I know it is difficult to believe but I was doing a little bit of background reading recently on Monospot tests (looking for heterophile antibodies to Epstein Barr Virus). On reading a guideline I came across this statement…
“The presence of heterophile antibodies in a symptomatic adolescent or young adult has a sensitivity of approximately 90%, and specificity of almost 100% for glandular fever.”
Now the question I have is what is wrong with this statement?
The answer is that sensitivity and specificity are functions of the test itself. Different tests for the same disease from different manufacturers may have different sensitivity and specificity.
However once you start applying the test to population cohorts such as symptomatic adolescents, then you need to start talking in terms of positive and negative predictive value.
….and the paradox is when you use a test such a Monospot, with a sensitivity of approximately 80%, in a high prevalence population such as symptomatic adolescents,then your negative predictive value will be relatively lower than in a low prevalence cohort, as there will be a significant amount of people who will test negative who actually have the disease (false negatives).
Sometimes we need to think about the science behind the statements in the guideline and make sure it makes sense in our heads.
Don’t believe everything you read, (especially when it is written by me!)
For a really nice presentation on sensitivity, specificity, PPV etc click here (5-10 minute read)
I have been thinking about hepatitis serology recently and more particularly, best practice when trying to diagnose a viral hepatitis using serological testing.
There are several viral causes of hepatitis, such as Hep A, B, C, D & E, Epstein Barr virus, cytomegalovirus, and HIV. (Toxoplasmosis often included in this group as well, even though not a virus!)
….and that is even before you get started on the more esoteric viral causes of hepatitis.
However all these viruses have varying clinical presentations, different incubation periods and particular risk factors. Some are acute and some are chronic.
I therefore find it a little frustrating when the request form asks for “hepatitis serology” without specifying the particular viruses that require testing, along with the clinical rationale.
As a laboratory profession, I don’t think we do ourselves or our patients any favours by accepting non-specific requests such as “hepatitis serology”, “viral hepatitis screen”, “hepatitis screen” etc etc. It is esssentially encouraging poor practice.
“Hepatitis serology” is not really a test request. It is more of a chapter in a textbook…..
Screening for cervical cancer is traditionally carried out by cytology of a cervical smear.
Over the next 5 years or so, this will change worldwide to primary screening with Human Papilloma Virus (HPV) testing by PCR. This has massive implications for cervical screening services and also staff that perform cervical cytology.
In 1975, Professor zur Hausen hypothesized that human papillomavirus was a necessary cause of cervical cancer. For this he won the Nobel prize in 2008.
There is little doubt that the evidence supporting primary screening (more or less) in place of cytology is overwhelming, but this raises a few secondary questions…
What is the optimal screening interval, given that it takes several years for cervical cancer to develop from primary HPV infection.
When is the optimal age to stop screening?
What future impact will the recently introduced HPV vaccine have on the cost effectiveness of screening?
This is where we need to be careful. The majority of research trials into HPV testing for cervical cancer screening are carried out/sponsored by the large corporate companies that produce and sell HPV tests. These companies have a clear conflict of interest in the answers to the three questions above. This is not their fault, but the conflict is there nevertheless. We just need to be very careful with anything we read (or are given to read) on the topic
HPV is a fascinating area. If I was setting an exam on molecular testing, then HPV would be more or less at the top of my list of questions, and I am afraid I would be a ruthless examiner!