Validation of sensor calibration
There is provided a method of validating a calibration of a sensor represented by a calibrated relationship between concentration of an analyte in a sample and measurements from a sensor of an optical property of a sensing substance, wherein the optical property of the sensing substance has a spectrum that varies with the concentration of the analyte in the sample, and the spectrum has an isosbestic wavelength at which the optical property does not vary with concentration of the analyte, the method comprising making measurements of the optical property at three or more wavelengths of light while the sensing substance is exposed to the sample, determining whether the measurements of the optical property are inconsistent with the calibrated relationship, and outputting a warning signal in response to the measurements of the optical property being inconsistent with the calibrated relationship.
1. A method of validating a calibration of a sensor represented by a calibrated relationship between concentration of an analyte in a sample and measurements from the sensor of an optical property of a sensing substance, wherein the optical property of the sensing substance has a spectrum that varies with the concentration of the analyte in the sample, and the spectrum has an isosbestic wavelength at which the optical property does not vary with concentration of the analyte,
the method comprising:
making measurements of the optical property at three or more wavelengths of light while the sensing substance is exposed to the sample;
determining whether the measurements of the optical property are inconsistent with the calibrated relationship; and
outputting a warning signal in response to the measurements of the optical property being inconsistent with the calibrated relationship.
2. A method according to claim 1 , wherein the method further comprises deriving a measure of concentration of the analyte in the sample from the measurements in accordance with the calibrated relationship.
3. A method according to claim 2 , wherein
the calibrated relationship comprises a relationship between the concentration and a ratio between measurements of the optical property at a pair of wavelengths, and
the step of deriving the measure of concentration comprises calculating a ratio between the measurements of the optical property at the two wavelengths and deriving the measure of concentration from the ratio in accordance with the calibrated relationship.
4. A method according to claim 3 , wherein the measure of concentration comprises a measure calculated using a ratio between two measurements of the optical property not made at the isosbestic wavelength.
5. A method according to claim 1 , further comprising:
determining a measure of deviation of the measurements of the optical property from the calibrated relationship; and
determining that the measurements of the optical property are inconsistent with the calibrated relationship if the measure of deviation is above a predetermined threshold.
6. A method according to claim 5 , further comprising deriving a plurality of measures of concentration of the analyte in the sample from the measurements at different wavelengths in accordance with the calibrated relationship, wherein the measure of deviation is a measure of variation between the plurality of measures of concentration.
7. A method according to claim 6 , wherein
the calibrated relationship comprises a relationship between the concentration and plural ratios between measurements of the optical property at respective pairs of wavelengths, and
the step of deriving a plurality of measures of concentration of the analyte in the sample comprises calculating ratios between the measurements of the optical property at the respective pairs of wavelengths and deriving the plural measures of concentration from the ratios in accordance with the calibrated relationship.
8. A method according to claim 6 , wherein the measure of variation is a coefficient of variation.
9. A method according to claim 1 , wherein one of the three or more wavelengths is the isosbestic wavelength.
10. A method according to claim 1 , wherein one of the three or more wavelengths is a wavelength at which the spectrum of the optical property of the sensing substance has a maximum or a minimum.
11. A method according to claim 1 , wherein the optical property is one of absorption and emission, and the spectrum is respectively one of an absorption spectrum and an emission spectrum.
12. A method according to claim 11 , wherein the optical property is emission, and making measurements of the optical property at three or more different wavelengths comprises for each one of the three or more wavelengths:
exciting the sensing substance using light of a first wavelength; and
measuring the intensity of light emitted by the sensing substance at the one of the three or more different wavelengths,
wherein the first wavelength is the same for each of the three or more wavelengths.
13. A method according to claim 11 , wherein the optical property is absorption, and making a plurality of measurements of the optical property at three or more different wavelengths comprises for each one of the three or more wavelengths:
exciting the sensing substance using light at the one of the three or more wavelengths;
measuring the intensity of light emitted by the sensing substance at a second wavelength,
wherein the second wavelength is the same for each of the three or more wavelengths.
14. A method according to claim 11 , wherein the optical property is absorption, and making a plurality of measurements of the optical property at three or more different wavelengths comprises for each one of the three or more wavelengths:
illuminating the sensing substance using light at the one of the three or more wavelengths; and
measuring the intensity of light transmitted by the sensing substance at the one of the three or more wavelengths.
15. A method according to claim 1 , wherein the sensing substance comprises two species in equilibrium, the equilibrium between the two species being dependent on the concentration of the analyte in the sample.
16. A method according to claim 1 , wherein the analyte is one of carbon dioxide, and hydrogen ions.
17. A method according to claim 1 , wherein the sample comprises blood or interstitial fluid.
18. A method according to claim 1 , wherein the sensing substance comprises a luminescent compound, optionally wherein the luminescent compound is a fluorescent compound.
19. A method according to claim 18 , wherein the sensing substance comprises 8-Hydroxypyrene-1,3,6-trisulfonic acid.
20. A method according to claim 1 , wherein making the measurements of the optical property comprises making raw measurements at a plurality of times and time-averaging the raw measurements.
21. A method according to claim 1 , further comprising an initial calibration step of determining the calibrated relationship between the concentration of the analyte and the measurements of the optical property.