Hi,
just because a sputum smear, or BAL smear are negative it doesnt mean TB is ruled out, the second part of the
test, which is the culture takes 6-8 wks to grow something, the AFB, or acid fast bacilli( which is the bacteria that can cause TB ) is slow growing. starting tb meds empirically is good in the sense that your doctor can always stop the medications if its completely ruled out, rather than waiting 6-8 wks and then finding out it is TB.