A server apparatus (102) for monitoring blood glucose in a wearer (106) of a wearable device (104) is configured to perform the first mode of operation of blood glucose monitoring for the wearer of the device using data obtained by a radio- frequency blood glucose monitoring module (124) in the wearable device. The server apparatus is also configured to perform a second mode of operation of blood glucose monitoring using data obtained by a light spectroscopy blood glucose monitoring module (126) in the wearable device. The server apparatus performs a threshold comparison between a reference blood glucose value for the wearer of the device and the threshold glucose value (314). A first one of the first mode of operation and the second mode of operation is selected in dependence of the threshold comparison.
