More information about text formats
Evidence-Based-Medicine and The Glass Test - A Spoof
Reductio ad Absurdum and EBM
A 15 year old girl from an all-girls boarding-school was found to be
pregnant. She denied a history of sexual intercourse. We did this study to
see if there is any relationship between pregnancy and sexual intercourse.
Structured clinical question
In girls (patient)...
Structured clinical question
In girls (patient) is sexual intercourse (intervention) and pregnancy
Search strategy and outcome
Medline 1966-February 2004 using PubMed : ‘pregnancy’ AND ‘sexual
intercourse’. Limits: Unlimited and All languages
No relevant or irrelevant paper were found
Apparent Clinical Bottom Line
No evidence was found to support the widely held belief that pregnancy
results from sexual intercourse.
We used strategy similar to that used by Parikh and Maconochie (1) who
found no evidence to support the ‘glass test’ in the diagnosis of
petechiae . We found no evidence to substantiate the widely held belief
that sexual intercourse is often a prelude to pregnancy.
Reductio ad Absurdum is a mode of argument that seeks to establish a
contention by deriving an absurdity from its denial, thus arguing that the
thesis must be accepted because its rejection would be untenable. We
contend that the EBM technique used for testing the glass test is
There is a small chance that a good double blind randomized placebo
controlled study was indeed done, looking at sexual intercourse and
pregnancy but it did not get past the peer review process and so we could
not retrieve it in our search of literature after 1966.
It is possible that the EBM penchant for excessive structure and
oversimplification (take for example the ‘structured clinical question’:
In a child with a rash (patient) does a positive glass tumbler test (test)
really pick up petechiae (outcome); It is worded as if feeble-minded
readers would confuse the patient and the test, unless identified in
parenthesis) leads to use of ‘search terms’ that are inappropriate. To
test if changing the search terms can makes a difference to the yield in
this form of arm-chair-research, we decided to look for the term ‘non-
blanching rash’ AND ‘purpura’ in a search limited to the archives of the
Archives of Disease in Childhood after 2000, and it yielded the paper by
Wells et al (2) where the authors specifically studied ‘non-blanching rash
(defined by them as a rash that does not blanch when pressed with a glass)
There is no evidence that because one EBM study was badly designed,
all EBM is bunkum. However there is old fashioned anecdotal evidence that
EBM reports do not change clinical practice. Published EBM reports are
used harmlessly in the curriculum vitae (CV) to enlarge ones list of
publications, and the same motivation probably prompted the authors of
this communication to undertake the study (declaration of conflict of
What are the odds that the editor will notice this letter among the
hundreds received as electronic-responses-to-published-articles, and pick
this one up for publication in the print version? Odds are low by the
simple rule of probability. PLAN: Send article as ‘fresh research on
pregnancy,’ to Bench Press.
What is the probability that the editor will send this paper for peer
review to an expert on EBM who by definition is biased against publication
of this article? Probability: Very high. PLAN: Send the article
simultaneously to the electronic-response site and refer to it in the CV
1. Parikh A, Maconochie I. What is the use of the glass test?
Arch Dis Child 2003; 88: 1135.
2. Wells LC, Smith JC, Weston VC, Collier J, and Rutter N
The child with a non-blanching rash: how likely is meningococcal disease?
Arch Dis Child 2001; 85: 218 - 222.