Complex sentence: Steroid use, malnutrition and sarcoidosis can also lead to false-negative results, because the immune system needs to be functional to mount a response to the protein derivative injected under the skin.
Split into simpler sentences: Steroid use, malnutrition and sarcoidosis can also lead to false negative results. This is because the immune system needs to be functional to mount a response to the protein derivative injected under the skin.