Near-infrared reflectance spectroscopy for noninvasive monitoring of metabolites.

An important class of substances in clinical chemistry are metabolites in body fluids, which are accessible by near-infrared spectroscopy without sample treatment using reagentless, fast and readily automated in vitro assays. Furthermore, noninvasive sensing systems are under development for the determination of blood glucose, especially for diabetic patients or for monitoring in intensive care and surgery. Near-infrared diffuse reflectance spectrometry of skin was employed allowing a certain tissue volume to be integrally probed. For calibration, the partial least-squares (PLS) algorithm was used either based on wide spectral intervals or using special spectral variable selection. Capillary blood glucose reference concentrations were obtained by finger pricking and an automated laboratory method (hexokinase/G6P-DH). Clear evidence is provided for the physical effect, as manifested by the spectral glucose absorptivities, underlying the individual single-person calibration models, which still require improvements in the methodology in the normo- and hypoglycemic concentration range. In extending the potential of noninvasive blood assays by infrared spectroscopy, a novel technique is presented for probing the intravascular fluid space by using fast spectral near-infrared measurements of skin tissue. The pulsatile blood spectrum can be derived from reflectance spectra of oral mucosa by Fourier analysis (near-infrared plethysmography). Future applications and prospects for noninvasive blood assays are discussed.

Human oral mucosa studies with varying blood glucose concentration by non-invasive ATR-FT-IR-spectroscopy.

During the last few years infrared spectrometry has been investigated as a non-invasive clinical tool for improved understanding of in-vivo processes. Oral mucosa has been suggested as an especially suited subject for drug delivery and in vivo monitoring of endogenous body metabolites due to histological and physicochemical reasons. The attenuated total reflection (ATR) technique was used to characterize the outmost epidermal layer of human oral mucosa by means of Fourier-transform infrared spectroscopy. The penetration depth of the probing radiation in the mid-infrared fingerprint region, using a ZnSe-crystal for the horizontal ATR accessory, is in the order of a few micrometers so that microlayer information can be obtained by such a technique. Spectra of outer human lip and saliva components are presented for comparison. For several test persons, lip spectra were recorded during oral glucose tolerance tests. The individually varying blood glucose concentration was followed by means of frequent blood testing. Variability of the outmost microlayer has been studied using factor analysis of the ATR inner-lip spectra. There is no clear evidence that blood glucose concentration can be followed by ATR-spectroscopy of oral mucosa. Non-invasive spectroscopic methods exploiting trace signals require special attention paid to the variability due to person-to-person differences and changes in physiological conditions.

Optical diffuse reflectance accessory for measurements of skin tissue by near-infrared spectroscopy.

An optimized accessory for measuring the diffuse reflectance spectra of human skin tissue in the near-infrared spectral range is presented. The device includes an on-axis ellipsoidal collecting mirror with efficient illumination optics for small sampling areas of bulky body specimens. The optical design is supported by the results of a Monte Carlo simulation study of the reflectance characteristics of skin tissue. Because the results evolved from efforts to measure blood glucose noninvasively, the main emphasis is placed on the long-wavelength near-infrared range where sufficient penetration depth for radiation into tissue is still available. The accessory is applied for in vivo diffuse reflectance measurements.

Noninvasive blood glucose sensors based on near-infrared spectroscopy.

Research into noninvasive devices for self-monitoring of blood glucose is mainly based on near-infrared spectroscopy. Such a device is particularly desirable in the intensive therapy of patients with diabetes mellitus to achieve optimal metabolic control through frequent glucose testing. The state of noninvasive assay technology is presented. Using diffuse reflectance spectra of mucous lip tissue has advantages and drawbacks compared with tissue transmittance experiments. Different approaches have been proposed in the patent literature; however, current technology requires further significant improvements, particularly within the lower normal and hypoglycemic glucose concentration ranges.

Multivariate determination of glucose in whole blood by attenuated total reflection infrared spectroscopy.

A spectral analysis of whole EDTA blood was undertaken by using attenuated total reflection and Fourier-transform infrared spectroscopy. The concentration of blood glucose was measured by an enzymatic method using glucose dehydrogenase and ranged between 40 and 290 mg/dL with an average concentration of 90.4 mg/dL. Multivariate calibration with the partial least-squares (PLS) algorithm was performed on spectral data between 1500 and 750 cm-1 showing a varying background from different unidentified interfering compounds. Cross validation was carried out for optimizing the PLS model. PRESS was 19.8 mg/dL, which was calculated on the basis of 127 standards, whereas the estimated standard deviation for the calibration fit was computed to be 11.9 mg/dL. Infrared spectroscopy can be used for monitoring glucose levels within the normal physiological range in a complex matrix like whole blood as an alternative to electrochemical sensors.

Multivariate calibration for assays in clinical chemistry using attenuated total reflection infrared spectra of human blood plasma.

A spectral analysis of human blood plasma was undertaken by use of a Fourier-transform infrared spectrometer with a circular attenuated total reflection cell. The concentrations of total protein, glucose, triglycerides, total cholesterol, urea, and uric acid were measured by chemical or enzymatic methods. For these constituents the partial least-squares (PLS) algorithm was used for a multivariate calibration including the infrared fingerprint region of the plasma spectra. Best results were achieved for total protein with an average prediction error (PRESS based on cross validation) of 2.1 g/L; other PRESS results were for glucose 22 mg/dL, triglycerides 33 mg/dL, cholesterol 31 mg/dL, urea 4.4 mg/dL, and uric acid 1.6 mg/dL.