Actress Jing Lusi revealed to What's on TV that a health issue is flagged up when Tara gives herself a test MRI scan.
She explained: "Tara knows that if it got out she could lose her job because she could be putting patients at risk. Also, she doesn't want charity or sympathy - she just wants to prove she's a normal doctor who's good at her job.
"Tara's always been so keen to impress at Holby because she knows she's a ticking time-bomb. She doesn't have time to gradually work her way up the ranks; if she can speed up the process to promotion she will. Because she has to."
Tara Lo's test results show up on the hospital computer, much to the alarm of consultant Hanssen.
Holby City's next episode 'Long Way Down' is screened on Tuesday, June 26 at 8pm on BBC One.