this is very important question even for me also. i insert 1.5 mL of PBS in a mice for BALF and i get average protein concentrion is 0.2 to 0.8 mg/mL. since in real time the total volume of surfactant of lungs would be less then 100 uL (cant say exactly) and we collect 1 to 1.5 mL total wash. so i am too waiting for answer of this question