How to read a environment variables with .crt in your path

Hi Guys, someone can help me with this problem.
i need to read a file .crt in my application but when i try to read, i receive on undefined.
this is my code for read.

// this variable is for to get a environment variable from my system
var KEY = process.env.MQTT_KEY;

var KEY_T = fs.readFileSync(${KEY});

I use this lib FS to read my file .crt but this not working