Tag Archives: PCR

“What we know is what we get”

The Science of Microbiology, ,

Myco_culture

We are all familiar with chlamydia, gonorrhoea, syphilis etc when thinking about STIs.

But when it comes to Mycoplasma genitalium, the knowledge base might be a little more patchy…

Mycoplasma genitalium has been associated with urethritis since the early 80s. However in those days, the only means of diagnosis was culture.

Have you ever tried to culture a Mycoplasma? It is not surprising that Mycoplasma genitalium was kept in the dark as a causative agent of Non-specific Urethritis (NSU) for a couple of decades. Quite simply, it was put in the ‘too difficult basket’ in terms of laboratory diagnosis.

It is only in the past few years that commercial PCR assays have become increasingly available for this pathogen. Consequently clinicians and microbiologists are becoming much more aware of it.

As a cause of urethritis it is more common than Neisseria gonorrhoeae, but less common than Chlamydia trachomatis.

Most labs still do not test for M. genitalium routinely, restricting testing to treatment failures or other special circumstances. However I think this will change in the future, and it may well be included in an NSU panel with C. trachomatis and N. gonorrhoeae.

If I was an examiner, and it is a relief to many that I am not, Mycoplasma genitalium would be one topic that I would ask about. I suspect it would be a good topic to separate the passes from the distinctions…

Michael

Click here for a short CDC review article on Mycoplasma genitalium (about a 5 minute read)

“Ignorance is bliss”

Confessions of a Microbiologist, , ,
B. hominis
B. hominis

PCR for enteric pathogens is starting to take hold, both locally and internationally.

One area where there is a significant difference between traditional methods and PCR is in the area of enteric parasites.

PCR is much more sensitive than microscopy at picking up faecal parasites. This has shown great benefit when trying to diagnose conditions like Entamoeba histolytica, (not only in faeces, but also in liver abscesses as well)

So far, so good,

but there are downsides as well….

PCR for Blastocystis hominis and Dientamoeba fragilis are generally much more sensitive methods than their microscopical counterparts. This leads to high numbers of “positives” being reported, particularly in children. However these two parasites are putative pathogens at best, potentially causing symptoms in a small and select number of patients only. Most of the time they are probably just there for the ride.

Using such a sensitive assay for parasites of uncertain significance like this can thus lead to misdiagnosis, over treatment and undue anxiety. More is not always better.

Sometimes ignorance is bliss…

Michael

“Ward based PCR”

Future of Microbiology, , ,

There are a couple of new systems just on the market which I suspect will revolutionise PCR testing, particularly where an urgent result is desirable. This is often the case when trying to diagnose Influenza and RSV in the acute care setting.

Cepheid and Roche have both released benchtop PCR systems for Influenza and RSV testing which can be placed in the ward/ED environment, requires minimal training and can give you a highly sensitive result for influenza/RSV in well under an hour.

“Highly sensitive” are the key words here. This is in contrast to rapid antigen tests for Influenza and RSV which although can be useful in certain circumstances, are limited in their clinical usefulness by sub-standard sensitivity. RSV and Influenza rapid antigen tests are now living on borrowed time….

The other highlight is “under an hour”. This allows real-time management of the patient in the ED./acute assessment ward (including treatment, isolation, cohorting, discharge etc.) based on the result of the PCR test.

The big downsides of course are the cost and the “one at a time access”. More expensive (the cost will come down)than traditional batched PCRs it might be a while before such testing systems become commonplace. (A lot depends on the funding model of your healthcare system.)

But they will, you can be sure of that.

Michael