Dear student,
A clinical thermometer is designed for measuring body temperature while lab thermometers are less specialized. Clinical thermometers have a range of 35-42 degree celsius while laboratory thermometers have mostly 0-100 degree celsius and they are used to measure immediate temperatur. Hence, after recording our body temperature, until we check it, mercury will fall and not give is an accurate reading. Thats why, we don't use laboratory thermometers to check human body temperture.
Regards