The present invention relates to a method for optically measuring the concentration of a specific component in a living body such as blood sugar level, water or cholesterol by measuring light reflected from the living body and a measuring device using it.
Hitherto, various methods for measuring a sample, a specific component in a living body in particular, with the use of an optical measuring device have been proposed, one of which is a method for measuring blood sugar level by bringing a transparent attenuated total reflectance (ATR) element having a pair of reflecting planes, which are arranged in parallel and facing each other, into contact with top and bottom lips (see Japanese Laid-Open Patent Publications Nos. Hei 9-113439 and Hei 11-178799).
To be more precise, according to the above method, light is introduced into an ATR element made of zinc selenide, silicon, germanium or the like being held in the mouth and pressed against the top and bottom lips. The incident light undergoes repeated total internal reflection between the reflecting planes of the ATR element, each reflecting plane being located at the interface between the lip and the ATR element, and then the light exits the ATR element. Analyzing this exiting light provides information concerning the concentration of a component contained in the lips.
Another proposed method is to introduce a laser beam with a wavelength of 9 to 11 micrometers into an ATR element made of ZnSe optical crystal, which is closely adhered to a mucous membrane of the lips, so as to have multiple reflection inside the ATR element. In this method, attenuated total internal reflection light, scattered reflected light or the like after the multiple reflection is analyzed to determine the blood sugar level or the concentration of ethanol in the blood.
These above methods realize realtime measurement of the concentration of a specific component such as glucose or cholesterol in a noninvasive manner. In these methods, evanescent wave (so-called penetrating wave) is applied to a quantitative analysis.
As described above, by measuring light reflected from a living body, it is possible to acquire information concerning the concentration of a component contained in a body fluid. This method, however, is accompanied by a problem that a fluid present at the interface between the optical element and a living body influences the accuracy of the measurement. It should be noted that the term “information concerning the concentration” includes a concentration itself and an absolute value of concentration as well as a change in concentration with time and the like.
A conventional optical measuring device using such method as described above also has the following problem. Light traveling through an ATR element slightly invades the lips when the light undergoes repeated total internal reflection between the reflecting planes of the ATR element, each reflecting plane being located at the interface between the lip and the ATR element, during which the light is affected by a component contained in a body fluid present at the interface.
Glucose, for example, has an absorption peak at a wavelength of around 1033 cm−1 and 1080 cm−1. Accordingly, if light having the above wavelength is applied to a living body, the amount of light absorbed will differ according to the concentration of glucose in the living body.
More specifically, in the case of measuring the concentration information of a component in the lips of a living body using an optical element, if a fluid such as saliva is present between the optical element and the lips, the amount of the light reaching the lips will differ according to the thickness of the fluid layer. As a result, the amount of signal to be detected based on the amount of the light varies significantly, giving varying measurement results. Therefore, stable measurement results cannot be obtained.
In the above example, if the concentration information is determined based only on absorption peak values of glucose, erroneous concentration information might be obtained. For example, a low absorbance might be obtained due to a thick saliva layer although, in fact, the living body has a high concentration of glucose. Conversely, a high absorbance might be obtained due to a thin saliva layer although, in fact, the living body has a low concentration of glucose.
Further, a similar problem also occurs when a plurality of living bodies are subjected to the measurement because the amount of the signal varies significantly due to differences in refractive index among individuals.
Furthermore, a similar problem also occurs when the measurement is taken at a body part other than the lips. This is because, if a body fluid such as sweat is present between the measuring part and an optical device, the amount of light reaching the measuring part will differ according to the thickness of the fluid layer, giving varying measurement results.
In the ATR technique, the depth to which evanescent wave penetrates into a measuring object is usually in an order of wavelength. The light penetrates a surface tissue layer extending from the surface of the measuring object to a wavelength order and then returns. The depth of the evanescent wave penetration is determined by a refractive index of the living body and an incident angle of the light entering the optical element.
In the case where a fluid is present between the measuring object and the optical element, a change in the thickness of the fluid present therebetween affects the depth of the evanescent wave penetrating into the measuring object. Likewise, the penetration depth is affected also in the case where a plurality of measuring objects are used for the measurement, because the measuring objects have a different refractive index. Therefore, the above problems are especially obvious in optically measuring devices that utilize an ATR element.
Even in a method using transmitted light, instead of evanescent wave, a fluid exists at the interface between a living body and an optical element. Accordingly, similar to the above, a change in the thickness of the fluid layer affects the wavenumber signal information.
As explained above, conventional measuring methods have a problem that a change in the state of the interface between a living body and the optical element affects the wavenumber signal information. In view of the above problems, the object of the present invention is to provide a method and a device for achieving easy and stable measurement for the concentration of a specific component contained in a living body even when a fluid such as water, saliva or sweat is present between an optical element and a living body or when the measurement is taken at a plurality of measuring parts of a living body.