C’est v Il / Elle est

Question

The lesson re the above clearly states that “If it/ he/ she is followed by a determinant you will use c’est.” Why then, after taking Lucie’s temperature does the doctor say,  “ Elle est un peu élevée” and when taking her blood pressure, “Elle est un peu basse”? I can understand why he would say “Elle est élevée” or “Elle est basse” but surely the given answer contradicts the rule.

Maarten Kamp · Accepted Answer

‘Un peu (adjective) ’ - ‘un peu ..’ is a locution adverbiale, and not a determiner. The fixed expression is treated grammatically as a unit.
It is the presence of a noun after ‘il or elle est…’ that is uncommon (but not never). Recognising a determiner is normally a good shortcut indication of a noun following, and also avoids confusion with the description of professions by ‘noun adjectives’ being caught up in the ‘prohibited list’. 
Unfortunately, no simple rule covers all the exceptions - nor the colloquial spoken use of the language ! 
https://www.wordreference.com/fren/un%20peu 
https://www.frenchtoday.com/blog/french-grammar/cest-versus-il-elle-est/