The test they conduct for the plague is quite simple. They take the blood of the person they want to test and then they take it to a "lab" and inject a rat with the blood. If the rat gets sick you are positive for the plague. If it doesn't then you don't have it.
As they took the blood from Icarus, all he could think about was how unfortunate it would be if he had the plague. He'd get thrown out, shun, and maybe killed. He really wouldn't survive in a world without everything he needed. He wouldn't survive in the woods where he had to hunt for everything he needed. Without a bed, without clean clothes. Wow it really would be terrible, it'd be bad news, but then again he was used to bad news, bad news like his closest friends dying, bad news like- "we are done here sir. We will have your results in a few days. I don't think it'll be bad, you don't look sick." Her mask slightly muffled her words but he could hear what she was saying, he just didn't care.
When he was given the option to get killed or face the torture of slowly dying in the woods and possibly being found by the superior and species killed in the most excruciating way possible, he understood why most chose to just die. But he decided that since he didn't look sick, or act sick, maybe he would find another city and live a normal life. Obviously the chances of this were low because the cities were well hidden to keep the "insects" out and the humans in. There were a few ways ti send a silent signal to the cities ti let them know that you're human, but he didn't know that because he never thought he would have to use those signals. He thought he'd always have a peaceful life and the plague would go away and everything would be fine by the time he was old enough to be exiled.
Right now all he could think of in this moment was how he was going to die the second he got out there.