Discrepencies Between Lab Techs - (Mar/31/2009 )

I've been trying to figure out for a while now why my co-worker and I get such different counts when we plate (we use PetriFilm). My counts are always quite a bit higher. When we plate side-by-side, we do everything the same. sanitize the counter, wash our hands, and start plating. The peculiar thing is when we do the American Proficiency testing every quarter. In the past I've had counts way higher than hers, but mine seem to be the average in comparison to all other facilities participating in the test. When she does it by herself, she tends to score 50-75% whereas I always score 100%. Is it possible she is somehow actually KILLING bacteria when she plates?? Some products that we test require us to aseptically cut off pieces of raw meat and stomach it with Peptone. The aseptic technique we use is scissors and forceps stored in alcohol. I always shake off excess alcohol prior to cutting afraid of killing something on my sample. In school I learned that alcohol sanitizes as it is drying and once it is dry, it is no longer effective. Therefore, by removing the excess, it allows the utensils to dry a bit before coming into contact with the meat. Does this sound like a possibility as I don't believe she does that prior to cutting?? I'm confused! If our test results from American Proficiency Institute were flip-flopped then I'd guess I wasn't being sanitary enough so that's what throws me off! Any ideas??? I'm stumped!

Lab Techie on Apr 1 2009, 05:12 AM said:

It is indeed possible that she uses to much alcohol like you said: that the scissors and forceps are still wet with the alcohol when she uses it. That could be an explenation.
You dont use a bunsenburner ?

We operate in a TINY lab. It's about the size of a small bedroom. Also, it is attached to the rest of the facility (a meat-packing plant) and it was deemed too much of a hazard to use bunsen burners so we had to convert to alcohol. Thank you much for your thoughts!!

Lab Techie on Apr 1 2009, 06:22 PM said:

one more question: who does the counting ? She counts her own plates and you count yours? Or you send the plates to some organization and they count it? (I am not familiar with the American Proficiency testing so do not exactly know what you mean)

I know this is a strange question, but a few months ago I had somewhat of a similar problem: someone had always a very low counting.. as we checked it we found out that the one with the low counting simply didnt count correctly (but that was with a burkner counting chamber). I do not really see that you can miscount on a plate? but still...

You know what, that is a good point! She is 20 years my senior and could very possibly not see everything that I can therefore giving me higher counts, because, yes, I count my own and she counts her own. Hmmm... that is very interesting. We do both use a lightbox to count and she also (usually) uses a magnifying glass, but those don't always work that great. I've seen her count before without a lightbox OR a magnifying glass! That cannot possibly be accurate! There are so many times when you get those pinpoint colonies. I might be tempted to discreetly clean the cover of her plates and re-count them to see what counts I get on the exact same plate. That could also explain the times we prepare plates side-by-side without the use of any utensils (such as with the proficiency test). I'll do some research into this concept and report back! Thanks so much!!!