Praise be to Allah.
Injections for medical purposes do not break the fast unless they are taking the place of nourishment, such as injections of vitamins and solutions that are injected into the blood. It says in a statement of the Islamic Fiqh Council no. 93 (1/10) concerning the things that break the fast in the field of medical treatment: The following things are not regarded as breaking the fast … Medical injections into the skin (intradermal), muscles (intramuscular) or veins (intravenous), with the exception of solutions and injections containing nutrients. End quote.
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) was asked about the ruling on injections into a muscle, a vein or the hip?
There is nothing wrong with injections into a vein, a muscle or the hip, and that does not break the fast, because these are not things that break the fast and do not come under the heading of things that break the fast. So they are not food or drink, and do not come under the same heading as food and drink. We have previously explained that this does not affect the fast. Rather what affects the fast is injecting the patient with that which replaces food and drink.
End quote from Fataawa as-Siyaam (p. 220).
Based on that, there is nothing wrong with testing for sensitivity by injecting a saline solution under the skin, because this does not serve to provide nutrients.
And Allah knows best.