MFF0005: Positive and negative pathology test results

So just because your doctor says you have a positive test for something, it doesn’t mean you have the associated disease. This is true for many microbiology results and especially when the method of testing is an indirect form of diagnosis.


MALDI-TOF Gary Lum

One of the common misconceptions about pathology test results is that a positive result means you have a disease and a negative result means you don’t have a disease.

I’ve written elsewhere about this and in the blog post, you’ll find links to a couple of posts I’ve written before.

This notion that we shouldn’t expect a positive result to mean a disease process is present is especially true for serology. It can also apply to nucleic acid amplification assays. Despite the claims of manufacturers and the theory of high specificity of PCR, it is not a panacea.

I’d like to cite a few examples:

Polymerase chain reaction

A common situation I come across is when a certain respiratory panel is used by one manufacturer and it cannot differentiate between rhinovirus and enterovirus. When the PCR result signals positive for this reaction, we still require a decent clinical and epidemiological history to make an accurate diagnosis.

MALDI-TOF

In a similar vein, this chemistry instrument which is used to identify bacteria and yeasts in microbiology medical testing laboratories, cannot differentiate between Escherichia coli and Shigella species.

Serology

Cross-reactions are common in serology. If we take, for example, Rickettsial infections, we usually see many cross-reactions. The patient is not going to be ‘positive’ for all these pathogens despite all the reactive serology results. We need to rely on the clinical features of the illness as well as pertinent epidemiological features.

While the result may be positive, it doesn’t mean the patient positively has the disease in question.

So just because your doctor says you have a positive test for something, it doesn’t mean you have the associated disease. This is true for many microbiology results and especially when the method of testing is an indirect form of diagnosis.

My final word is that tests used should be fit for purpose. A test should only be used for patients with the typical clinical presentation of the illness being tested. Using a test for patients with nonspecific symptoms will likely give a higher proportion of false reactive rather than true reactive results. In this situation, it is important the referring medical practitioner and the specialist pathologist agree that testing in such situations is inappropriate and can do more harm than good.

Recommended reading

Diagnostic test limitations

https://garydavidlum.com/2015/09/04/diagnostic-test-limitations/

Reactive versus positive serology

https://garydavidlum.com/2014/12/23/reactive-versus-positive-serology/

If you disagree with anything in these podcasts or if you would like to voice a different view, please feel free to write a comment. If I have said something incorrect I welcome correction. Please also feel free to share your comments on social media.

You can find me on Twitter and Facebook. Please follow me on Twitter and like my Facebook page.

MFF0004: The gonococcus

In truth, my experience in the Northern Territory of Australia is that we could grow gonococci from high vaginal swabs, low vaginal swabs, introital swabs and pus that was found on the inner thigh.


Medical Fun Facts logo from Gary Lum

As I mentioned in episode 3, the gonococcus is closely related to the meningococcus, in fact, in many ways, they could be regarded as a single species much in the same way some people consider Escherichia coli and Shigella spp. to be one species.

The formal name for the gonococcus is Neisseria gonorrhoeæ. It causes gonorrhoea or the clap.

In medical school, we were taught that the gonococcus is a very delicate and fastidious microorganism however that is not always the case. While it is virtually impossible to be infected from gonococcal pus on a toilet seat, it is possible to get gonococcal conjunctivitis from flies (see the reference below).

Neonatal ophthalmic infection can occur during passage through the birth canal and there is speculation about autoinoculation from infected genitalia to eyes/conjunctivæ.

I expect the fly-borne infection relates to the biological burden of infected sites with flies landing on areas contaminated by discharge or purulence and then landing on the conjunctivæ of someone close by.

On the subject of biological burden, we are taught that ideally specimens should be sampled from areas of columnar epithelium-like urethral and endocervical cells rather than stratified squamous epithelium. In truth, my experience in the Northern Territory of Australia is that we could grow gonococci from high vaginal swabs, low vaginal swabs, introital swabs and pus that was found on the inner thigh. We could also grow gonococci from first pass urine that had been collected for nucleic acid amplification assays.

Another nice thing about gonococci is while most have an outer surface capsule, some strains do not and they are resistant to the effects of complement so they can disseminate and then form infections in joints or the heart or the brain. Disseminated gonococcal infection is not that uncommon. In women when the infection is left untreated, we sometimes see infection spread via the fallopian tubes into the peritoneum and then coating the surface of the liver and spleen to cause Curtis Fitz Hugh syndrome.

One thing that is becoming more common apart from urethral and cervical infection is rectal and throat gonorrhoea.

Thank you for listening to Medical Fun Facts.

If you disagree with anything in these podcasts or if you would like to voice a different view, please feel free to write a comment. If I have said something incorrect I welcome correction.

Please also feel free to share your comments on social media.

You can find me on Twitter and Facebook. Please follow me on Twitter and like my Facebook page.

An outbreak of non-sexually transmitted gonococcal conjunctivitis in Central Australia and the Kimberley region. Rex Matters, Ignatius Wong, and Donna Mak. Comm Dis Intell 1998;22:52–58

MFF0003: The meningococcus

One of my greatest peeves and something that annoys me no end is when an illness is referred to as ‘meningococcal’ and that word is used as a noun when it is an adjective, that is, a describing word.


