 nm. Absorbance maxima of hemoglobin derivatives are oxyhemoglobin, 541, 568-572 nm; reduced hemoglobin, 555 nm; carboxyhemoglobin, 537, 568-572 nm; and methemoglobin 540, 578, 630 nm.
Generally, optical type of CO-oximeters measure the absorbance of the blood sample at multiple wavelengths on the spectrum. Ultimately, based on the known CO-ox fraction absorption wavelengths regions, CO-oximeters analyze blood samples by the collection of absorption data at specific wavelengths. The data are then typically recorded and a process called multicomponent analysis is used to simultaneously calculate the concentrations of the each of hemoglobin CO-ox fraction present in the blood sample.
CO-oximeter instruments are typically designed to measure CO-ox fractions with values broader than the ranges observed in patient samples. For example, instruments may have the capability to measure CO-ox fractions in the defined instrument ranges of: about 4 to about 25 g/dL of THb, about 30% to about 98% oxyhemoglobin (O.sub.2 Hb); 0 to about 50% carboxyhemoglobin (COHb); 0 to about 40% methemoglobin (MetHb); and 0 to about 50% reduced hemoglobin (HHb), with all CO-ox fraction percentages herein based on the total amount of hemoglobin. Within the range of instrument capability is a clinically meaningful CO-ox fraction range and a normal physiological CO-ox fraction range. The clinically meaningful range is defined herein as: total hemoglobin 8 to 20 g/dL, 60 to 98% oxyhemoglobin (O.sub.2 Hb); 0 to 20% carboxyhemoglobin (COHb); 0 to about 20% methemoglobin (MetHb); and 0 to about 20% reduced hemoglobin (HHb). The normal physiological range is defined herein as: total hemoglobin 14 to 17 g/dL for men and 12 to 15 g/dL for women, oxyhemoglobin 94 to 98%, carboxyhemoglobin 0 to about 1% for nonsmoker, methemoglobin 0 to about 1.5%, and reduced hemoglobin from about 1 to about 5%.
Reference materials generally function to validate the performance of a diagnostic instrument. For CO-oximeter instrument systems, ideal quality control standard materials are formulated to provide pre-determined CO-ox fraction values not only within the broad instrument capability range, but also within the normal physiological range.
The prior art teaches two general types of CO-oximeter quality control standard materials. The first type are aqueous dye-based materials, where dyes are used in an attempt to match the spectrum of blood. The second type are blood-based materials, where the presence blood allows for the direct match of the spectrum of blood. Both types of materials have been associated with a variety of problems as discussed below.
In developing dye-based quality control standard materials for CO-ox instrument systems, to more closely approximate the spectrum of blood, combinations of dyes have been used. The combinations of dyes have been used because no single synthetic dye has a spectrum sufficiently similar to the absorption bands of a blood spectrum. Because the spectrum of blood has multiple distinctive bands of absorption, it is challenging to prepare a quality control standard material to mimic these characteristics. Although one dye might contribute an absorption characteristic that is present in the normal blood spectrum, it may also present other absorption characteristics that are dissimilar to blood at other portions of the spectrum.
Prior art methods teach dye combinations that only partially simulate blood's visible spectrum. Consequently, the CO-ox fraction values of the prior art quality control standard materials are often not clinically meaningful. For example, U.S. Pat. No. 4,843,013 teaches a combination of dyes for a CO-ox quality control standard, however, as shown in Cols. 6 and 7 of U.S. Pat. No. 4,843,013, the described quality control standard provides negative values for some of the CO-ox fractions. A negative value for a CO-ox fraction would never appear in a blood sample and thus these control standards have limited usefulness in qualifying CO-oximeters in a clinical setting. There has been a long-felt commercial need for dye-based quality control materials that may be formulated in a predictable manner to provide pre-determined clinically meaningful CO-ox fractions, particularly in the normal physiological range.
Blood-based reference materials provide clinically meaningful CO-ox fraction values, however, numerous limitations have been associated with using blood in the quality control standard materials. For example, a significant problem encountered by users of the proteinaceous blood-based materials is that blood-based materials are very susceptible to bacterial contamination and have a limited shelf life. Consequently, in most cases, users of blood-based materials must refrigerate the products during storage. Additionally, blood-based reference materials are generally classified as biohazardous materials, thus requiring the user to take additional safety precautions.
There is a need to provide non-proteinaceous dye-based reference materials that provide clinically and physiologically meaningful CO-ox fraction ranges.