Maybe the eaisest way to avoid missed samples is to use a ring buffer like Serial. Put the binary ADC values in the buffer in the ISR and remove and format the values in the background.
Using sprintf is faster than Arduino print. I suggested an improved smaller version of print that is faster and smaller but I guess the Arduino group has no intrest.
You should use the format string "%i\r\n" so you can use one write.
Look at the Arduino hardware serial driver for details on using the ring buffer.