Although the decrease of sensitivity could be estimated, it is advised that a glucose standard such as urine spiked with 25 mM glucose (with sodium azide at a final concentration of 0.05% w/w to prevent microbial degradation) be included at regular intervals to check the sensitivity. It is good practice to replace the nitrocellulose and glucose oxidase after approx- imately 3 days of use. This interval would correspond to approximately 100−200 samples, depending on the intensity of use.