I am a mold consultant and use Air-O-Cell cassettes to collect air samples.  I then send them to a laboratory to conduct mold counts.  The laboratory I use charges $35/sample and reports that they count 100% of the sample trace. Some laboratories charge as little as $25/sample; others as high as $60/sample.  They count only about 25% of the sample.  Does the analysis cost and percent of sample trace counted make any difference in accuracy and the reliability of the results that I receive?-Anon. 

            I have surveyed a number of commercial laboratories that provide mold spore sampling services.  They vary from a low of $20/sample to a high of $60/sample.  Most are somewhere in between (i.e. $30-40/sample).  In almost any service that we use, cost and the apparent reputation of the provider are important factors in which provider we use.  We hope that those charge less do as good (or better) of a job as those who charge more.  That, of course, is a leap of faith and unless we get burned by our choice, we generally do not know whether we are getting what we hope we are paying for.

            In the matter of mold test results reported by commercial laboratories, a recent study we conducted showed a very strong correlation between analyses charges of laboratories and count values reported.  Those laboratories that charged $50-60 per sample typically reported the highest count values for samples collected in the same location and the same time.  This relationship can be seen in the following graph.

 

 

 

 

 

 

 

 

 

 

            These results are based on six individual sample collections sent to 10 different laboratories.  Based on comparisons to counts that this investigator conducted, those laboratories which had the highest cost/fee per sample had the more accurate and thus more reliable test results.

            Increasing the percent of sample counted does not increase the accuracy of a count.  It can increase the likelihood that mold types that are present in low numbers can be detected and theoretically decrease count variability.  Doing a 100% scan for large spore species such as Stachybotrys, Chaetomium, Cladosporium herbarum, Alternaria, Pithymyces, Epicoccum, etc. at 400 or 600X magnification makes sense.  This is done by most laboratories including those that report counts as being based on 20-30% of the total sample trace. 

            Conducting and reporting counts based on 100% of the deposition trace may have its downsides.  It should in theory increase the amount of time that an analyst takes to do a count and thus reduces the analyst’s productivity and thus increases a laboratory’s cost.  There is no indication that this is the case that is counts may simply be conducted more rapidly and less carefully. Based on my own experience, a good count on 5% of the deposition area generally takes 15-20 minutes.  In a number of commercial laboratories, a 30-100% count at 600X generally takes about 7-10 minutes. At that rate, the potential for an undercount is high.

            Getting back to cost, let me share with you an experience that we had with one of the very low cost high volume laboratories (used by many home inspectors, etc.).  We sent them lots of two samples at three different times.  Of six samples five were miscalculated.  Once the values were recalculated, reported count concentrations were twice as high as those initially reported.

“caveat emptor” 

October 7, 2005