Calibration in vivo

In contrast to the subjects of other fields, biological systems always run the risk of being physiologically unbalanced as a result of sampling. The consequence of this disturbance is that, however reliable the analysis may be, one never measures the actual value of the parameter of interest. This problem is lessened by performing in situ or in vivo measurements, which, however, require sterilization and calibration. In vivo measurements can be classified according to whether they are made under a static or a dynamic flow. 

Intensified metabolic control, especially in case of diabetes, demands minimal-invasive or non-invasive methods of analytical measurement. For this goal, a method has been developed to measure the blood glucose content in vivo, in direct contact with the skin, by means of diffuse reflection near infrared (NIR) spectroscopy on the basis of multivariate calibration and neural networks (Muller et al. [1997] Fischbacher et al. [1997] Danzer et al. [1998]). Because no patients with any standard blood glucose value are available in principle, a method of indirect calibration has... 

Although a few amperometric pH sensors are reported [32], most pH electrodes are potentiometric sensors. Among various potentiometric pH sensors, conventional glass pH electrodes are widely used and the pH value measured using a glass electrode is often considered as a gold standard in the development and calibration of other novel pH sensors in vivo and in vitro [33], Other pH electrodes, such as metal/metal oxide and ISFETs have received more and more attention in recent years due to their robustness, fast response, all-solid format and capability for miniaturization. Potentiometric microelectrodes for pH measurements will be the focus of this chapter. 

G. Velho, P. Froguel, D. R. Thevenot, and G. Reach, In vivo calibration of a subcutaneous glucose sensor for determination of subcutaneous glucose kinetics, Diabetes Nutr. Metab Clin, Exp. 1, 227-233 (1988). 

Previously, in vitro recovery was the most commonly used method for estimating ECF concentrations of a substance (Benveniste, 1989 Stable et al., 1991). To determine in vitro recovery, the probe is immersed in a known concentration of the analyte, preferably at brain temperature, and perfused with a medium free of the analyte. Percent recovery (or relative recovery) is defined as the ratio between two measures (a) the concentration of the analyte that is recovered from the probe and (b) the known concentration. In vitro calibration is limited and no longer considered appropriate, because it fails to factor in physiological factors, such as extracellular tortuosity and neurochemical reuptake, which iirfluence in vivo but not in vitro recovery (Benveniste, 1989 Benveniste and Huttemeier, 1990 Bungay et al., 1990 Hsiao et al., 1990 Morrison et al., 1991 Parsons et al., 1991b Parsons and Justice, 1992 Stable, 2000). 

MenacherryS, Hubert W, Justice JB Jr. 1992. In vivo calibration of microdialysis probes for exogenous compounds. Anal Chem 64(6) 577-583. 

Figure 3.1 Correlation between passive permeability and in vivo BAV in Sprague Dawley rats (N=m). The PAMPA F(%) values in the y-axis were derived from the passive permeability measurements in a PAMPA assay [19] using a calibration curve with reference compounds of known fraction absorbed.

Fig. 2.9. A Average Raman spectra of hyperplastic (n = 20 solid line) and adenomatous (n = 34 broken line) colon polyps collected ex vivo (power = 200 mW 30-s collection time). B Average Raman spectra of hyperplastic (n = 9 solid line) and adenomatous (n = 10 broken line) colon polyps collected in vivo (power = 100mW 5-s collection time). The spectra have been intensity corrected, wavelength calibrated, and fluorescence background subtracted (modified from [25], with permission)...

This chapter discusses the use of Raman spectroscopy for analysis of biofluids, specifically blood and urine. After a brief overview of the clinical motivations for analyzing biofluids, the benefits of optical approaches in general and Raman spectroscopy in particular are presented. The core of the chapter is a survey of equipment, data-processing, and calibration options for extracting concentration values from Raman spectra of biofluids or, in the in vivo cases, volumes that include biofluids. The chapter finishes with a discussion of fundamental limits on how accurately concentrations can be determined from Raman measurements and how closely current experiments approach that limit. 

Fig. 16.4. Three methods of obtaining Raman-based estimates of biofluid concentrations in vivo, a Confocal isolation of a subsurface volume occupied by a blood vessel, enabling direct measurement of a blood spectrum, b Difference measurement between tissue in two states, one with more blood in the sampling volume (in this case, due to pressure modulation by the subject [6]). Computing the difference removes the bulk tissue contributions to the spectral measurement and emphasizes the contribution from blood, c Statistical correlation approach of measuring many volunteers tissue in a region where sufficient blood is present (e.g., the forearm as shown here) and obtaining a correlated reference value from a blood sample drawn at the same time. Multivariate calibration is then used to find correlations between the reference value and the spectral data vector. Unlike the previous two methods, this does not intrinsically isolate the blood chemicals Raman signatures from those of the surrounding tissue volume...

The literature is rather vague concerning the question of what happens to the sensitivity of a sensor when it is implanted in a biological fluid. In our hands, subcutaneous sensors immediately and rapidly lose sensitivity, a process that takes place in minutes. Despite losses in sensitivity of 10-30%, these sensors will function satisfactorily over periods in excess of 4 days. Indeed, the performance of the sensor frequently improves with time. The origin of this sensitivity loss has been studied in detail and some results are shown in Figure 1.5.55 If the sensor is removed from the tissue and quickly calibrated in buffer solution (10 min), essentially the same in vitro sensitivity is obtained as for the in vivo value. Further incubation in buffer causes the sensitivity to rise until eventually the original in vitro value is obtained. The important... 

Csoregi E, Quinn CP, Schmidtke DW, Lindquist SE, Pishko MV, Ye L, Katakis I, Hubbell JA, Heller A. Design, characterization, and one-point in vivo calibration of a subcutaneously implanted glucose electrode. Analytical Chemistry 1994,66, 3131-3138. 

USE OF OXYGEN SENSORS AS A SURROGATE GLUCOSE SENSOR FOR IN VIVO TESTING AND IMPORTANT ISSUES RELATED TO IN VITRO SENSOR CALIBRATION... 

Sensors implanted in experimental subjects need to be carefully calibrated prior to testing in vivo. Immediately following testing, calibration validation needs to be performed to demonstrate that the calibration was not lost during the testing procedure. It is important to maintain proper calibration to eliminate any variability produced by the sensors themselves. If sensors are not appropriately calibrated prior to experimentation, it will not be possible to get an accurate measure of the variability due to tissue effects. 

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