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. 2016 Feb 17;7:10776. doi: 10.1038/ncomms10776

Figure 3. Using the blood absorption index to identify suitable tissue sites for quantitative measurement of the ZnPP/haem ratio.

Figure 3

(a) For whole-blood concentrations between 2 and 8% (whole blood of a healthy subject diluted by saline), the measured ZnPP fluorescence remains little changed despite this large variation in the blood concentration. (b) Illustration of the blood absorption index. The magnitude of the decrease in the tissue-background fluorescence spectrum resulting from blood absorption, termed the ‘blood absorption index', is continuously analysed during the measurement by evaluating three spectral bands centred at 562, 576 and 593 nm, around a blood absorption peak. (c) The blood absorption index in relation to the relative deviation of the measured values in vivo from HPLC reference values in vitro. Data from 10 measurement sites from each of 14 subjects are shown. To guide the examiner to a site on the lower lip with an adequate blood absorption index, feedback by an indicator light is given as red, red-orange, orange and green. Measurements are only started at sites with a blood absorption index >0.7 × 10−2 (green) to ensure that the relationship between the detected red blood cell ZnPP fluorescence and the ZnPP/haem ratio is preserved.