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UK C-diff testing "inaccurate and inconsistent"

OnMedica staff

Friday, 31 October 2008

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Current hospital tests for the bacterium C difficile are not accurate enough and a new two-stage process should be introduced to avoid misdiagnosis and its consequences

New research by Professor Sanjeev Krishna from St George’s Healthcare NHS Trust, London and colleagues published today Online and in the December edition of Lancet Infectious Diseases, says a new process, although initially more expensive, would save money and lives by ensuring appropriate treatment.

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 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).

The authors propose a new two-stage testing system starting with a rapid, highly sensitive screening assay done on the day of receipt of the patient’s stool sample to 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.”

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