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Why would I want to talk about the meningococcus? It’s not so much the bacterium although it is a delightful microorganism. It’s more the derived words associated with the meningococcus.

Let me explain. The formal name for the meningococcus is Neisseria meningitidisNeisseria meningitidis causes nasty infections like meningitis, bacteræmia, septicæmia, and occasionally pneumonia. These are often described as meningococcal meningitis, meningococcal septicæmia and meningococcal disease.

One of my greatest peeves and something that annoys me no end is when an illness is referred to as ‘meningococcal’ and that word is used as a noun when it is an adjective, that is, a describing word. We learn these things in grade three. Surely it is not too much to ask of journalists and healthcare practitioners and health policy advisers and politicians to know the difference between an adjective and a noun. That’s the rant part of this podcast over, now to get to some fun facts about the meningococcus.

It is almost identical to the gonococcus and sometimes the meningococcus can cause urethral and throat infection as a sexually transmitted pathogen.

We commonly find the meningococcus in throat specimens, conjunctival specimens and sputum, but just because we isolate it from these sites doesn’t make it a finding of public health importance. The meningococcus can be a commensal organism and do no harm.

I have been involved in at least two patients from whom we have isolated a meningococcus from the blood and in these cases the person was well, afebrile, with no headache and no rash. I recall one patient was seen in the emergency department for vomiting and had blood collected for culture despite a lack of fever. To be safe though, because we grew the meningococcus from the blood each patient received appropriate antimicrobial treatment.

I will cover the gonococcus soon because that is truly a gorgeous microorganism.

If you disagree with anything in these podcasts or if you would like to voice a different view, please feel free to write a comment. If I have said something incorrect I welcome correction. Please also feel free to share your comments on social media.

You can find me on Twitter and Facebook. Please follow me on Twitter and like my Facebook page.

MFF0002: Golden staph

If you read a newspaper or listen to the news on the radio or watch the news on TV you’d think golden staph was a so-called superbug.


Staph and Gary Lum
My favourite golden staph

If you read a newspaper or listen to the news on the radio or watch the news on TV you’d think golden staph was a so-called superbug.

I hate calling bacteria bugs. Bugs have legs. Bugs are eucaryotic creatures. Bacteria are bacteria, some of them can be super virulent but that does not make them super resistant to treatment.

There’s a view amongst journalists without medical training that any bacterium which is resistant to multiple antibiotics is super. I disagree. In my mind, something super would have special powers in terms of virulence. A super bacterium wouldn’t only be resistant to common antimicrobial agents it would have a super penetrative capability, it would have super persistence, it would be resistant to the body’s cellular and humoral immune system.

Put simply golden staph is any run of the mill Staphylococcus aureus. It’s called staph because microscopically the cocci are arranged in grape-like clusters. It’s called golden because on horse or sheep blood agar, colonies of Staphylococcus aureus have a golden or yellow colouration.

The fact is these days most strains we see in the clinical laboratory are not golden but tend to be white. Which is a bit confusing because in the old days we’d call staphylococci that formed white colonies ‘Staph. albus’.

Golden staph prefers temperatures of about 30 °C and that’s why you’ll find it in your nostrils, armpits and groins. Because of the coagulase enzyme, it is good at forming abscesses which can range from a zit or a pimple to a large loculated carbuncle. I really like carbuncles, they are so cool to look at and even better to incise and drain. Mmm… pus!

As far as antimicrobial resistant golden staph is concerned the best-known group is MRSA or methicillin-resistant Staphylococcus aureus. MRSA can be broken down into multi or nonmulti resistant. Then there is vancomycin-resistant Staphylococcus aureus, VRSA is a particularly nasty bugger.

For the majority of mild superficial skin infections caused by golden staph, you do not need antimicrobials. Scrub the wound, keep it clean and dress it properly.

If you disagree with anything in these podcasts or if you would like to voice a different view please feel free to write a comment. If I have said something incorrect I welcome correction. Please also feel free to share your comments on social media.

You can find me on Twitter and Facebook. Please follow me on Twitter and like my Facebook page.

MFF0001: The pilot


I started Medical Fun Facts after a friend at work asked that whenever I visit her at her workstation I share with her a medical fun fact.

Given my knowledge-base is focused mainly on medical microbiology, infectious diseases, some aspects of pathology and a little bit about health emergency management, this podcast will probably reflect these areas. It’s highly unlikely I will delve into psychiatry or endocrinology, not because they are not interesting, but mainly because I don’t have much to do with those areas of medicine.

I will hopefully soon have this podcast on iTunes and Stitcher. Please bear with me and feel free to send me feedback via the contact page.

You can find me on Twitter and Facebook. Please follow me on Twitter and like my Facebook page.

First blog post

This is the first post on “The Medical Fun Facts Podcast” blog.


This is the first post on “The Medical Fun Facts Podcast” blog. The aim of this website is to showcase the MFF podcast which was originally hosted on a Squarespace site at drgarylum.com

I decided it would be better to start a WordPress.com version because of the greater flexibility of the WordPress.com platform. I hope this podcast will quickly get a place in iTunes and Stitcher so that iOS and Android users will be able to listen.

 

Medical Fun Facts logo from Gary Lum