Improving the reliability of smartphone-based urine colorimetry using a colour card calibration method

OBJECTIVE: Urine colorimetry using a digital image–based colorimetry is potentially an accessible hydration assessment method. This study evaluated the agreement between urine colorimetry values measured with different smartphone brands under various lighting conditions in patients with dengue fever. METHODS: The urine samples were photographed in a customized photo box, under five simulated lighting conditions, using five smartphones. These images were analyzed using Adobe Photoshop to obtain urine Red, Green and Blue (RGB) values with and without colour correction. A commercially available colour calibration card was used for colour correction. Using intraclass correlation coefficient (ICC), inter-phone and intra-phone agreements of urine RGB values were analyzed. RESULTS: Without colour correction, the various smartphones produced the highest agreement for Blue and Green values under the ‘daylight’ lighting condition. With colour correction, ICC values showed ‘exceptional’ inter-phone and intra-phone agreement for the Blue and Green values (ICC > 0.9). Red values showed ‘poor’ (ICC < 0.5) agreement with and without colour correction in all lighting conditions. Out of the five phones compared in this study, Phone 4 produced the lowest intra-phone agreement. CONCLUSIONS: Colour calibration using photo colour cards improved the reliability of smartphone-based urine colorimetry, making this a promising point-of-care hydration assessment tool using the ubiquitous smartphone.