Monthly Archives: March 2013

“The Dark Art of Serology”

Confessions of a Microbiologist, ,

Just as much as with reading bacterial culture plates, there is art in serology. I am not saying that the results should be created from thin air, more that the interpretation of serology results needs to handled very carefully. I think you learn to get a feel for when a result is right or wrong, which is not very scientific really.

serology tube

I usually say that serology is half result, half interpretation. Knowledge of sensitivity, specificity and positive predictive value are key.

There are (quite) a few ways in which serological results can lead us up the garden path:

  • Cross-reactivity. The truth is that any serological test can cross-react and give a false positive result. The most common scenarios however are when testing for Herpeviruses or Arboviruses. Over the past few months, I have seen quite a few cases of false positive CMV IgM results due to cross reaction from EBV antigens circulating in the patient. This may not be that important in an infectious mononucleosis illness in a fit healthy young adult, but a false positive diagnosis of CMV is much more critical in pregnancy. I now think twice before releasing a positive CMV result before knowing the EBV serology status.
  • Prolonged presence of IgM. Toxoplasma is the classical example of this, with IgM persisting for up to two years after the initial infection.
  • Rapid appearance of IgG. My impression of the sophisticated and sensitive assays that we use nowadays is that IgG can be picked up almost as quickly as IgM. We really need to throw the rather misleading textbooks out the window when it comes to both appearance and duration of IgM and IgG.
  • Geographically specific serotypes. Make sure that the serotypes that your serology assay looks for actually occur in your locality. Leptospirosis assays are a good example of this.
  • Low reactive results. Some sort of serological reaction is picked up by the EIA but is insufficient to meet the criteria of being positive. What do these mean and how are they interpreted?

Generally clinicians regard laboratory results as “the gospel truth” without particular regard to the multiplicity of factors, including those above which can render them dubious, most of which are outwith our control.

This demonstrates therefore the importance of appropriate comments (which I call disclaimers) attached to serological results, to show that although we think this result is correct, one can never be totally sure with serology….

Michael

“Pandering: Bending our Microbiological Principles”

Confessions of a Microbiologist,

This article has nothing to do with prostitution! I am talking here about microbiological pandering. Ie to facilitating a microbiological request made by another person/group, especially one of questionable appropriateness.

Ther are lots of examples of microbiological pandering i.e.:

  •  Accepting a request for extra susceptibility testing when the antibiotic can easily be inferred from one already tested, or when the result is a foregone conclusion.
  •  Accepting a request for a serological test which does not fit with the clinical picture and thus will have poor positive predictive value.
  •  Doing extra tests on a patient just because he/she is famous or well known in the community.
  •  Using a diferent methodology for one consultant’s patients because this is his/her personal preference.

and many many more…

There are several reasons why we pander; To maintain relationships, to avoid criticism, because of the seniority of the requestor, to try and please everyone, to “pay back a favour”….

I suspect we all pander to some extent. I am as guilty as anyone else of the occasional pander.

However “overpandering” leads to chaotic and unprincipled methods and approaches, and is ultimately disadvantageous to the patient.

Conversely, an organisation or individual which doesn’t pander at all may be seen as unwilling to listen, not prepared to negotiate.

I think we need to “live with” a bit of minor pandering here and there, but first and foremost we need to stand up to our microbiological principles and apply them as consistently as we possibly can…..

Michael

 

Microbiology Picture Quiz: Number 2.

Microbiology Picture Quizzes,

An Injecting Drug User presents to ED with the lesion (as shown) on his arm. He also has a fever.

anthrax2

1) What disease is very important to exclude as a cause of his lesion?

2) The clinician phones the microbiology laboratory asking what samples to take. What advice do you give?

3) How would you diagnose this condition in the microbiology laboratory?

 

Click here for sample answers with brief explanations.