On 07/Jan/2015 / In Articles
As a profession, we are used to having the answers: glucose values, compatible units of blood, pathogens in a urine culture. As simple as this seems to outsiders who see lab techs as mere button pushers, we know there is a lot of judgment involved. In many cases “the answer” is ambiguous or at least far from simple. We have to answer, “Does it matter what it is?”
For example, a few weeks ago our micro techs were looking at a follow up throat culture on selective strep media to confirm a negative screen and noted a pure growth of a fine lawn of weakly beta hemolytic colonies at 24 hours. It didn’t appear to be Streptococcus pyogenes by Taxo A disc, but what was it? Did it matter?
Excellent questions. Experience teaches that asking the physician is always answered with “Yes, work everything up.” So much for asking the doc.
Arcanobacterium haemolyticum is a Gram positive bacillus that can easily be confused with Streptococcus macroscopically, first described in 1946 in cases of sore throat in US servicemen in the South Pacific. While its morphotype should be evident microscopically, it is catalase negative and often Gram variable at 24 hours.
According to Medscape:
A. haemolyticum may be missed on routine throat cultures because of the use of rapid group A streptococcal antigen assays and the use of special culture media for optimal isolation of group A streptococcal species. Most cultures for pharyngitis are evaluated at 24 hours, at which point A haemolyticum colonies are very small and demonstrate minimal hemolysis, and the cultures may be discarded.
A. haemolyticum causes 0.5-2.5% of pharyngitis cases, especially in adolescents. Rare complications include an inability to swallow, peritonsillar abscess, and sepsis. Most strains are easily treated with antibiotics e.g. macrolides such as erythromycin.
I suspect A. haemolyticum was our bug in this case, although we didn’t find out for sure because it was a screen for Group A beta hemolytic strep. It’s one of those cases where the answer to “Does it matter what it is?” is “probably not.”
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