Attention: Iím not a teacher.
There are some definitions concerning the word in question. In your practical example the doctor say that he will order (tell, ask) a blood test for support of the primary speculation by the visual medical check. He want to check the patientís blood as well as to make complete chemical studies of his blood.
order = make a request for something
order somebody something
I'll order you a taxi.
to tell someone that they must do something, especially using your official power or authority:
The court ordered his release from prison.
order somebody to do something
Tom was ordered to pay £300 as compensation.
Her doctor had ordered her to rest for a week.