Pitfalls in Using Pallor of the Eye to Detect Anaemia with Digital Cameras.

Anaemia - a reduced concentration of functional haemoglobin in the blood - affects around 20% of the global population. As a result, there is focus on developing non-invasive methods to screen for anaemia. A promising group of techniques uses smartphone imaging of external tissues. Because haemoglobin preferentially reflects red light, by measuring the colour ('redness') of blood vessels (or tissues as a whole), it is suggested that blood haemoglobin may be estimated. However, a potential issue is that 'redness' might depend not just on the severity of anaemia, but also upon the 'physiological structure' - i.e. the vessel density, vessel depth, and vessel diameter. We performed Monte-Carlo tissue optics simulations to investigate the potential impact of these structural factors on measurement of colour in the scleral conjunctiva. We found that within reasonable ranges of physiological structure, it is possible to get similar redness between severely anaemic (2.35 g/dL) and non-anaemic (15 g/dL) blood haemoglobin concentrations. This suggests that simple measurement of the redness of external tissues may not be suitable for detecting anaemia.Clinical RelevancePallor of external tissue is commonly used as a hallmark of anaemia. These results suggest that pallor may arise not only from anaemia, but also from variation in the physiological structure of the eye.