Consider the age of the patient. Many physicians still advise using a rectal thermometer for children up to three years of age, but recommend other types of thermometers for older children and adults who will usually object to the use of a rectal thermometer. If the person whose temperature you're taking struggles to stay still during a reading, minimize the struggle by using a faster or less-invasive thermometer.
When do readings usually take place? For a sick newborn, parents may want to take their temperature during the night when the infant is calmer. Older children and adults may prefer to have their temperatures taken during the day. If you're in the habit of taking temperatures at night, keep in mind that several features can affect the thermometer's suitability, including how invasive the thermometer is and how loudly it beeps to confirm a good reading (which might wake others in the room).
Do you need to document temperatures over time? If you're using a digital thermometer for fertility tracking or for a chronic illness that requires ongoing documentation, look for a thermometer that stores many past readings in internal memory. Being able to store 20 or 30 readings saves you from having to constantly document your readings one at a time, especially if you're taking temperatures at night or in the early morning.
Will you be using the thermometer for more than one person? If you're looking for a digital thermometer for the whole family, look for a model with disposable probe covers or an easy-to-clean probe. This is especially important if you use the same thermometer for rectal temperatures and measuring other sites, although most users quite rightly prefer to hold one thermometer in reserve for rectal measurements only, and use another one for taking temperatures at other sites.
We see two parallel trends happening in the world of digital thermometers. The first is a gradual but undeniable push toward "smart" thermometers that interface with an app and unlock a world of tracking and sharing features. Syncing and device compatibility can sometimes be a problem, though. Once those usability issues are addressed, we expect smart thermometers to be a higher-quality -- if also higher-priced -- alternative to today's cheap, disposable digital thermometers.
The other trend is toward wearable thermometers for babies, like the wearable Fridababy FeverFrida iThermometer (Est. $70), a sticker that adheres to your child's underarm area, then continuously transmits his or her temperature to your smartphone. Most users love the idea but again, unreliable Bluetooth pairing and syncing are deal-breakers for many. Once that is ironed out, we expect this sort of technology to become very popular with parents.