Variation of haemoglobin extinction coefficients can cause errors in the determination of haemoglobin concentration measured by near-infrared spectroscopy.

Near-infrared spectroscopy or imaging has been extensively applied to various biomedical applications since it can detect the concentrations of oxyhaemoglobin (HbO(2)), deoxyhaemoglobin (Hb) and total haemoglobin (Hb(total)) from deep tissues. To quantify concentrations of these haemoglobin derivatives, the extinction coefficient values of HbO(2) and Hb have to be employed. However, it was not well recognized among researchers that small differences in extinction coefficients could cause significant errors in quantifying the concentrations of haemoglobin derivatives. In this study, we derived equations to estimate errors of haemoglobin derivatives caused by the variation of haemoglobin extinction coefficients. To prove our error analysis, we performed experiments using liquid-tissue phantoms containing 1% Intralipid in a phosphate-buffered saline solution. The gas intervention of pure oxygen was given in the solution to examine the oxygenation changes in the phantom, and 3 mL of human blood was added twice to show the changes in [Hb(total)]. The error calculation has shown that even a small variation (0.01 cm(-1) mM(-1)) in extinction coefficients can produce appreciable relative errors in quantification of Delta[HbO(2)], Delta[Hb] and Delta[Hb(total)]. We have also observed that the error of Delta[Hb(total)] is not always larger than those of Delta[HbO(2)] and Delta[Hb]. This study concludes that we need to be aware of any variation in haemoglobin extinction coefficients, which could result from changes in temperature, and to utilize corresponding animal's haemoglobin extinction coefficients for the animal experiments, in order to obtain more accurate values of Delta[HbO(2)], Delta[Hb] and Delta[Hb(total)] from in vivo tissue measurements.