Current Clostridium difficile hospital tests not accurate enough and new 2-stage process needed

October 31, 2008

Current hospital tests for the bacterium Clostridium difficile (C. difficile) are not accurate enough and a new two-stage process should be introduced to avoid misdiagnosis and its consequences. These are the conclusions of a Review on the effectiveness of commonly used C. difficile test kits published early Online and in the December edition of The Lancet Infectious Diseases. The paper was written by Professor Sanjeev Krishna, St George's University of London and St George's Healthcare NHS Trust, London, UK, and colleagues.

C. difficile is a hospital-acquired infection that can cause diarrhoea and severe bowel inflammation, leading to death in 6-15% of cases. Patients on antibiotics are at high risk because C. difficile can thrive when the normal balance of the gut is upset. The elderly are particularly at risk, with 80% of C. difficile cases affecting people over 65. The consequences of a misdiagnosis can be severe -- tests that give false positive results can lead to antibiotic treatment for other conditions being stopped, patients treated inappropriately for C. difficile infection, and isolation with other C. difficile patients which can then lead to them contracting the condition. Tests that give a false negative result will mean the patient will not get the C. difficile treatment they require, leading to more serious illness for them and possible infection for other patients close to them.

Most brands of hospital testing kits detect a C. difficile toxin (CDT) in a patient's stool sample. However the researchers show in the Review that current tests on the market can have proportions of positive results that are false from 3-45%, and proportions of true positives that are missed from 5-24%.* The authors say: "No assay reliably fulfilled the criteria we preset for an acceptable single test to detect CDT." Whilst the tests were similar in their accuracy, some were more likely to return a false positive result ,whereas others were more likely to miss a case -- such variation makes comparisons of C. difficile prevalence in different UK health trusts difficult (since different trusts use different kits).

To improve diagnostic accuracy, the authors propose a new two-stage testing system. Stage one would be an initial, rapid, highly sensitive screening assay done on the day of receipt of the patient's stool sample -- this would detect nearly all positives and mean that confirmed negative results are issued promptly. The second stage would then be a confirmatory test that would weed out the false positives using the current, reference method and provide a definitive result within 2-5 days. The authors say: "We are currently evaluating such a testing scheme in our department."

They conclude by saying that the additional costs caused by the proposed new first stage could be offset by reducing number of time-consuming reference method tests that would be necessary in the second stage. Estimating the extra first-stage costs at £10,000-£30,000 per year, they add: "We predict the savings in antibiotic costs, the enhanced use of isolation facilities, reduced burden on infection control, and reduced cases of C. difficile associated diarrhoea across the hospital will offset these additional laboratory expenditures."

As a result of huge public interest in infections of this nature, The Lancet Infectious Diseases will be hosting a conference on healthcare-associated infections featuring experts in the field from around the globe. The meeting will take place from December 11-12 at the QEII Conference Centre, London, UK**.

John McConnell, editor of The Lancet Infectious Diseases, says: "Modern medicine faces few greater challenges than that of healthcare-associated infections (HAIs). In the UK, one of the worst-affected countries in Europe, HAIs are estimated to cost the National Health Service at least £1 billion per year. Media attention has increased public pressure to tackle HAIs -- a pressure that clinicians, researchers, and policy makers must deal with. The conference programme has been designed to combine state-of-the-art lectures with the opportunity for delegates to discuss management of HAIs with key opinion leaders."
For Professor Sanjeev Krishna, St George's University of London and St George's Healthcare NHS Trust, London, UK, please contact Tamsin Starr, Communications T) +44(0) 208 725 1139 / +44 (0) 7786 982028 E) / /

The Lancet Infectious Diseases Press Office T) +44 (0) 20 7424 4949 E)

Notes to editors:

For full Review please see:

**please see for more information on the conference

*Effectiveness of the six most commonly used tests (prepared by St George's University of London Press Office):

This is a digest of the information available in Table 2 and Figure 4 of the study. Please note these are approximate figures after the results of several studies have been combined and there was considerable variation between studies.

* please see Table 2 - column "Sensitivity "- this number is equivalent to (1-median sensitivity) expressed as a percentage

† this is information from figure 4. We assume a prevalence of C. difficile of 10% - which is approximately the current situation. The number here is (1-positive predictive value) expressed as a percentage. Read off the graph at a prevalence of 10%


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