You are correct that house made formalin and other reagents can contain AFB 
if the water is contaminated - that is one reason that we always used a 
negative control from the current day's workload.  This is a check on fixation and 
processing as well as on the staining procedure.  I would suggest that you 
repeatedly centrifuge 50 ml aliquots of tap water, decanting and centrifuging 
another 50 ml until you can see some sediment in the centrifuge tube.  Take this to 
microbiology and have them culture for AFB. 

In researching this problem, we noted that AFB has also been reported in 
paraffin (the old Tissue Mat wafers) and growing in a 40 gal. formalin tank.  It 
is a problem that is not taken into consideration often enough.

Freida

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