A method for use in monitoring for low values of a physiological characteristic of a subject, with a low value of the physiological characteristic indicating that the subject has, or the degree to which the subject has, a physiological condition, the method comprising: receiving a value of the physiological characteristic (R e) of the subject; determining whether the received physiological characteristic value is normal using a range of normal variability of the physiological characteristic for the subject calculated using a set of previous values of the physiological characteristic for the subject, by: comparing the physiological characteristic value to upper and lower bounds (LB, UB) that define the range of normal variability; characterised in that if the received physiological characteristic value is below the lower bound, comparing the number of consecutive received physiological characteristic values (C below) that are below the lower bound to a first threshold value; and determining that the received physiological characteristic value is abnormal when the number of consecutive received physiological characteristic values that are below the lower bound is equal to or greater than the first threshold value; determining the received physiological characteristic value to be normal if the number of consecutive received physiological characteristic values below the lower bound is less than the first threshold value; and in the event that the received physiological characteristic value is determined to be normal, updating the range of normal variability using the received physiological characteristic value. receiving a value of the physiological characteristic (R e) of the subject; determining whether the received physiological characteristic value is normal using a range of normal variability of the physiological characteristic for the subject calculated using a set of previous values of the physiological characteristic for the subject, by: comparing the physiological characteristic value to upper and lower bounds (LB, UB) that define the range of normal variability; characterised in that if the received physiological characteristic value is below the lower bound, comparing the number of consecutive received physiological characteristic values (C below) that are below the lower bound to a first threshold value; and determining that the received physiological characteristic value is abnormal when the number of consecutive received physiological characteristic values that are below the lower bound is equal to or greater than the first threshold value; determining the received physiological characteristic value to be normal if the number of consecutive received physiological characteristic values below the lower bound is less than the first threshold value; and in the event that the received physiological characteristic value is determined to be normal, updating the range of normal variability using the received physiological characteristic value. comparing the physiological characteristic value to upper and lower bounds (LB, UB) that define the range of normal variability; characterised in that if the received physiological characteristic value is below the lower bound, comparing the number of consecutive received physiological characteristic values (C below) that are below the lower bound to a first threshold value; and determining that the received physiological characteristic value is abnormal when the number of consecutive received physiological characteristic values that are below the lower bound is equal to or greater than the first threshold value; determining the received physiological characteristic value to be normal if the number of consecutive received physiological characteristic values below the lower bound is less than the first threshold value; and in the event that the received physiological characteristic value is determined to be normal, updating the range of normal variability using the received physiological characteristic value.